68-�

Report on Comments F2006 — Copyright, NFPA NFPA 68 Report of the Committee on

Explosion Protection Systems

Samuel A. Rodgers, ChairHoneywell, Incorporated, VA [U]

Luke S. Morrison, SecretaryProfessional Loss Control Incorporated, Canada [SE]

Joe R. Barton, Fountaintown, IN [SE]Kenneth L. Cashdollar, US Department of Health & Human Services, PA [RT]Michael Davies, PROTEGO (USA) Incorporated, SC [M]Alexi I. Dimopoulos, ExxonMobil Corporation, VA [U] Rep. American Petroleum InstituteRobert J. Feldkamp, Nordson Corporation, OH [M]Larry D. Floyd, Ciba Specialty Chemicals Corporation, AL [U]Joseph P. Gillis, Westboro, MA [SE]John E. Going, Fike Corporation, MO [M]Stanley S. Grossel, Process Safety & Design, Incorporated, NJ [SE]Dan A. Guaricci, ATEX Explosion Protection L.P., FL [M]Michael D. Hard, Hard Fire Suppression Systems, Incorporated, OH [IM] Rep. Fire Suppression Systems AssociationDavid D. Herrmann, E. I. DuPont de Nemours & Company, DE [U]David C. Kirby, Baker Engineering & Risk Consultants, WV [SE]Richard S. Malek, Eastman Kodak Company, NY [U]Steven A. McCoy, National Starch & Chemical Company, IN [U] Rep. NFPA Industrial Fire Protection SectionRobert W. Nelson, Pocasset, MA [I] Rep. Swiss Re, Global Asset Protection ServicesJames O. Paavola, DTE Energy/Detroit Edison Company, MI [U]Mitchel L. Rooker, BS&B Safety Systems, LLC, OK [M]Joseph A. Senecal, Kidde-Fenwal, Incorporated, MA [M]Bill Stevenson, Cv Technology, Incorporated, FL [M]Stephen M. Stuart, Marsh USA Incorporated, MI [I]Erdem A. Ural, Loss Prevention Science & Technologies, Incorporated, MA [SE]Bert von Rosen, Natural Resources Canada, Canada [E]Robert G. Zalosh, Worcester Polytecnic Institute, MA [SE]

Alternates

Geof Brazier, BS&B Safety Systems, LLC, OK [M] (Alt. to Mitchel L. Rooker)Gary A. Chubb, Chubb Engineering, LLC, KS [M] (Voting Alt. for Columbian TecTank) David G. Clark, E. I. DuPont de Nemours & Company, DE [U] (Alt. to David D. Herrmann)Martin P. Clouthier, Marsh Canada Limited, Canada [I] (Alt. to Stephen M. Stuart)Ettore Contestabile, Natural Resources Canada, Canada [E] (Alt. to Bert von Rosen)

Randal R. Davis, Kidde-Fenwal, Incorporated, MA [M] (Alt. to Joseph A. Senecal)Todd A. Dillon, Swiss Re, Global Asset Protection Services, OH [I] (Alt. to Robert W. Nelson)Kirk W. Humbrecht, Phoenix Fire Systems, Incorporated, IL [IM] (Alt. to Michael D. Hard)Edward L. Jones, Nordson Corporation, OH [M] (Alt. to Robert J. Feldkamp)Peter J. McWilliams, Eastman Kodak Company, NY [U] (Alt. to Richard S. Malek) Richard F. Schwab, Honeywell, Incorporated, NJ [U] (Alt. to Samuel A. Rodgers)Jef Snoeys, Fike Corporation, Belgium [M] (Alt. to John E. Going)

Nonvoting

Franz Alfert, Inburex Consulting, Germany [SE]Laurence G. Britton, Neolytica, WV [SE]Vladimir Molkov, University of Ulster, Northern Ireland, UK [SE]Harry Verakis, US Department of Labor, WV [E]Walter B. Howard, Omaha, NE [SE] (Member Emeritus)

Staff Liaison: Guy R. Colonna

Committee Scope: This Committee shall have primary responsibility for documents on explosion protection systems for all types of equipment and for buildings, except pressure venting devices designed to protect against overpressure of vessels such as those containing flammable liquids, liquefied gases, and compressed gases under fire exposure conditions, as now covered in existing NFPA standards.

This list represents the membership at the time the Committee was balloted on the text of this edition. Since that time, changes in the membership may have occurred. A key to classifications is found at the front of this book.

This portion of the Technical Committee Report of the Committee on Explosion Protection Systems is presented for adoption.

This Report on Comments was prepared by the Technical Committee

on Explosion Protection Systems, and documents its action on the comments received on its Report on Proposals on NFPA 68, Guide for Venting of Deflagrations, 2002 edition, as published in the Report on Proposals for the 2006 Fall Meeting.

This Report on Comments has been submitted to letter ballot of the Technical Committee on Explosion Protection Systems, which consists of 27 voting members. The results of the balloting, after circulation of any negative votes, can be found in the report.

68-2

Report on Comments F2006 — Copyright, NFPA NFPA 68 ____________________________________________________________68-� Log #CC3 Final Action: Accept(Entire Document)____________________________________________________________Submitter: Technical Committee on Explosion Protection Systems Comment on Proposal No: 68-3Recommendation: Delete unit conversion for the 20 L test sphere wherever it appears in the document. Only show the test apparatus as 20 L with no conversion to other units, such as gallons.Substantiation: US units are not appropriate to describe the test vessel as it is always referred to as the 20 L sphere, so the conversion to gallons needs to be deleted to reflect this convention.Committee Meeting Action: AcceptNumber Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Comment on Affirmative: URAL, E.: A. Replace Figure B.2.4.� (which was taken from Bartknecht �978) with the updated figure from Bartknecht �993 shown below:

Existing figure is the source of a misconception that Kst is independent of particle size as long as the latter is below �00 microns or 20 microns. This is contradicted by available data. B. Replace 3.3.2 Combustible Dust definition with the definition of word “Combustible” alone. Alternatively define the following terms used in the proposed standard: combustible gas, combustible vapor, combustible mixture, combustible mist, and combustible materials. NFPA 654 definition adopted here is misleading (See the explanation for my NFPA 69 ROP negative ballot.) C. Remove the word “cloud” from Kg definition in 3.3.�8. D. Replace the word “cloud” in Kg definition in 3.3.�9 with “when suspended in air.” E. Revise 3.3.26 as follows: 3.3.26 Rate of Pressure Rise (dP/dt). The derivative of the smoothed pressure time curve with respect to time. The increase in pressure divided by the time interval necessary for that increase to occur. F. Include a new definition for “Building” for which the proposed standard imposes the 70 percent dust explosion vent area penalty. G. Reword Pes definition in 3.3.30.� to eliminate contradictions with Section 4.3. H. Shouldn’t the ultimate strength definition in 3.3.30.2 include non-structural components that can create casualties when failed? I. Delete 7.�.3 as it contradicts 7.2.2.2. J. Revise 7.2.2.3 as follows: 7.2.2.3 The design of deflagration venting for mists shall be based on the venting parameter for propane, or 7.2.2.2, whichever is more conservative. K. Add Reference (��5) just before Equation 8.�. L. Add Reference (��5) just before Equation 8.2. M. Add Reference (��6, zalosh halifax) just before Equation 8.3. N. Add Reference (��6, zalosh halifax) just before Equation 8.4A. O. Add Reference (��6, zalosh halifax) just before Equation 8.4B. P. Reference (��7) Mitch Rooker’s work in 8.2.7, 8.2.8, 7.2.2.5, and 7.2.2.6. Q. Replace Figure B.2.4.2.� (unpublished data) with the data published by ASTM E27 committee members in the 2004 Loss Prevention Symposium shown in the next column:

Add appropriate reference.

____________________________________________________________68-2 Log #3 Final Action: Accept(3.3.x Friction Factor and A.3.3.x (New))____________________________________________________________Submitter: Samuel A. Rodgers, Honeywell, Inc.Comment on Proposal No: 68-3Recommendation: Add a definition for Friction Factor, using the D’Arcy form, and provide annex material as follows: 3.3.x* Friction Factor, fD. D’Arcy friction factor relating pressure drop in a straight duct to velocity and wetted surface area, dimensionless.

2

2 hD

D Pf

U L⋅ ∆

=ρ ⋅ ⋅

Where:

Dh = hydraulic diameter

∆P = pressure loss across the duct

ρ = fluid density

U = fluid velocity (shown here as U to avoid confusion with volume)

L = duct length

A.3.3.x At least two friction factors are in common usage, the D’Arcy friction factor as used in this document and the Fanning friction factor. The two forms differ by a factor of 4, as seen below:

Equation A.3.3.x(a) 22

hF

D Pf

U L⋅ ∆

=ρ ⋅ ⋅

, the Fanning friction factor

Equation A.3.3.x(b) 2

2 hD

D Pf

U L⋅ ∆

=ρ ⋅ ⋅

, the D’Arcy friction factor

Equation A.3.3.x(c) fD = 4fF

The equivalent velocity head loss for straight duct is expressed as

Equation A.3.3.x(d) 4 F

h

f LK

D⋅

= when using the Fanning friction factor

Equation A.3.3.x(e) D

h

f LK

D⋅

= when using the D’Arcy friction factor

D’Arcy friction factors are presented in Moody diagrams and can be calculated from equations which represent the diagrams. See 750, Standard on Water Mist Fire Protection Systems, for a Moody diagram. Similar diagrams are also available to provide Fanning friction factors. In order to be sure that the appropriate diagram is being used, the user can examine the laminar region. In the laminar region, i.e., low Reynolds number, the D’Arcy friction factor equals 64/Re. The Fanning friction factor in the laminar region equals �6/Re.

68-3

Report on Comments F2006 — Copyright, NFPA NFPA 68 Colebrook equations model the friction factor using implicit equations, which must be solved iteratively. The factor of 4 difference can be seen in the similar equations below:

Equation A.3.3.x(f) 10

1 1.2554 log

3.7 hF FDf R f

ε= − +

for the Fanning

friction factor

Equation A.3.3.x(g) 10

1 2.512log

3.7 hD DDf R f

ε= − +

for the D’Arcy

friction factor

Where:

ε is the Absolute Roughness

R is the dimensionless Reynolds Number

Note that ε/D is the dimensionless Relative Roughness

When applied to venting, the friction factor is evaluated at fully turbulent conditions, meaning a very large Reynolds number. For these conditions, the D’Arcy form of the Colebrook equation is rearranged and simplified to allow a direct solution as below:

10

12log ( 0)

3.7 hD Df

ε= − + ≈

10

11.14 2log

hD Df

ε= −

Equation A.3.3.x(h) f

D

D

h

=−

1

1 14 2

2

. log10

ε

Substantiation: The text references to friction factor do not clearly differentiate between the commonly used variations of this parameter. By providing a definition and annex material, proper application will be ensured.Committee Meeting Action: AcceptNumber Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Comment on Affirmative: FLOYD, L.: Gee, I have always heard it referred to as “Darcy Weisbach” or “Darcy” for short. Learn something new every day!

____________________________________________________________68-3 Log #�2 Final Action: Accept(Chapter 4, 5.3.2 and 5.3.3, and Chapter 6)____________________________________________________________Submitter: Samuel A. Rodgers, Honeywell Inc.Comment on Proposal No: 68-3Recommendation: Reorder Chapters as follows: Chapter 4 becomes Chapter 6. Chapter 5 becomes Chapter 4. Chapter 6 becomes Chapter 5. Revise 5.3.2 and 5.3.3 as shown: 5.3.2 Performance-Based Design. A performance-based design shall be in accordance with Chapter 6 5 of this standard. 5.3.3 Prescriptive-Based Design. A prescriptive-based design shall be in accordance with Chapter 4 and Chapter 7 6 through Chapter �� of this standard.Substantiation: This reorganization moves all the prescriptive requirements to appear in order, by moving the performance-based requirements first, followed by all the prescriptive requirements.Committee Meeting Action: AcceptNumber Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.

____________________________________________________________68-4 Log #�0 Final Action: Accept in Principle(4.1.2 and 8.1.1)____________________________________________________________Submitter: Samuel A. Rodgers, Honeywell Inc.Comment on Proposal No: 68-3Recommendation: Add 4.�.2.� to permit ISO Methods for KSt, deleting 8.�.�. Relocate A.8.�.� to be new A.4.�.2. Add sampling guidance to the end of A.4.�.2 as follows: 4.�.2* For dusts, KSt and Pmax shall be determined in approximately spherical calibrated test vessels of at least 20 L (5.3 gal) capacity per ASTM E �226, Standard Test Method for Pressure and Rate of Pressure Rise for Combustible Dusts. 4.�.2.� It shall be permitted to determine KSt and Pmax per ISO 6�84/�, Explosion Protection Systems - Part �: Determination of Explosion Indices of Combustible Dusts in Air or CEN... 8.�.�* The variable KSt is a measure of the deflagration severity of a dust and shall be as established by the test requirements of ASTM E �226, Standard Test Method for Pressure and Rate of Pressure Rise for Combustible Dusts or ISO 6�84/�, Explosion Protection Systems. Part �: Determination of Explosion Indices of Combustible Dusts in Air.A.4.�.2 (relocated existing A.8.�.�) Current vent sizing methodology is based upon KSt as determined by ASTM E �226 or the similar ISO 6�84/�. Determination of KSt values by methods other than these would be expected to yield different results. Data from the Hartmann apparatus should not be used for vent sizing. Also, the 20 liter test apparatus is designed to simulate results of the � m3 chamber, however, the igniter discharge makes it problematic to determine KSt values less than 50 bar-m/sec. Where the material is expected to yield KSt values less than 50 bar-m/sec, testing in a � m3 chamber might yield lower values. The KSt value needs to be verified by specific test of a dust that has been created by the process that created the dust. There are reasons why this needs to be done. The shape and particle size distribution of the dust is affected by the mechanical abuse that the material has undergone by the process which has created the dust in the first place. An example of this is a polymeric dust created by the suspension polymerization of styrene (in water) results in particle shapes which are spherical (resembling small spheres). A polymeric dust created by sending a bulk polymerized polystyrene block through a hammermill results in a dust that has been fractured and has many sharp edges and points. Even if the sieve size distribution of the two types of particles are similar, the specific surface area of the spherical particles can be much smaller than the particles generated by hammermill. The KSt values for these 2 samples will be different. The rate of pressure rise for the spherical particles will be slower than the dust sample created by the hammermill operation. Guidance for representative particulate sampling procedures can be found in ASTM D5680a, Standard Practice for Sampling Unconsolidated Solids in Drums or Similar Containers, or Guidelines for Safe Handling of Powders and Bulk Solids, Section 4.3.� (CCPS).Substantiation: The standard should define KSt at the first usage, and in only one place, with an indication as to the preferred and alternate methods. The existing annex for 8.�.� is relevant to both test methods and should be retained in support of the combined requirement in Chapter 4, however, more guidance is needed with regard to appropriate sampling methods.Committee Meeting Action: Accept in Principle Add 4.�.2.� to permit ISO Methods for KSt, deleting 8.�.�. Relocate A.8.�.� to be new A.4.�.2. Add sampling guidance to the end of A.4.�.2 as follows: 4.�.2* For dusts, KSt and Pmax shall be determined in approximately spherical calibrated test vessels of at least 20 L (5.3 gal) capacity per ASTM E �226, Standard Test Method for Pressure and Rate of Pressure Rise for Combustible Dusts. 4.�.2.� It shall be permitted to determine KSt and Pmax per ISO 6�84/�, Explosion Protection Systems - Part �: Determination of Explosion Indices of Combustible Dusts in Air.or CEN... 8.�.�* The variable KSt is a measure of the deflagration severity of a dust and shall be as established by the test requirements of ASTM E �226, Standard Test Method for Pressure and Rate of Pressure Rise for Combustible Dusts or ISO 6�84/�, Explosion Protection Systems. Part �: Determination of Explosion Indices of Combustible Dusts in Air.A.4.�.2 (relocated existing A.8.�.�) Current vent sizing methodology is based upon KSt as determined by ASTM E �226 or the similar ISO 6�84/�. Determination of KSt values by methods other than these would be expected to yield different results. Data from the Hartmann apparatus should not be used for vent sizing. Also, the 20 liter test apparatus is designed to simulate results of the � m3 chamber, however, the igniter discharge makes it problematic to determine KSt values less than 50 bar-m/sec. Where the material is expected to yield KSt values less than 50 bar-m/sec, testing in a � m3 chamber might yield lower values. The KSt value needs to be verified by specific test of a dust that has been created by the process that created the dust. There are reasons why this needs to be done.

68-4

Report on Comments F2006 — Copyright, NFPA NFPA 68 The shape and particle size distribution of the dust is affected by the mechanical abuse that the material has undergone by the process which has created the dust in the first place. An example of this is a polymeric dust created by the suspension polymerization of styrene (in water) results in particle shapes which are spherical (resembling small spheres). A polymeric dust created by sending a bulk polymerized polystyrene block through a hammermill results in a dust that has been fractured and has many sharp edges and points. Even if the sieve size distribution of the two types of particles are similar, the specific surface are of the spherical particles can be much smaller than the particles generated by hammermill. The KSt values for these 2 samples will be different. The rate of pressure rise for the spherical particles will be slower than the dust sample created by the hammermill operation. Guidance for representative particulate sampling procedures can be found in ASTM D5680a, Standard Practice for Sampling Unconsolidated Solids in Drums or Similar Containers, or Guidelines for Safe Handling of Powders and Bulk Solids, Section 4.3.� (CCPS).Committee Statement: US units are not appropriate to describe the test vessel as it is always referred to as the 20 L sphere, so the conversion to gallons has been deleted to reflect this convention. Otherwise, the Committee accepted the intent of the submitter.Number Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Comment on Affirmative: FLOYD, L.: Is there value in referencing either ASTM E789 “Standard Test Method for Dust Explosions in a �.2-Litre Closed Cylindrical Vessel”, or National Materials Advisory Board document 353-3 to further describe the Hartmann apparatus? ZALOSH, R.: The underlined sentence in A4.�.2 refers to two references as being alternative references on sampling, and allows the reader to choose which reference to use. Since the two references do not contain the same information, I suggest the following minor editorial change: change “or Guidelines for Safe Handling …” to “and Guidelines for Safe Handling…”

____________________________________________________________68-5 Log #�� Final Action: Accept(4.1.2.1)____________________________________________________________Submitter: Samuel A. Rodgers, Honeywell Inc.Comment on Proposal No: 68-3Recommendation: Revise 4.�.2.� to read: 4.�.2.�* It shall be permitted for the owner/user to test the dust with moisture content and particle size that deviates from those the recommended conditions established by the method described in 4.�.2 or 4.�.2.�(new) provided a documented risk assessment acceptable to the authority having jurisdiction has been performed prior to using these KSt and Pmax values to determine vent sizing.Substantiation: Clarify the ASTM E �226 does not require the normal particle size and moisture contents that have been used to develop the Committee data. Since the majority of the data is based upon tests using the recommended conditions, it is these conditions that form the basis for the Committee correlations.Committee Meeting Action: AcceptNumber Eligible to Vote: 27Ballot Results: Affirmative: 23 Negative: � Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Explanation of Negative: URAL, E.: Remove the words “...for the owner/user...”. Also the substantiation of this proposed change is incorrect. ASTM E�226 states “Tests may be run on an as received sample.”Comment on Affirmative: ZALOSH, R.: I suggest the following sentence be added to 4.�.2.� for clarification and emphasis: The assessment shall include an explanation for any deviation from the standard test conditions.

____________________________________________________________68-6 Log #CC�2 Final Action: Accept(4.2.2.2)____________________________________________________________Submitter: Technical Committee on Explosion Protection Systems Comment on Proposal No: 68-3Recommendation: Revise 4.2.2.2 as follows: 4.2.2.2 Where the dust mixture composition is not certain, the vent size shall be based on the highest KSt of all components and the highest Pmax of all components.Substantiation: This revision clarifies the intention for determining protection for dust mixtures based upon the key dust hazard properties, KSt and Pmax.Committee Meeting Action: AcceptNumber Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.

____________________________________________________________68-7 Log #6 Final Action: Accept in Principle(4.2.6.3 (New) )____________________________________________________________Submitter: Bill Stevenson, CV Technology, Inc.Comment on Proposal No: 68-3Recommendation: Add new text to read as follows: 4.2.6.3 The provision of a deflector shall be permitted to be used where the deflagration vent openings are located closer than the distances determined by 7.6.4 and Section 8.8. The deflector approximately halves the distance calculated, which defines the limit for safe distance using this technique. 4.2.6.4 The deflector shall meet the following minimum criteria: (a) Be �.75 × the dimensions of the vent and at least 3 times the area. (b) The deflector shall be inclined 45 degrees to 60 degrees from the vent axis to deflect the ejected flame. (c) The distance from the vent opening to the deflector shall be �.5 D, where D is the equivalent diameter of the vent. (d) The deflector plate shall be mounted so as to withstand the force exerted by the vented explosion, which can be calculated by multiplying Pred × Deflector Plate Area. (e) The enclosure volume limit is not greater than 20 cubic meters.

Explosion panel

EnclosureExclusion distance

Strongly mounted deflector plate

D45°–60°

1.5 D

FIGURE 5 Design of a blast deflector plate.

Substantiation: The use of explosion vents is limited if there is not a safe distance from the vent opening to nearby structures. The use of a deflector plate to redirect the flame and pressure was tested by the Health and Safety Laboratory, Harpur Hill, Bruxton, UK, final report “Effect of Deflector Plates on Vented Dust Explosions, EC/98/29, dated 20 November, �998, provided the basis for the text.Committee Meeting Action: Accept in Principle Replace existing 4.6.2.3 with the following and renumber as needed: 4.6.2.3* Where a deflector is provided in accordance with sections 4.6.2.4 and 4.6.2.5, it shall be permitted to reduce the axial (front-centerline) hazard distance to 50% of the value calculated in 7.6.4 or 8.8.2. This method shall not be used to reduce the radial hazard distance as defined in 7.6.4.2 and 8.8.2.2. [��6] A.4.6.2.3 A deflector is considered to be a specific subset of the general concept of a barrier. Walls or 3-sided containment constructions are used to minimize the hazard of fragments and flame impingement from a deflagration; however, if the wall is too close or if the containment volume is too small Pred will increase and pressure will build between the barrier and the vent. The effectiveness of the wall is limited to the area immediately behind it. Pressure and flame effects will reform at some point downstream of the wall. 4.6.2.4* A deflector design shall meet all of the following criteria. (�) The deflector for a rectangular vent shall be geometrically similar to the vent and sized with a linear scale factor of at least �.75. For a round vent the deflector shall be square shaped and at least �.75 times the vent diameter. (2) The deflector shall be inclined 45 degrees to 60 degrees from the vent axis, see Figure 4.6.2.4. (3) The centerline of the deflector shall be coincident with the vent axis. (4) The distance from the vent opening to the deflector on the vent axis shall be �.5 D, where D is the equivalent diameter of the vent. (5) The deflector plate shall be mounted so as to withstand the force exerted by the vented explosion, calculated as Pred times Deflector Area. (6) Deflector location shall not interfere with the operation of hinged vent closures.

68-5

Report on Comments F2006 — Copyright, NFPA NFPA 68

Explosion panel

EnclosureExclusion distance

Strongly mounted deflector plate

D45°–60°

1.5 D

Figure 4.6.2.4 Design for an installation of a blast deflector plate.

A.4.6.2.4 Other deflector designs are possible, but design information is not available at this time. An alternative could be to use a vent duct consisting of a long radius elbow, accounting for the effect of vent area according to Chapter 8 for dusts. A vertical barrier wall could result in higher Pred or larger radial hazard distance than an angled deflector and no design guidance can be given. 4.6.2.5* A deflector to limit flame length shall not be used: (�) For enclosure volume >20 cubic meters, or (2) With a tethered or translating vent closure. A.4.6.2.5 A deflector inclined at 45 to 60 degrees can be applied to larger vessels to protect personnel as long as it is installed more than �.5 D from the vent opening so as to not increase Pred. The ability of this deflector to limit flame length for these larger vessels is uncertain. Add the following to Annex K: ��6 Health and Safety Laboratory, Harpur Hill, Bruxton, UK, final report “Effect of Deflector Plates on Vented Dust Explosions, EC/98/29, dated 20 November, �998.Committee Statement: The Committee accomplished the intent of the submitter with the addition of requirements applicable to deflectors. The Committee also distinguished deflectors from barriers. The Committee enhanced and clarified the requirements recommended by the submitter with the addition of annex support material for several of the new paragraphs.Number Eligible to Vote: 27Ballot Results: Affirmative: 22 Negative: 2 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Explanation of Negative: STEVENSON, B.: There is no design basis for the committee action to remove the 20 cubic meter volume limit. URAL, E.: There are several serious problems with the proposed text, including: A) Dimensioning of the deflector is not required to be done on its projection on a plane normal to the vent discharge direction. B) The proposed �.75 factor covers vent discharge expansion+derivations of up to �4 degrees. This factor may not be adequate for typical (low) Pred applications where substantial flow velocity (momentum) might develop in the vent plane inside the enclosure. It is not clear the particular design was validated for such applications. C) Where hinged vents or asymmetrically opening panels are used, fireball may be directed to miss the deflector altogether. D) The deflector may conceivably act as a mixing or flame-holding baffle and thus enhancing the likelihood and severity of external explosions for initially fuel rich mixtures. It is not clear the particular design was validated for venting of fuel rich mixtures.

____________________________________________________________68-8 Log #38 Final Action: Accept in Principle in Part(4.5 and 4.8)____________________________________________________________Submitter: Mitchel L. Rooker, BS & B Safety SystemsComment on Proposal No: 68-3Recommendation: Add text to read: Snow and ice accumulation shall be prevented by a weather cover designed for that purpose, deicing plan, or accounted for within an approved risk analysis calculation. Weather covers shall have a burst pressure less than 50 percent of the vent Pstat. Weather covers shall have a mass less than 50 percent of MT or the vent mass.Substantiation: These are obvious requirements that are too often ignored. VDI 3673 Section 9.2 has mentioned this issues for at least �0 years.

Committee Meeting Action: Accept in Principle in Part Add new text as 4.5.2.3 to read: 4.5.2.3 To prevent snow and ice accumulation, where the potential exists, and to prevent entry of rain water and debris, the vent or vent duct exit shall not be installed in the horizontal position, unless alternative methods in 4.5.2.3.� are followed. 4.5.2.3.� Any of the following alternative methods of protection for horizontal vent or vent duct exits shall be permitted: �. Fixed rain hats where Pred effects on vent area are included in accordance with 8.5 and restraint design includes maximum force from Pred applied over the area. 2. Weather covers mounted at an angle sufficient to shed snow, with restraints designed and tested to prevent the cover from becoming a free projectile, where inertia effects of the additional weather cover mass and Pstat of the cover are included. 3. Deicing provisions such as a heated vent closure. Based upon action in Comment 68-3 (Log #�2), this becomes Chapter 6.Committee Statement: The Committee applied the more general design criteria for vent inertia and Pstat already contained in the standard, rather than include the specific additional criteria proposed by the submitter. The Committee’s action accomplishes the intent of the submitter to emphasize the need to prevent snow and ice and rain water accumulation and provides alternative methods to accomplish this objective.Number Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Comment on Affirmative: URAL, E.: Replace all occurrences of “rain hat” with “rain cover”.

____________________________________________________________68-9 Log #27 Final Action: Accept(6.1.1 and A.6.1.1 (New) )____________________________________________________________Submitter: Samuel A. Rodgers, Honeywell Inc.Comment on Proposal No: 68-3Recommendation: Revise text to read as follows: 6.�.�* Qualifications. The performance-based design shall be prepared by a person with qualifications acceptable to the authority having jurisdiction. Add annex to 6.�.� as follows: A.6.�.� The person(s) or organization performing these assessments should have experience in the technologies presented in this document, knowledge of explosion dynamics, the effects of explosions on structures, and alternative protection measures.Substantiation: The annex material has been added to clarify that the relevant experience with these technologies is essential.Committee Meeting Action: AcceptNumber Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.

____________________________________________________________68-�0 Log #23 Final Action: Accept in Principle(6.2.3.3)____________________________________________________________Submitter: Samuel A. Rodgers, Honeywell Inc.Comment on Proposal No: 68-3Recommendation: Revise to read as follows: 6.2.3.3 The area into which deflagration vents discharge shall not expose personnel or the public to an unacceptable risk from flames, hot gases, hot particles, or projectiles.Substantiation: Clarify that the venting should not impact areas where personnel are present both in the facility and external to the facility (such as to a public way).Committee Meeting Action: Accept in Principle See committee action on Comment 68-�� (Log #4�).Committee Statement: See committee action and statement on Comment 68-�� (Log #4�).Number Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.

____________________________________________________________68-�� Log #4� Final Action: Accept in Principle(6.2.3.3 and A.6.2.3.3)____________________________________________________________Submitter: Luke S. Morrison, Professional Loss ControlComment on Proposal No: 68-3Recommendation: Revise text to read: 6.2.3.3* Access to spaces into which deflagration vents discharge shall be restricted so as to minimize, to an acceptable level, the risk of injury to personnel from flame, hot gases, hot particles or projectiles. A.6.2.3.3 Deflagration vents should be located to discharge into spaces where they will not present a hazard to personnel. It is acknowledged that it may be impractical to achieve this safety objective in some cases such as

68-6

Report on Comments F2006 — Copyright, NFPA NFPA 68 existing plants. In these cases appropriate warning signs should be posted and the risk should be minimized using an ALARP or other acceptable risk mitigation principle.Substantiation: The requirement as written is overly restrictive and could be interpreted as prohibiting access to a fairly large space. This could be an impractical limitation particularly for existing facilities where plant layout did not consider this restriction. A warning of potential harm is required but it is suggested that a risk based approach should be acceptable.Committee Meeting Action: Accept in Principle Revise text to read: 6.2.3.3* Access to spaces into which deflagration vents discharge shall be restricted so as to minimize, to an a level acceptable to the authority having jurisdiction level, the risk of injury to personnel from flame, hot gases, hot particles or projectiles. A.6.2.3.3 Deflagration vents should be located to discharge into spaces where they will not present a hazard to personnel. It is acknowledged that it may be impractical to achieve this safety objective in some cases such as existing plants. In these cases appropriate warning signs should be posted and the risk should be minimized using an ALARP or other acceptable risk mitigation principle.Committee Statement: The Committee added that the AHJ controls the acceptable level of risk in this application. The word “personnel” was deleted to make the applicability more broad and thus include any person.Number Eligible to Vote: 27Ballot Results: Affirmative: 23 Negative: � Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Explanation of Negative: STEVENSON, B.: There is no justification on the basis of old facilities or otherwise to condone or imply to condone that explosion vents can discharge into areas occupied by people. It would be one thing if there were not practical alternatives, but there are several options that address this problem.

____________________________________________________________68-�2 Log #9 Final Action: Accept in Principle(7.2)____________________________________________________________Submitter: Samuel A. Rodgers, Honeywell Inc.Comment on Proposal No: 68-3Recommendation: Proposal: Delete or relocate indicated redundant statements (Chapter 4 and Chapter 7) as noted. Correct the example to indicate toluene as a specific material for the calculation and provide guidance on how to design for a range of materials. Relocate the example to be annex material for 7.2.4 and correct figure and equation references. Modify existing Section 7.2 as follows:

Relocate Annex example to 7.2.4.

7.2.4* Calculation of Internal Surface Area.

Retitle and modify 7.2.6:

7.2.6 Vent Design. See also Sections 4.5 4.3 through 4.7.

7.2.6.� The vent closure shall be designed, constructed, installed, and maintained so that it releases readily and moves out of the path of the combustion gases. The closure shall not become a hazard when it operates.

7.2.6.2* The total weight of the closure assembly, including any insulation or hardware, shall comply with the requirements in 4.7.

7.2.6.3* The construction material of the closure shall be compatible with the environment to which it is to be exposed.

7.2.6.4* For low-strength enclosures Pred shall exceed Pstat by at least 0.024 bar (0.35 psi).

7.2.6.5 If an enclosure is subdivided into compartments by walls, partitions, floors, or ceilings, then each compartment that contains a deflagration hazard shall be provided with its own vent.

7.2.6.6 Each closure shall be designed and installed to move freely without interference by obstructions such as ductwork or piping. Such a design ensures that the flow of combustion gases is not impeded by an obstructed closure. (See 4.5.�.)

7.2.6.7* Guarding shall be provided to prevent personnel from falling against vent closures.

7.2.6.8* Measures shall be taken to protect the closures against accumulations of snow and ice.

7.2.6.9* A lightweight roof shall be permitted to be considered sacrificial, provided its movement can be tolerated and provided its movement is not hindered by ice or snow.

7.2.6.�0 In such cases, it shall be necessary to strengthen the structural members of the compartment so that the reduced vent area available is equivalent to the vent area needed.

7.2.6.�� The minimum pressure needed for the weakest structural member shall be obtained by substituting the values for the available area, the internal surface area, and the applicable C value for the variables in Equation 7.� and then calculating Pred, the maximum allowable pressure.

7.2.6.�2* The vent area shall be distributed as evenly as possible over the building’s skin.

Change reference of annex items and delete as indicated:

A.7.2.2.2 Use of Figure 7.2.2.2 provides a way to interpolate between he vent parameters previously provided to accommodate a range of fuels. Methane (previously included in the vent parameter table) has been left out of the curve deliberately since flame speeds in methane/air mixtures do not accelerate as much with turbulence as with other hydrocarbons with similar fundamental burning velocity. The shape of the curve beyond 46 centimeters per second was developed based on limited data with fuels of higher burning velocity.

(No change to remainder of A.7.2.2.2)

A.7.2.6.2 See 4.8 for restrictions on vent closure weight where using Equation 7.� without consideration for vent closure efficiency.

A.7.2.6.3 Some closures, on activation, are blown away from their mounting points Brittle materials can fragment, producing projectiles. Each installation should be evaluated to determine the extent of the hazard to personnel from such projectiles. Additionally, it should be recognized that the vented deflagration can discharge burning gases, posing a personnel hazard.

A.4.5.2.� A.7.2.6.3 For further information, see National Association of Corrosion Engineers Handbook.

A.7.2.6.8 The criteria for the design of roof mounted closures are basically the same as those for wall closures.

A.7.2.6.�2 A.7.2.6.9 Situations can arise in which the roof area or one or more of the wall areas cannot be used for vents, either because of the location of equipment, or because of exposure to other buildings or to areas normally occupied by personnel.

A.7.2.4 Sample Calculations of Internal Surface Areas. (edit. Figures to be renumbered also)

Problem. Consider a 20 ft × 30 ft × 20 ft (6.� m × 9.2 m × 6.� m) (length × width × height) dispensing room for toluene, a Class I flammable liquids. The anticipated flammable liquids have fundamental burning velocityies of toluene is 4� cm/sec less than �.3 times that of propane [see Table DC.�(a)]. Figure 7.2.2.2 specifies a venting equation constant, C, of 0.�7. If more than one flammable liquid could be processed in this room, the designer should consider the material with the highest burning velocity when designing the vent. The room is located against an outside wall and, in anticipation of deflagration venting requirements, the three inside walls are designed to withstand a Pred value of 0.05 bar (0.69 psi). For most flammable liquids, Figure 7.2.2.2 specifies a venting equation constant, C, of 0.�7. The internal surface area of the room, 297 m2 (3200 ft2), is determined by the following equation:

2 21/2

(0.17)(3200)655 ft (61 m )

0.69vA = =

This area is more than is available in the outside wall, so modification is necessary.

Step 2. If the wall strength were increased to resist a Pred of 0.072 bar (�.04 psi), a vent area of 50 m2 (533 ft2) would be needed. This wall strength can usually be achieved and is recommended over the common wall strength intended to resist a Pred of 0.048 bar (0.69 psi).

Step 3. Consider the building illustrated in Figure A.7.2.46.9(�), for which deflagration venting is needed. The building is to be protected against a deflagration of a hydrocarbon vapor that has the burning characteristics of propane. The maximum Pred that this building can withstand has been determined by structural analysis to be 3.45 kPa (0.5 psi).

FIGURE A.7.2.6.9(1) Building Used in Sample Calculation (Not to Scale) (Version I). (Existing FIGURE A.7.2.6.9(a) as shown in the ROP

Draft)

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Report on Comments F2006 — Copyright, NFPA NFPA 68 Step 4. Divide the building into sensible geometric parts (Parts � and 2) as shown in Figure A.7.2.46.9(2).

FIGURE A.7.2.6.9(2) Building Used in Sample Calculation (Not to Scale) (Version II). (Existing FIGURE A.7.2.6.9(b) as shown in the ROP

Draft)

Step 5. Calculate the total internal surface area of each part of the building.

Part 1 Surface Area (AS1)

Floor = 5�.8 m × 9.�5 m = 474 m2 (�70 ft × 30 ft = 5�00 ft2)

Roof = 5�.8 m × 9.65 m = 499 m2 (�70 ft × 3�.6 ft = 5372 ft2)

Rear wall = 5�.8 m × 6.� m = 3�6 m2 (�70 ft × 20 ft = 3400 ft2)

Front wall = (36.6 m × 9.�5 m) + (�5.25 m × 3.05 m) = 38� m2 [(�20 ft × 30 ft) + (50 ft × �0 ft)] = 4�00 ft2

Side walls (rectangular part)

= 2 × 9.�5 m × 6.� m = ��� m2 (2 × 30 ft × 20 ft = �200 ft2)

Side walls (triangular part)

= 9.�5 m × 3.05 m = 28 m2 (30 ft × �0 ft = 300 ft2)

Total Part �: AS1 = �809 m2 (�9,472 ft2)

Part 2 Surface Area (AS2)

Floor = �5.25 m × 9.�5 m = �39 m2 (50 ft × 30 ft = �500 ft2)

Roof = �5.25 m × 9.�5 m = �39 m2 (50 ft × 30 ft = �500 ft2)

Front wall = �5.25 m × 6.� m = 93 m2 (50 ft × 20 ft = �000 ft2)

Side walls = 2 × 9.�5 m × 6.� m = ��� m2 (2 × 30 ft × 20 ft = �200 ft2)

Total Part 2: AS2 = 483 m2 (5200 ft2)

Step 6. Thus, the total internal surface area for the whole building, AS, is expressed as follows:

As = �809 m2 (�9,472 ft2) + 483 m2 (5200 ft2) = 2292 m2 (24,672 ft2)

Step 7. Calculate the total vent area, Av, needed using Equation 6.�:

( )1/2

sv

red

C AA

P=

where:

C = 0.045 bar�/2 (0.�7 psi)�/2 from Figure 7.2.2.2 Table 6.2.2

AS = 2292 m2 (24,672 ft2)

Pred = 3.45 kPa (0.5 psi)

Step 8. Substituting these values:

( )( ) ( )2 21/2

0.17 24,672551 m 5932 ft

0.5vA = =

Step 9. The total vent area needed of 55� m2 (5932 ft2) should be divided evenly over the outer surface of the building and should be apportioned between the parts in the same ratio as their surface area.

Step 10. Total vent area of Part �:

2 211

19,4725932 435 m (4682 ft )

24,672s

v vs

AA A

A

= = =

Step 11. Total vent area of Part 2:

2 222

52005932 116 m (1250 ft )

24,672s

v vs

AA A

A

= = =

Step 12. Check to determine whether sufficient external surface area on the building is available for venting.

Step 13. In Part �, the vent area needed [435 m2 (4682 ft2)] can be obtained by using parts of the front, rear, and side walls or by using the building roof.

Step 14. In Part 2, the vent area needed [��6 m2 (�250 ft2)] can be obtained by using parts of the front and side walls or by using the building roof.

Step 15. Note that only the outer “skin” of the building can be used for vent locations; a deflagration cannot be vented into other parts of the building.Substantiation: Some material in Section 7.2 is redundant to material in Chapter 4. Having this material in two places in slightly different terminology is confusing to the user. The example calculation incorrectly references the earlier venting parameter for the range of materials with burning velocities less than �.3 times that of propane, instead of a particular design material. There is no guidance on what to do with a range of materials with different fundamental burning velocities.Committee Meeting Action: Accept in Principle Delete or relocate indicated redundant statements (Chapter 4 and Chapter 7) as noted. Correct the example to indicate toluene as a specific material for the calculation and provide guidance on how to design for a range of materials. Relocate the example to be annex material for 7.2.4 and correct figure and equation references. Modify existing Section 7.2 as follows: Relocate Annex example to 7.2.4.

7.2.4* Calculation of Internal Surface Area. Retitle and modify 7.2.6: 7.2.6 Vent Design. See also Sections 4.5 4.3 through 4.7. 7.2.6.� The vent closure shall be designed, constructed, installed, and maintained so that it releases readily and moves out of the path of the combustion gases. The closure shall not become a hazard when it operates. 7.2.6.2* The total weight of the closure assembly, including any insulation or hardware, shall comply with the requirements in 4.7. 7.2.6.3* The construction material of the closure shall be compatible with the environment to which it is to be exposed. 7.2.6.4* For low-strength enclosures Pred shall exceed Pstat by at least 0.024 bar (0.35 psi). 7.2.6.5 If an enclosure is subdivided into compartments by walls, partitions, floors, or ceilings, then each compartment that contains a deflagration hazard shall be provided with its own vent. 7.2.6.6 Each closure shall be designed and installed to move freely without interference by obstructions such as ductwork or piping. Such a design ensures that the flow of combustion gases is not impeded by an obstructed closure. (See 4.5.�.) 7.2.6.7* Guarding shall be provided to prevent personnel from falling against vent closures. 7.2.6.8* Measures shall be taken to protect the closures against accumulations of snow and ice. 7.2.6.9* A lightweight roof shall be permitted to be considered sacrificial, provided its movement can be tolerated and provided its movement is not hindered by ice or snow. 7.2.6.�0 In such cases, it shall be necessary to strengthen the structural members of the compartment so that the reduced vent area available is equivalent to the vent area needed. 7.2.6.�� The minimum pressure needed for the weakest structural member shall be obtained by substituting the values for the available area, the internal surface area, and the applicable C value for the variables in Equation 7.� and then calculating Pred, the maximum allowable pressure. 7.2.6.�2* The vent area shall be distributed as evenly as possible over the building’s skin. Change reference of annex items and delete as indicated: A.7.2.2.2 Use of Figure 7.2.2.2 provides a way to interpolate between he vent parameters previously provided to accommodate a range of fuels. Methane (previously included in the vent parameter table) has been left out of the curve deliberately since flame speeds in methane/air mixtures do not accelerate as much with turbulence as with other hydrocarbons with similar fundamental burning velocity. The shape of the curve beyond 46 centimeters per second was developed based on limited data with fuels of higher burning velocity. (No change to remainder of A.7.2.2.2) A.7.2.6.2 See 4.8 for restrictions on vent closure weight where using Equation 7.� without consideration for vent closure efficiency. A.7.2.6.3 Some closures, on activation, are blown away from their mounting points Brittle materials can fragment, producing projectiles. Each installation should be evaluated to determine the extent of the hazard to personnel from such projectiles. Additionally, it should be recognized

68-8

Report on Comments F2006 — Copyright, NFPA NFPA 68 that the vented deflagration can discharge burning gases, posing a personnel hazard. A.4.5.2.� A.7.2.6.3 For further information, see National Association of Corrosion Engineers Handbook. A.7.2.6.8 The criteria for the design of roof mounted closures are basically the same as those for wall closures. A.7.2.6.�2 A.7.2.6.9 Situations can arise in which the roof area or one or more of the wall areas cannot be used for vents, either because of the location of equipment, or because of exposure to other buildings or to areas normally occupied by personnel. A.7.2.4 Sample Calculations of Vent Area. (edit. Figures to be renumbered also) Problem. Step �. Calculate the internal surface area. If you consider a 20 ft × 30 ft × 20 ft (6.� m × 9.2 m × 6.� m) (length × width × height) dispensing room for toluene, a Class I flammable liquids, the internal surface area of the room is 3200 ft2 (297 m2). Next, the anticipated flammable liquids have fundamental burning velocityies of toluene is 4� cm/sec less than �.3 times that of propane [see Table DC.�(a)]. Figure 7.2.2.2 specifies a venting equation constant, C, of 0.�7. If more than one flammable liquid could be processed in this room, the designer should consider the material with the highest burning velocity when designing the vent. The room is located against an outside wall and, in anticipation of deflagration venting requirements, the three inside walls are designed to withstand a Pred value of 0.05 bar (0.69 psi). For most flammable liquids, Figure 7.2.2.2 specifies a venting equation constant, C, of 0.�7. Now the vent area, Av, can be determined using the internal surface area of the room, 297 m2 (3200 ft2), is determined by the following equation:

2 21/2

(0.17)(3200)655 ft (61 m )

0.69vA = =

This area is more than is available in the outside wall, so modification is necessary. Step 2. If the wall strength were increased to resist a Pred of 0.072 bar (�.04 psi), a vent area of 50 m2 (533 ft2) would be needed. This wall strength can usually be achieved and is recommended over the common wall strength intended to resist a Pred of 0.048 bar (0.69 psi). Step 3. Consider the building illustrated in Figure A.7.2.46.9(�), for which deflagration venting is needed. The building is to be protected against a deflagration of a hydrocarbon vapor that has the burning characteristics of propane. The maximum Pred that this building can withstand has been determined by structural analysis to be 3.45 kPa (0.5 psi).

FIGURE A.7.2.46.9(1) Building Used in Sample Calculation (Not to Scale) (Version I). (Existing FIGURE A.7.2.6.9(a) as shown in the ROP

Draft)

Step 4. Divide the building into sensible geometric parts (Parts � and 2) as shown in Figure A.7.2.46.9(2).

FIGURE A.7.2.46.9(2) Building Used in Sample Calculation (Not to Scale) (Version II). (Existing FIGURE A.7.2.6.9(b) as shown in the ROP

Draft)

Step 5. Calculate the total internal surface area of each part of the building.

Part 1 Surface Area (AS�)

Floor = 5�.8 m × 9.�5 m = 474 m2 (�70 ft × 30 ft = 5�00 ft2)

Roof = 5�.8 m × 9.65 m = 499 m2 (�70 ft × 3�.6 ft = 5372 ft2)

Rear wall = 5�.8 m × 6.� m = 3�6 m2 (�70 ft × 20 ft = 3400 ft2)

Front wall = (36.6 m × 9.�5 m) + (�5.25 m × 3.05 m) = 38� m2 [(�20 ft × 30 ft) + (50 ft × �0 ft)] = 4�00 ft2

Side walls (rectangular part)

= 2 × 9.�5 m × 6.� m = ��� m2 (2 × 30 ft × 20 ft = �200 ft2)

Side walls (triangular part)

= 9.�5 m × 3.05 m = 28 m2 (30 ft × �0 ft = 300 ft2)

Total Part �: AS� = �809 m2 (�9,472 ft2)

Part 2 Surface Area (AS2)

Floor = �5.25 m × 9.�5 m = �39 m2 (50 ft × 30 ft = �500 ft2)

Roof = �5.25 m × 9.�5 m = �39 m2 (50 ft × 30 ft = �500 ft2)

Front wall = �5.25 m × 6.� m = 93 m2 (50 ft × 20 ft = �000 ft2)

Side walls = 2 × 9.�5 m × 6.� m = ��� m2 (2 × 30 ft × 20 ft = �200 ft2)

Total Part 2: AS2 = 483 m2 (5200 ft2)

Step 6. Thus, the total internal surface area for the whole building, AS, is expressed as follows:

As = �809 m2 (�9,472 ft2) + 483 m2 (5200 ft2) = 2292 m2 (24,672 ft2)

Step 7. Calculate the total vent area, Av, needed using Equation 6.�:

( )�/ 2= s

vred

C AA

P

where:

C = 0.045 bar �/2 (0.�7 psi) �/2 from Figure 7.2.2.2 Table 6.2.2 AS = 2292 m2 (24,672 ft2) Pred = 3.45 kPa (0.5 psi)

Step 8. Substituting these values:

( )( ) ( )2 2�/ 2

0.�7 24,67255� m 5932 ft

0.5= =vA

Step 9. The total vent area needed of 55� m2 (5932 ft2) should be divided evenly over the outer surface of the building and should be apportioned between the parts in the same ratio as their surface area.

Step 10. Total vent area of Part �:

2 2��

�9,4725932 435 m (4682 ft )24,672

= = =

s

v vs

AA AA

Step 11. Total vent area of Part 2:

2 222

52005932 ��6 m (�250 ft )24,672

= = =

s

v vs

AA AA

Step 12. Check to determine whether sufficient external surface area on the building is available for venting.Step 13. In Part �, the vent area needed [435 m2 (4682 ft2)] can be obtained by using parts of the front, rear, and side walls or by using the building roof.Step 14. In Part 2, the vent area needed [��6 m2 (�250 ft2)] can be obtained by using parts of the front and side walls or by using the building roof.Step 15. Note that only the outer “skin” of the building can be used for vent locations; a deflagration cannot be vented into other parts of the building.Committee Statement: The Committee accepted the submitter’s recommendation with some editorial changes to clarify the procedures for Step � in the example calculation and some edits to the figures.Number Eligible to Vote: 27Ballot Results: Affirmative: 22 Abstain: 2Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Explanation of Abstention: URAL, E.: This item is very difficult to evaluate in the format it is presented. Some of the errors propagating from the previous editions need to be fixed. I volunteer to proofread this section once a clean copy becomes available.

Here are some obvious fixes and suggestions:7.2.6.4* For low-strength enclosures Pred shall exceed Pstat by at least

0.024 bar (0.35 psi).

68-9

Report on Comments F2006 — Copyright, NFPA NFPA 68 Add to the annex material a statement that the weird numbers originate

from the psf units preferred for building design in the US.A.7.2.2.2 Use of Figure 7.2.2.2 provides a way to interpolate between

the venting equation constants parameters previously provided in previous editions to accommodate a range of fuels. (DELETE THE FOLLOWING SENTENCE AS IT CREATES A FALSE IMPRESSION THAT PEOPLE CAN STILL USE THE OLD C VALUE) Methane (previously included in the vent parameter table) has been left out of the curve deliberately since flame speeds in methane/air mixtures do not accelerate as much with turbulence as with other hydrocarbons with similar fundamental burning velocity. The shape of the curve beyond 46 centimeters per second was developed based on limited data with fuels of higher burning velocity.

(No change to remainder of A.7.2.2.2)A.7.2.4 Sample Vent Sizing Calculations of Internal Surface Areas.

(edit. Figures to be renumbered also)Problem Example A. Consider a 20 ft × 30 ft × 20 ft (6.� m × 9.2 m

× 6.� m) (length × width × height) dispensing room for toluene, a Class I flammable liquids. (I recommend picking another fuel that will give a C value different than the old tables for better illustration of the change). The anticipated flammable liquids have fundamental burning velocityies of toluene is 4� cm/sec less than �.3 times that of propane [see Table DC.�(a)]. Figure 7.2.2.2 specifies a venting equation constant, C, of 0.�7. If more than one flammable liquid could be processed in this room, the designer should consider the material with the highest burning velocity when designing the vent. The room is located against an outside wall and, in anticipation of deflagration venting requirements, the three inside walls (and the ceiling ?) are designed to withstand a Pred value of 0.05 bar (0.69 psi). For most flammable liquids, Figure 7.2.2.2 specifies a venting equation constant, C, of 0.�7. The internal surface area of the room

As = 2*(2*20*30+20*20)= 297 m2(3200 ft2). The vent are, Av, is determined by the following equation:

22�/2 (0.�7)(3200) 655 ft (6� m )0.69 v A = =This area is more than is available in the outside wall, so modification

is necessary.Step 2. If the wall and ceiling (?) strength were increased to resist a Pred of 0.072 bar (�.04 psi), a vent area of 50 m2 (533 ft2) would be needed.

This wall strength can usually be achieved and is recommended over the common wall strength intended to resist a Pred of 0.048 bar (0.69 psi).

Example B Consider the building illustrated in Figure A.7.2.46.9(�), for which deflagration venting is needed. The building is to be protected against a deflagration of a hydrocarbon vapor that has the burning characteristics of propane. The maximum Pred that this building can withstand has been determined by structural analysis to be 3.45 kPa (0.5 psi).

FIGURE A.7.2.6.9(1) Building Used in Sample Calculation (Not to Scale) (Version I). (Existing FIGURE A.7.2.6.9(a) as shown in the ROP Draft) (ADD AN ISOMETRIC SKETCH)

Step 1 4. Divide the building into sensible geometric parts (Parts � and 2) as shown in Figure A.7.2.46.9(2).

FIGURE A.7.2.6.9(2) Building Used in Sample Calculation (Not to Scale) (Version II). (Existing FIGURE A.7.2.6.9(b) as shown in the ROP Draft)

Step 2 5. Calculate the total internal surface area of each part of the building.

Part 1 Surface Area (AS1)Floor = 5�.8 m × 9.�5 m = 474 m2 (�70 ft × 30 ft = 5�00 ft2)Roof = 5�.8 m × 9.65 m = 499 m2 (�70 ft × 3�.6 ft = 5372 ft2)Rear wall = 5�.8 m × 6.� m = 3�6 m2 (�70 ft × 20 ft = 3400 ft2)Front wall = (36.6 m × 9.�5 m) + (�5.25 m × 3.05 m) = 38� m2 [(�20 ft

× 30 ft) + (50 ft × �0 ft)] = 4�00 ft2

DON’T FORGET TO SUBTRACT THE AREA OF THE OPENING)Side walls(rectangular part) = 2 × 9.�5 m × 6.� m = ��� m2 (2 × 30 ft × 20 ft =

�200 ft2)Side walls(triangular part) = 9.�5 m × 3.05 m = 28 m2 (30 ft × �0 ft = 300 ft2)Total Part �: AS1 = �809 m2 (�9,472 ft2)

Part 2 Surface Area (AS2)Floor = �5.25 m × 9.�5 m = �39 m2 (50 ft × 30 ft = �500 ft2)Roof = �5.25 m × 9.�5 m = �39 m2 (50 ft × 30 ft = �500 ft2)Front wall = �5.25 m × 6.� m = 93 m2 (50 ft × 20 ft = �000 ft2)Side walls = 2 × 9.�5 m × 6.� m = ��� m2 (2 × 30 ft × 20 ft = �200 ft2)Total Part 2: AS2 = 483 m2 (5200 ft2)

Step 3 6. Thus, the total internal surface area for the whole building, AS, is expressed as follows:

As = �809 m2 (�9,472 ft2) + 483 m2 (5200 ft2) = 2292 m2 (24,672 ft2)

Step 4 7. Calculate the total vent area, Av, needed using Equation 6.�:( )�/ 2svredC AAP=where:C = 0.045 0.051 bar�/2 (0.�7 0.20 psi)�/2 from Figure 7.2.2.2 Table 6.2.2

FIX THE REST OF THE NUMBERS.

AS = 2292 m2 (24,672 ft2)Pred = 3.45 kPa (0.5 psi)

Step 5 8. Substituting these values:( )( ) 2( 2)�/ 20.�7 24,67255� m 5932 ft0.5 v A = =

Step 6 9. The total vent area needed of 55� m2 (5932 ft2) should be divided evenly over the outer surface of the building and should be apportioned between the parts in the same ratio as their surface area. (REFER TO OLD SECTION 6.2.3)

Step 7 10. Total vent area of Part �:� 2 2�5932 �9,472 435 m (4682 ft )24,672sv vsA A AA= = =

Step 8 11. Total vent area of Part 2:2 2 225932 5200 ��6 m (�250 ft )24,672sv vsA A AA= = =

Step 9 12. Check to determine whether sufficient external surface area on the building is available for venting.

Step 10 13. In Part �, the vent area needed [435 m2 (4682 ft2)] can be obtained by using parts of the front, rear, and side walls or by using the building roof.

Step 11 14. In Part 2, the vent area needed [��6 m2 (�250 ft2)] can be obtained by using parts of the front and side walls or by using the building roof.

Step 12 15. Note that only the outer “skin” of the building can be used for vent locations; a deflagration cannot be vented into other parts of the building.

68-�0

Report on Comments F2006 — Copyright, NFPA NFPA 68 ZALOSH, R.: The examples and revisions to the figures are not sufficiently clear to cast a vote. Furthermore, the June 28th email correspondence belatedly pointing out the change in the C factor for propane is too important a change to be included as editorial in nature. In the absence of any Committee discussion of this change, I can neither support the current action item, nor reject it and be left with the unfinished result of the ROP ballot.

____________________________________________________________68-�3 Log #�5 Final Action: Reject(7.2.2.5, 7.2.2.6, 7.3.3.5, and 7.3.3.6)____________________________________________________________Submitter: S. Dorofeev, H. Febo, Norwood, MAComment on Proposal No: 68-3Recommendation: All of these sections should be removed.Substantiation: The correlation for panel inertia effects derived for dusts are not applicable for gas explosions.Committee Meeting Action: RejectCommittee Statement: The equations questioned by the submitter represent an improvement over previous editions. The equations are conceptually consistent with the pertinent physical phenomena, and are in agreement with most of the data reviewed by the Committee. The Committee also notes that discussion in A.4.7.2 includes some recognized exceptions and the equations still represent conservative vent designs for those exceptions. Number Eligible to Vote: 27Ballot Results: Affirmative: 23 Abstain: �Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Explanation of Abstention: STEVENSON, B.: It is not clear to me if the equations “represent conservative vent designs” where gas explosions are concerned.

____________________________________________________________68-�4 Log #CC2 Final Action: Accept(7.2.2.5, 7.2.2.6, 7.3.3.6, 7.3.3.7, and A.7.1.3)____________________________________________________________Submitter: Technical Committee on Explosion Protection Systems Comment on Proposal No: 68-3Recommendation: Add the following to 7.2.2.5: 7.2.2.5* Effects of Panel Inertia. 7.2.2.5.� When the mass of the vent panel is less than or equal to 40 kg/m2 and KG is less than or equal to �30 bar-m/sec then Equation 7.2.2.5 shall be used to determine if an incremental increase in vent area is needed and the requirements of 7.2.2.6 shall be used to determine the value of that increase. Existing 7.2.2.5 becomes 7.2.2.5.2; revise KG less than or equal �30. Revise A.7.2.2.5 as follows: A.7.2.2.5 replace with existing A.7.2.2.6 with KG modified to less than or equal to �30 bar-m/sec as per the change in 7.2.2.5.�. Delete A.7.2.2.6. Revise 7.2.2.6 by deleting the condition M < or equal to 40 kg/m2. Add the following to 7.3.3.6: 7.3.3.6* Effects of Panel Inertia. 7.3.3.6.� When the mass of the vent panel is less than or equal to 40 kg/m2 and KG is less than or equal to �30 bar-m/sec then Equation 7.3.3.6 shall be used to determine if an incremental increase in vent area is needed and the requirements of 7.3.3.7 shall be used to determine the value of that increase. Existing 7.3.3.6 becomes 7.3.3.6.2. Revise A.7.3.3.6 as follows: A.7.3.3.6 replace with existing A.7.3.3.7 as modified for KG from 250 to �30 based upon the change in 7.3.3.6.�. Delete existing A.7.3.3.5. Delete A.7.3.3.7. Revise 7.3.3.7 by deleting the condition M < or equal to 40 kg/m2. Add a new A.7.�.3 A.7.�.3 By test, KG of propane is �00 bar-m/sec with a burning velocity of 46 cm/sec. For gases of unknown KG it is possible to estimate KG from burning velocity using Equation E.�.Substantiation: The conditions for use of the vent panel mass correlation need to be stated as part of the initial requirements for threshold mass. In the ROP draft these conditions were in a later section and have now been moved to the appropriate place for this application. The threshold for KG was revised from 250 bar-m/sec to �30 bar-m/sec to limit the application to materials with no more than �.3 times the fundamental burning velocity of propane.Committee Meeting Action: AcceptNumber Eligible to Vote: 27Ballot Results: Affirmative: 23 Negative: � Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Explanation of Negative: URAL, E.: There appears to be a potential loophole here for applications where SIGMA>40 or Kg>�30. These sections must specify what a user should do in such cases.

____________________________________________________________68-�5 Log #�9 Final Action: Accept in Principle(Figure 7.4.1)____________________________________________________________Submitter: S. Dorofeev, H. Febo, Norwood, MAComment on Proposal No: 68-3Recommendation: Curve captions should be changed as follows: <2.57*(Av)

�/2 3m (�0 ft) duct length 2.57*(Av)

�/2 to 5.�4*(Av)�/2 3 m to 6 m (�0 ft to 20 ft)

Substantiation: Duct length should be scaled with duct area. Otherwise the correlation will be overly conservative for large vent areas and not conservative for small vent areas. The correlations for vent duct effects in 7.4.2.3 are based on data from W. Bartknecht, Explosion-shutz - Grundladen and Anwendung, Springer-Verlag, �993, the proposed rescaling takes into account the actual duct area of Av = �.36 m2, so that 2.57*(Av)

�/2 = 3 m.Committee Meeting Action: Accept in Principle Revise Figure 7.4.2 as follows: Revise labels on 2 indicated curves by replacing labels as follows: Existing curve labeled as < 3 m is to be labeled “Curve A” Existing curve labeled as 3 m to 6 m to be labeled as “Curve B” Add explanation for Curve A and Curve B as follows: Curve A to be used for duct length < 3 m (�0 ft) and less than 4 duct hydraulic diameters.   Curve B to be used for duct length of 3 m to 6 m (10 ft to 20 ft) or ≥ 4 duct hydraulic diameters. Curve B is not valid for duct lengths greater than 6 m (20 ft). Note for both curve A and B: Unlike a piping system described in Chapter 9 where flammable vapor is presumed present, in this situation flammable vapor is not initially present in the vent duct.

P′ re

d w

ithou

t duc

t (E

qs. 6

.8 &

6.9

)

0.6Pred — maximum vented pressure — (bar) with duct

1 4.21.4 1.8 2.2 2.6 3 3.4 3.8

1.8

2

1.6

1.4

1.2

1

0.8

0.6

0.4

0.2

Curve

ACur

veB

FIGURE 7.4.2 Maximum Pressure Developed during Venting of Gas, with and without Vent Ducts.

Committee Statement: The Committee reviewed additional data beyond the reference to Bartknecht and determined that an L/D value of 4 was more representative of the data reviewed by the Committee. The Committee also retained the limit of 3 m to maintain a conservative approach for large enclosures.Number Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Comment on Affirmative: URAL, E.: Must limit the upper L/D for use with curve B.

____________________________________________________________68-�6 Log #�8 Final Action: Accept in Principle(7.4.2)____________________________________________________________Submitter: S. Dorofeev, H. Febo, Norwood, MAComment on Proposal No: 68-3Recommendation: Revise text to read: 7.4.2 Duct lengths shorter than 2.57*(Av)

�/2 3 m (�0 ft), shall be treated as 2.57*(Av)

�/2 3 m (�0 ft) in length for calculation purposes.Substantiation: Duct length should be scaled with duct area. Otherwise the correlation will be overly conservative for large vent areas and not conservative for small vent areas. The correlations for vent duct effects in 7.4.2.3 are based on data from W. Bartkneht, Explosion-shutz - Grundladen and Anwendung, Springer-Verlag, �993, the proposed rescaling takes into account the actual duct area of Av = �.36 m2, so that 2.57*(Av)

�/2 = 3 m.

68-��

Report on Comments F2006 — Copyright, NFPA NFPA 68 Committee Meeting Action: Accept in Principle Revise text to read: 7.4.2 Duct lengths shorter than 3 m (�0 ft) and 4 duct hydraulic diameters in length shall be treated using Curve A in Figure 7.4.2. For ducts exceeding either of the limitations above, use Curve B.Committee Statement: See committee statement on Comment 68-�5 (Log #�9).Number Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Comment on Affirmative: URAL, E.: Must limit the upper L/D for use with curve B. ZALOSH, R.: I suggest that the sentence referring to Curve B be revised as follows for clarification: For duct lengths less than 6 m, and exceeding either of the two preceding limitations, use Curve B.

____________________________________________________________68-�7 Log #�6 Final Action: Accept in Principle(7.4.2.3)____________________________________________________________Submitter: S. Dorofeev, H. Febo, Norwood, MAComment on Proposal No: 68-3Recommendation: Revise text to read: 7.4.2.3 For vent ducts with lengths of less than 2.57*(Av)

�/2 3 m (�0 ft), the following equation shall be used to determine P’red:Substantiation: Duct length should be scaled with duct area. Otherwise the correlation will be overly conservative for large vent areas and not conservative for small vent areas. The correlations for vent duct effects in 7.4.2.3 are based on data from W. Bartknecht, Explosion-shutz - Grundladen and Anwendung, Springer-Verlag, �993, the proposed rescaling takes into account the actual duct area of Av = �.36 m2, so that 2.57*(Av)

�/2 = 3 m.Committee Meeting Action: Accept in Principle Revise text to read: 7.4.2.3 For vent ducts with lengths of less than 3 m (�0 ft) and 4 duct hydraulic diameters, the following equation shall be used to determine P’red:

( )1.1610.779red redP P′ =

Committee Statement: See committee statement on Comment 68-�5 (Log #�9).Number Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.

____________________________________________________________68-�8 Log #�7 Final Action: Accept in Principle(7.4.2.4)____________________________________________________________Submitter: S. Dorofeev, H.Febo, Norwood, MAComment on Proposal No: 68-3Recommendation: Revise text to read: 7.4.2.4 For vent ducts with lengths of less than 2.57*(Av)

�/2 to 5.�4*Av)�/2 3 m to 6 m (�0 ft to 20 ft), the following equation shall be determined P’red:Substantiation: Duct length should be scaled with duct area. Otherwise the correlation will be overly conservative for large vent areas and not conservative for small vent areas. The correlations for vent duct effects in 7.4.2.3 are based on data from W. Bartknecht, Explosion-shutz - Grundladen and Anwendung, Springer-Verlag, �993, the proposed rescaling takes into account the actual duct area of Av = �.36 m2, so that 2.57*(Av)

�/2 = 3 m, and 5.�4* (Av)�/2 = 6 m.

Committee Meeting Action: Accept in Principle Revise text to read: 7.4.2.4 For vent ducts with lengths of 3 m to 6 m (�0 ft to 20 ft) or 4 duct hydraulic diameters, the following equation shall be used:

( )1.9360.172red redP P′

Committee Statement: See committee statement on Comment 68-�5 (Log #�9).Number Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Comment on Affirmative: URAL, E.: Must limit the upper L/D for use with curve B.

____________________________________________________________68-�9 Log #CC8 Final Action: Accept(7.6.4, 8.8.2, A.7.6.4, and A.8.8.2)____________________________________________________________Submitter: Technical Committee on Explosion Protection Systems Comment on Proposal No: 68-3Recommendation: Revise 7.6.4 and 8.8.2 as follows: n = number of evenly distributed vents Add the following new Annex material: A.7.6.4 The number of vents, n, should be those vents whose discharge directions are separate and evenly distributed around the circumference of a vessel or along the central axis. If multiple vent panels cover a single vent opening, they should not be treated as separate for this purpose. Add the following to existing A.8.8: A.8.8 See also A.7.6.4. Substantiation: The Committee clarified that the value of “n” represents the vents that are evenly distributed and added annex commentary to further clarify the application of these requirements.Committee Meeting Action: AcceptNumber Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Comment on Affirmative: URAL, E.: The phrase “evenly distributed” sounds much more restrictive than we intended to be here. Evenly distributed may be construed to mean all vents are identical and all spacings are exactly equal. The committee should come up with a better description of our intent.

____________________________________________________________68-20 Log #CC�5 Final Action: Accept(7.6.4 and 8.8)____________________________________________________________Submitter: Technical Committee on Explosion Protection Systems Comment on Proposal No: 68-3Recommendation: Add units for the variables in equations found in paragraphs 7.6.4 and 8.8 as shown: 7.6.4 Fireball Dimensions. 7.6.4.1 The hazard zone from a vented gas deflagration shall be calculated by the following equation: (7.6.4.�) D = 3.� (V/n)0.402 Where: D = axial distance (front-centerline) from vent, m V = volume of vented enclosure, m3

8.8 Fireball Dimensions. Measures shall be taken to reduce the risk to personnel and equipment from the effects of fireball temperature and pressure. 8.8.1 A documented risk assessment shall be permitted to be used to reduce the hazard distances calculated in 8.8.2 and 8.8.3. 8.8.2* In the case of dust deflagration venting, the distance, D, shall be determined based on Equation 8.�2. (8.8.2) D = K (V/n)�/3 Where: D = axial distance (front) from the vent, m K = flame length factor; K = �0 for metal dusts, K= 8 for chemical and agricultural dusts V = volume of vented enclosure, m3

Substantiation: This is an editorial correction as the ROP draft did not show units for the variables in the 2 equations.Committee Meeting Action: AcceptNumber Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.

____________________________________________________________68-2� Log #CC4 Final Action: Accept(Figure 8.1)____________________________________________________________Submitter: Technical Committee on Explosion Protection Systems Comment on Proposal No: 68-3Recommendation: Update figure as shown: DUST EXPLOSION VENT SIZING CALCULATION FLOW CHART FOR CHAPTER 8 Chapter 8 provides a number of equations and calculation procedures which shall be used to treat a variety of vent sizing applications. A general flowchart given in Figure 8.� demonstrates how these different tools fit together. Flowchart is shown on the following page.Substantiation: The figure shown in the ROP was not complete and has been revised and reconfigured.Committee Meeting Action: AcceptNumber Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.

68-�2

Report on Comments F2006 — Copyright, NFPA NFPA 68

Determine appropriate input parameters (e.g., KSt, Pmax, Pstat, Pinitial , enclosure

volume and L/D, vent cover area density).

Are the input parameters within the applicability

limits specified in 8.2.2.1?

Calculate minimum vent area for the

enclosure (Eq. 8.1).

Calculate minimum vent area for the

enclosure (Eq. 8.9).

Apply enclosure L/D correction (Eq. 8.2) if the enclosure L/D > 2.

Apply high turbulence corrections for high-velocity equipment or for buildings (Eq. 8.4A or 8.4B).

Increase Av, using Eq. 8.6 if the vent mass per area exceeds the limit of Eq. 8.5.

Reduce Av, using the partial volume procedure described in Section 8.3 if the maximum size of the dust cloud is limited by design or housekeeping procedures.

UseChapter 5 or 9 or perform test.

Are vent ducts present?

Yes

No

Apply procedure to account for the vent duct

effects (Section 8.5).

Is Pinitial > 0.2 barg?

Av

Av

No

Yes

YesNo Av

Av

FIGURE 8.1

____________________________________________________________68-22 Log #CC�0 Final Action: Accept(8.1.3 and 8.13.1)____________________________________________________________Submitter: Technical Committee on Explosion Protection Systems Comment on Proposal No: 68-3Recommendation: Revise 8.�.3 and 8.�.3.� as shown: 8.�.3 Where actual material is not available for test, vent sizing shall be permitted to be based upon KSt values for similar composition materials of particle size no greater than the specified particle size range per the chosen standard, either ASTM E�226 or ISO 6�84/�. 63 microns. 8.�.3.� Where the actual material intended to be produced is smaller than the size determined by 8.�.3 63 microns, tests shall be performed near the intended particle size.Substantiation: The revision clarifies the intent for selecting particle sizes according to either of two standards rather than establishing a specified particle size limitation.Committee Meeting Action: AcceptNumber Eligible to Vote: 27Ballot Results: Affirmative: 23 Negative: � Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Explanation of Negative: URAL, E.: Based on new Bartknecht (�993) Figure B.2.4.�, the sub-63 microns requirement does make a difference, is more conservative hence is more prudent. Furthermore, ASTM E�226 recommends, but does not require testing sub-74 micron samples. The sub-63 micron sample test practice also makes economical sense as the results can be used throughout the world.

____________________________________________________________68-23 Log #40 Final Action: Reject(8.2.3)____________________________________________________________Submitter: Franco Tamanini, Norwood, MAComment on Proposal No: 68-3Recommendation: The reference to Equation (8.2) and the equation itself should be deleted from the section in question (8.2.3). The equation yields a very low estimate of the vent area requirement due to L/D effects in the low range of reduced pressures (Pred < 0.4 barg). This aspect, which is not supported by available evidence, causes the L/D effect to be underestimated in a range of design pressures that is very important for practical applications.Substantiation: The following table shows that the proposed formula (Equation 8.2) produces values for the fractional vent area increase that are roughly in line with those of other methods for reduced pressures of 0.4 barg. One could show that the same holds approximately true for Pred > 0.4 barg. However, the current VDI standard and the 2002 version of NFPA 68, both predict increasing values for the additional area as Pred drops to 0.� barg. This trend is confirmed by available data and by the results of an analysis of this effect carried out at FM Global. The low values predicted by (Equation 8.2) for low PredS unrealistically underestimate the effect and are unjustified.

Vent Area Incr.∆Av/Av

Pred [barg]

0.1 0.2 0.4

VDI�3673 (2002) 3.9 2.9 �.9

NFPA 68 (2002) 4.8 3.� �.9

FM Global 3.� 2.4 �.4NFPA 68 (2006) �.7 �.6 �.5

Committee Meeting Action: RejectCommittee Statement: The Committee reviewed the data and found that there is sufficient support for the current correlation so the Committee has concluded to retain the current form of the correlation as presented in the ROP.Number Eligible to Vote: 27Ballot Results: Affirmative: 23 Abstain: �Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Explanation of Abstention: STEVENSON, B.: Both the submitter and the committee have reviewed data that I have not and have reached opposing conclusions. It is not possible to reach a conclusion from the data at my disposal.

____________________________________________________________68-24 Log #CC� Final Action: Accept(8.2.7 and 8.2.8)____________________________________________________________Submitter: Technical Committee on Explosion Protection Systems Comment on Proposal No: 68-3Recommendation: Revise 8.2.7 as follows: Add the following to 8.2.7: 8.2.7* Effects of Panel Inertia. 8.2.7.� When the mass of the vent panel is less than or equal to 40 kg/m2 and KSt is less than or equal to 250 bar-m/sec then Equation 8.2.7.2 shall be used to determine if an incremental increase in vent area is needed and the requirements of 8.2.8 shall be used to determine the value of that increase. Renumber existing 8.2.7 as 8.2.7.2 as shown: 8.2.7.2 The vent area determined by Equation 8.2.2 shall be adjusted for vent mass when the vent mass exceeds MT as calculated in Equation 8.2.7.2. (8.2.7.2) MT = (6.67 Pred

0.2 n0.3 V/KSt0.5)�.67

Where: N = number of panels MT = threshold mass (kg/m2) Pred = bar KSt ≤ 250.  Revise A.8.2.7 as follows - replace with existing A.8.2.8 and change Annex F to Annex G. A.8.2.7 Where M is greater than 40 kg/m2 or KSt > 250 bar-m/sec see Annex G for guidance. Delete A.8.2.8.   Revise 8.2.8 by deleting the condition M ≤ 40 kg/m2. 8.2.8* If M > MT, the vent area shall be increased by adding the calculated area, ΔAi from equation 8.2.8:   (8.2.8) ΔAi = Av (.0075) M0.6 KSt

0.5/n0.3 V Pred0.2

68-�3

Report on Comments F2006 — Copyright, NFPA NFPA 68 Where: Av = vent area calculated by Equation 8.2.2 M = mass of vent panel (kg/m2) M ≤ 40 kg/m2

Paragraph 8.2.8.� is unchanged.Substantiation: The conditions for use of the vent panel mass correlation need to be stated as part of the initial requirements for threshold mass. In the ROP draft these conditions were in a later section and have now been moved to the appropriate place for this application.Committee Meeting Action: AcceptNumber Eligible to Vote: 27Ballot Results: Affirmative: 22 Negative: 2 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Explanation of Negative: STUART, S.: I suggest that the Annex material attached to 8.2.7 actually be attached to 8.2.7.�. This might adequately address Erdem Ural’s negative vote and comment. Also, since the vent panel inertia correction is applied after the L/D correction according to Figure 8.�, Annex G should be modified to demonstrate how to incorporate L/D into the calculation. This can be done for a given vent area, Av by calculating an Av0 based on iterations of Pred, for use in the first and second peak calculations. I have a draft Annex G calculator with this feature that needs some peer review. URAL, E.: There appears to be a potential loophole here for applications where SIGMA>40 or Kst>250. These sections must specify what a user should do in such cases. (Annex G).

____________________________________________________________68-25 Log #39 Final Action: Reject(8.2.7 and 8.2.8 and [Equations (8.5) and (8.6)])____________________________________________________________Submitter: Franco Tamanini, Norwood, MAComment on Proposal No: 68-3Recommendation: The two sections in question (8.2.7 and 8.2.8) should be deleted as the proposed formulas they introduce are seriously in error.Substantiation: The scaling of the correlations in Equations (8.5) and (8.6) can be shown to be incorrect, based on the method in Annex G. More specifically a preliminary analysis of that method shows that MT should depend on Pred

5/3, V�/3 and K-5/2, as opposed to Pred�/3, V5/3 and K-5/6

of Equation (8.5). Should this scaling argument be considered inadequate, a numerical example will drive the point home. Consider a 60-m3 vessel, designed to withstand Pred = 0.2 barg for K = �50 bar m/s. Equation (8.5) would predict MT = �95 kg/m2 (40 lb/ft2)! For this panel mass, the method in Annex G indicates that venting is not possible. Furthermore, MT predictions are even more staggering (i.e., clearly unrealistic) for larger volumes. The same conditions, in the case of a 600-m3 volume, would yield MT = 9070 kg/m2 (�,860 lb/ft2), a result that is obviously wrong.Committee Meeting Action: RejectCommittee Statement: The Committee recognized that the ROP draft did not include the intended limit on panel density at the appropriate point (i.e., in paragraph 8.2.7) thus leading to the unreasonable result that can be derived for MT. The Committee has addressed that through its action on Committee Comment 68-24 (Log #CC�) which moves the limits into paragraph 8.2.7. The Committee retained the equations 8.2.7 and 8.2.8 as presented due to acceptable agreement using the data available to the Committee.Number Eligible to Vote: 27Ballot Results: Affirmative: 23 Negative: � Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Explanation of Negative: STUART, S.: After spending a few days crafting Excel spreadsheets to perform the calculations set forth in NFPA 68 (2007) for dust explosion venting, I think that the Committee was hasty in rejecting Dr. Tamanini’s comments regarding MT. He is correct in saying that the proposed formula for MT results in large values as vessel volumes increase. His example of the 60 m3 vessel is correct. Higher Pred’s also magnify the MT value. I worked three sample problems using 8.2.7 (old 8.5) and 8.2.8 (old 8.6), and with Annex G. Here are graphical results for three Pred’s: 0.20 bar, 0.60 bar and �.0 bar. Other parameters are:

V 60 m3

Pstat 0.05 barKSt �50 bar-m/sPmax 9.� barL/D vessel �Lduct 0 m

For Pred = 0.20 bar, MT = �95 kg/m2 as Franco indicates. Here is the graph:

The large MT forces NFPA 68 (2007) to calculate a constant Av for all M’s up to 40 kg/m2. Above 40 kg/m2, the user is directed to Annex G, resulting in the large discontinuity at that value. The kink at higher M results from the first peak dominating over the second peak.

Here is a graph for Pred = 0.60 bar (MT = 282 kg/m2):

And for Pred = �.0 bar (MT = 333 kg/m2):

After numerous other simulations, I conclude that the vent panel inertia correction defined by Equ. 8.2.8 is applicable to a limited number of scenarios due to the high values of MT calculated by Equ. 8.2.7. The high MT values calculated for many larger volume vessels, and for higher Pred’s cause the behavior shown in the above graphs. There is also a logical contradiction in directing the user to Annex G if the vent panel mass is greater than 40 kg/m2 and then saying that no vent panel inertia correction is needed if the MT is less than a value that exceeds 40 kg/m2.

68-�4

Report on Comments F2006 — Copyright, NFPA NFPA 68 Possible ways to address the above concerns are:

Derive a new equation for MT that gives values that are less than 40 kg/m2. A slight regrouping of variables in the current formula seems to do this, (V/KSt)

0.5 instead of V/KSt0.5. For the above examples:

Pred MT old MT new

0.2 bar �95 kg/m2 6.44 kg/m2

0.6 bar 282 kg/m2 9.29 kg/m2

�.0 bar 333 kg/m2 ��.0� kg/m2

Perhaps the developer of the equations will have more insights and comments.

Use Annex G for all cases where the Committee thinks that vent panel inertia requires a correction.

Go back to using 2.5 kg/m2 as MT and use Annex G for all cases where vent panel mass exceeds 2.5 kg/m2.

Derive a new “simplified” equation for M that merges smoothly with Annex G at 40 kg/m2.

I am also still bothered by the differences between Annex G and Equation 8.2.8 for low Pred’s, but cannot offer an alternative except to abandon use of Equation 8-2-8.

____________________________________________________________68-26 Log #�4 Final Action: Accept(8.2.8, 8.2.9, 8.3.1, 8.3.2, and 8.5)____________________________________________________________Submitter: Samuel A. Rodgers, Honeywell Inc.Comment on Proposal No: 68-3Recommendation: In 8.2.8 revise equation 8.6 as follows: Av3 = [� + (.0075)M0.6 KSt

0.5/n0.3V Pred0.2]Av2

Add new text and renumber:   8.2.9 When M ≤ MT, then Av3 = Av2 In 8.3.�+revise equation 8.7 (which is graphics) as follows: Change Avpv to Av4; change Av0 to Av3 Add new text and renumber as needed: 8.3.2.� When partial volume is not applied, then Av4 = Av3. Add new text and renumber after 8.4: If no vent ducts, then Av4 is equal to Avf. In 8.5 revise equation 8.�0 change Av6 to Avf and Av2 to Av4; also revise the equations in 8.�0a, 8.�0b, and the example in the annex to the same convention.Substantiation: Editorial changes are needed to carry the same numbering convention for Vent Area throughout this chapter. In this way, all values of Av are determined sequentially based on the prior sub-numbered value.Committee Meeting Action: AcceptNumber Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.

____________________________________________________________68-27 Log #8 Final Action: Accept(8.3.3.4)____________________________________________________________Submitter: Samuel A. Rodgers, Honeywell Inc.Comment on Proposal No: 68-3Recommendation: Add a new 8.3.3.4 after existing and renumber 8.3.3.4 to be 8.3.3.5 as follows: 8.3.3.4 In applications involving spray dryers where a partial volume venting is calculated in accordance with Equation 8.7, the vent shall be mounted within the chosen partial volume zone of the dryer that contains the driest fraction of material.Substantiation: Venting would be less efficient if applied remote from combustible zone.Committee Meeting Action: AcceptNumber Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Comment on Affirmative: URAL, E.: Isn’t the reduced efficiency effect already taken into account with the enclosure L/D correction and maximum credible flame path-length considerations?

____________________________________________________________68-28 Log #2 Final Action: Accept in Principle(8.5)____________________________________________________________Submitter: Samuel A. Rodgers, Honeywell, Inc.Comment on Proposal No: 68-3Recommendation: Add language to indicate the solution is iterative and to clarify that D’Arcy friction factor is used. Add statement to definition of Av2 to clarify where to determine Av2.8.5* Effects of Vent Ducts. The effect of vent ducts shall be calculated from the following equation. This solution is iterative, as E1, and E2 are both functions of Av6.

( )0.8 0.46 2 1 21 1.18v v

KA A E E Ko= + ⋅ ⋅ (8.10)

where:

Av2 = vent area after adjustment for turbulence (m2), per section 8.2.6.6 through 8.2.6.8

Av6 = vent area required when a duct is attached to the vent opening

(m2)

6

1

v ductA L

VE⋅

= (8.10a) 4

62 4 /3 3 /4

10(1 1.54 )

v

stat St

AP K VE

⋅=

+ ⋅ ⋅ ⋅ (8.10b)

Pstat = nominal static opening pressure of the vent cover (bar)

V = enclosure volume (m3)

Lduct = vent duct overall length (m)

K0 = �.5, the resistance coefficient value assumed for the test configurations that generated the data used to validate Equations 8.� and 8.2

212

Dinlet elbows outlet

h

f LK K K K

DU

∆ρ ⋅≡ = + + + +

⋅ρ ⋅ (8.10c)

K = overall resistance coefficient of the vent duct application

Kelbows, Koutlet – resistance coefficients for fittings

U = fluid velocity

Dh = vent duct hydraulic diameter (m)

fD = D’Arcy friction factor for fully turbulent flow, see annex for typical formula [��5] Substantiation: The current text does not indicate to the user that the area determination with ducts is an iterative solution, nor is the friction factor clearly identified as the D’Arcy form. Committee Meeting Action: Accept in Principle In addition to the submitter’s recommendation, also add the following new text 8.5.2 and 8.5.3 as shown: 8.5* Effects of Vent Ducts.

8.5.1 The effect of vent ducts shall be calculated from the following equation. This solution is iterative, as E1, and E2 are both functions of Av6.

( )0.8 0.46 2 � 2� �.�8= + ⋅ ⋅v v

KA A KoE E (8.10)

where:

Av2 = vent area after adjustment for turbulence (m2), per section 8.2.6.6 through 8.2.6.8

Av6 = vent area required when a duct is attached to the vent opening

(m2)

6

�

⋅= v ductA L

VE (8.10a)

46

4/3 3/ 42

�0(� �.54 )

⋅=

+ ⋅ ⋅ ⋅v

stat St

AP K VE (8.10b)

68-�5

Report on Comments F2006 — Copyright, NFPA NFPA 68 Pstat = nominal static opening pressure of the vent cover (bar)

V = enclosure volume (m3)

Lduct = vent duct overall length (m)

K0 = �.5, the resistance coefficient value assumed for the test configurations that generated the data used to validate Equations 8.� and 8.2

2�2

∆ ⋅≡ = + + + +

⋅ ⋅

Dinlet elbows outlet

h

f LK K K KDU

ρ

ρ (8.10c)

K = overall resistance coefficient of the vent duct application

Kelbows, Koutlet – resistance coefficients for fittings

U = fluid velocity

Dh = vent duct hydraulic diameter (m)

fD = D’Arcy friction factor for fully turbulent flow, see annex for typical formula [��5]

8.5.2 (new) Under certain circumstances there can be two solutions for vent area. In these cases the smaller vent area shall be used.

8.5.3 (new)There are circumstances where these equations do not produce a solution for vent area. In these cases, the design shall be modified by decreasing the vent duct length or strengthening the vessel to contain a higher Pred or both.Committee Statement: The Committee noted the need to provide guidance on the application where either 2 solutions can be obtained or no solutions can be obtained. The Committee accepted the original recommendation as submitted.Number Eligible to Vote: 27Ballot Results: Affirmative: 23 Abstain: �Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Explanation of Abstention: ZALOSH, R.: I can’t support an extremely complicated calculation procedure that can lead to multiple solutions in some cases and no valid solutions in other cases. On the other hand, the procedure itself is a fait a complete after the ROP ballot.

____________________________________________________________68-29 Log #CC5 Final Action: Accept(Table 8.5.9)____________________________________________________________Submitter: Technical Committee on Explosion Protection Systems Comment on Proposal No: 68-3Recommendation: Update table as shown to revise values for panel density, where panel density is less than MT and less than or equal to 40 kg/m2.

Table 8.5.9 Combination Rules and Limitations for NFPA 68 Dust Models (from errata)Model ApplicationVent Ducts 0.8 ≤ P0 ≤ �.2 bar abs

Panel density < MT and ≤ 40 Kg/m2

Allow Partial Volume� ≤ L/D ≤ 6(calculate vent duct effect last)

Partial Volume Allow Vent DuctPanel density < MT and ≤ 40 Kg/m2

0.8 ≤ P0 ≤ �.2 bar abs� ≤ L/D ≤ 6(calculate vent duct effect last)

Elevated Initial Pressure No Vent DuctPanel density < MT and ≤ 40 Kg/m2

0.2 ≤ P0 ≤ 4 bar gFull Volume Deflagration� ≤ L/D ≤ 6(calculate elevated initial pressure effect last)

Panel Inertia 0.8 ≤ P0 ≤ �.2 bar-aNo Vent DuctPanel density < MT and ≤ 40 Kg/m2

Allow Partial Volume� ≤ L/D ≤ 6

Substantiation: The revision to the panel density limitations is necessary to be consistent with the action taken by Committee Comment 68-�4 (Log #CC2).Committee Meeting Action: Accept

Number Eligible to Vote: 27Ballot Results: Affirmative: 22 Negative: 2 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Explanation of Negative: STUART, S.: Under “Panel Inertia”, Table 8.5.9 does not allow use of a vent duct, yet under “Vent Ducts” it gives criteria for panel density. Also, Figure 8.�, the flow chart for calculating dust explosion vents, directs the user to the vent duct correction after applying the panel inertia correction. My suggestion is to delete “No Vent Duct” from the right-hand column for “Panel Inertia”. URAL, E.: Remove phrases in parentheses (calculate _______ effect last) from the table. Limited Panel density to 200 kg/m2. Remove <MT requirement as it is redundant. Add a footnote to explain the use of the equations in the main body of the standard versus the use of Annex G.

____________________________________________________________68-30 Log #5 Final Action: Accept(8.7)____________________________________________________________Submitter: Bill Stevenson, CV Technology, Inc.Comment on Proposal No: 68-3Recommendation: Revise Section 8.7 as follows: 8.7.�(�) Locate all of the venting area below the bottom of the bags, filters, or cartridges, as shown in Figure 8.7.�(a). For this case, the vent area shall be permitted to be calculated on the basis of the dirty side only; that is, calculate the volume below the tube sheet, and subtract out the volume occupied by the bags. (�)a. When the spacing between bags is less than or equal to the radius of the bag, filter, or cartridge, the vent area shall be permitted to be calculated on the basis of the volume below the lower end of the bags. (�)b. When the spacing between bags is greater than the radius of the bag, filter, or cartridge, the vent area shall be permitted to be calculated on the basis of the dirty side only; that is, calculate the volume below the tube sheet, and subtract out the volume occupied by the bags.Add diagram showing plan view with spacing between bags.Substantiation: This wording was developed by the technical committee to clarify the options. Committee Meeting Action: AcceptNumber Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Comment on Affirmative: CHUBB, G.: In 8.7.�(3), the very last sentence is contradictory. The area below the tube sheet in a dust collector is the dirty air side and the area above the tube sheet is the clean air side. Therefore, the very last words “below the tube sheet” should be stricken Editorial: The equation in 4.3.�.3.2(2) is mis-numbered. URAL, E.: The proposed revision should also apply to 8.7.�(2) and to 8.7.�(3).

____________________________________________________________68-3� Log #7 Final Action: Accept in Principle(8.9.1)____________________________________________________________Submitter: Samuel A. Rodgers, Honeywell Inc.Comment on Proposal No: 68-3Recommendation: Add the following new text: 8.9.� It shall be permitted to use a lower value of the coefficient shown in the equation in Section 8.9(�) where experimental data is available.Substantiation: The equation shown in the equation in Section 8.9(�) is based upon the tests submitted with the original approval documentation. Limited tests and private communication from a manufacturer have indicated that there are conditions where a lower value of the coefficient can be applied.Committee Meeting Action: Accept in Principle Add the following proposed text as 8.9.3: Add the following new text: 8.9.3 It shall be permitted to use a lower value of the coefficient shown in the equation in Section 8.9.2(�) where experimental data is available to substantiate the lower value.Committee Statement: The Committee clarified that the experimental data must substantiate the lower value before it can actually be applied in the equation.Number Eligible to Vote: 27Ballot Results: Affirmative: 23 Negative: � Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Explanation of Negative: URAL, E.: The coefficient (�.74) has not been validated in publicly available documents as a worst case for all flameless venting devices. Therefore, this equation should be taken out, and users should rely only on specific experimental data for specific devices.

68-�6

Report on Comments F2006 — Copyright, NFPA NFPA 68 ____________________________________________________________68-32 Log #20 Final Action: Accept in Principle(9.1.4 (New) )____________________________________________________________Submitter: S. Dorofeev, H.Febo, Norwood, MAComment on Proposal No: 68-3Recommendation: Add new text to read: 9.�.4 This chapter does not apply to oxidants other than air, and to mixtures at elevated initial temperatures, which are greater than 330K.Substantiation: Data used are for fuel-air mixtures only. These data are not applicable for other oxidants and for oxygen-enriched air. Empirical correlations are used that do not account for changes of the mixture sensitivity to undergo DDT with variations of initial temperature.Committee Meeting Action: Accept in Principle Add new text to read: 9.�.4 This chapter does not apply to oxidants other than air, and to mixtures at elevated initial temperatures, which are greater than 57°C (�34°F).Committee Statement: The temperature was changed from Kelvin to equivalent Celsius, which becomes 57°C (�34°F).Number Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.

____________________________________________________________68-33 Log #2� Final Action: Reject(9.2.10.2.1 through 9.2.10.2.2)____________________________________________________________Submitter: S. Dorofeev, H. Febo, Norwood, MAComment on Proposal No: 68-3Recommendation: These sections shall be removed including Figure 9.2.�0.2.�.� and Figure 9.2.�0.2.2.�. Instead, these data may be included into Annex A with indication of the inconsistencies that limit application.Substantiation: Data of Figure 9.2.�0.2.�.� is not consistent with Figure 9.2.�0.�: according to Figure 30 (D = 0.4 m), and≈�4 (D = �.6 m), L/D ≈9.2.�0.�, DDT can be expected at L/D 45 (D = 0.2 m); and the corresponding overpressures can exceed≈L/D significantly those given in Figure 9.2.�0.2.�.�. Data of Figure 9.2.�0.2.2.� is not consistent with Figure 9.2.�0.�: according to Figure 9.2.�0.�, L/D for DDT depends on tube diameter, while Figure 9.2.�0.2.2.� does not reference tube diameter. In Figure 9.2.�0.� for KSt = 200, for example, DDT can be expected at L/D from 20 to 40 for D from � to 3 m; and the corresponding overpressures can exceed significantly those given in Figure 9.2.�0.2.2.�.Committee Meeting Action: RejectCommittee Statement: The Committee recognizes that there appears to be a misunderstanding in the interpretation of the Figure 9.2.�0.� (which is currently 8.4.3 in the 2002 edition). The figure is not intended to indicate the distance to DDT but is intended to provide a more conservative maximum spacing distance for vents in order to prevent DDT. In comparison, Figures 9.2.�0.2.�.� and 9.2.�0.2.2.� show anticipated Pred values which are less than would be expected during DDT. Number Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.

____________________________________________________________68-34 Log #22 Final Action: Accept in Principle(9.2.12)____________________________________________________________Submitter: S. Dorofeev, H. Febo, Norwood, MAComment on Proposal No: 68-3Recommendation: Revise text to read: 9.2.�2 For systems having an initial flow velocity greater than 20 m/sec, for gases having a burning velocity more than �.3 times that of propane, or for dusts with KSt > 300, vents shall be placed no more than 2 m (6.6 ft) apart placement shall be determined by appropriate tests.Substantiation: The requirement to place vents 2 m apart is not sufficiently conservative for sensitive mixtures, e.g., for hydrogen-air, or acetylene-air, and especially for gas flows with velocities of greater than 20 m/s. There are numerous experimental data that DDT run-up distances may be smaller than 2 m under these extreme conditions. See, e.g., “M. A. Nettleton, Gaseous detonations: their nature, effects and control, Chapman and Hall, London, New York, �987”, Moen IO, Bjerketvedt D, Jenssen A, Thibault PA (�985) Transition to Detonation in a Large Fuel-Air Cloud. Combustion and Flame 6�:285-29�”.Committee Meeting Action: Accept in Principle Revise text to read: 9.2.�2 For systems having an initial flow velocity greater than 20 m/sec, for gases having a burning velocity more than �.3 times that of propane, or for dusts with KSt > 300, vents shall be placed no more than 2 m (6.6 ft) apart placement shall be determined by appropriate tests.Committee Statement: The change is only editorial to remove unenforceable wording.Number Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.

____________________________________________________________68-35 Log #CC�� Final Action: Accept(10.4.4)____________________________________________________________Submitter: Technical Committee on Explosion Protection Systems Comment on Proposal No: 68-3Recommendation: Move the following paragraphs to the annex for this chapter and renumber as needed. Add paragraphs �0.4.3 and �0.4.4 through �0.4.4.4 to new Annex material for A.�0.4:

Move these sections to the Annex of 10.4A.10.410.4.3* Where the vent closure panel is a double-wall type (such as an

insulated sandwich panel), single-wall metal vent panel restraint systems shall should not be used.

10.4.4* The restraint system shown in Figure A.�0.4.4 shall should be used for double-wall panels.

10.4.4.1 The panel area shall should be limited to 3.� m2 (33 ft2), and its mass shall should be limited to �2.2 kg/m2 (2.5 lb/ft2).

Vent panel

Bar washer

Blind rivet

Sheet-metal subgirt (10 ga)

Girt

Roof girder

Wire rope clips

203.2 mm

101.6 mm

Close-up of shock absorber

6.35 mmdiam through-bolt

12.7 mm diam forged eye bolt

6.35 mmdiamfail-safetether, 0.61 mlong

Shock absorber (4.8 mm thick) — freedom to move through 90-degree arc

6.35 mm diam, 1.2 m long galv. wire rope tether

12.7 mm diam bolts

241.3 mm

FIGURE A.10.4.4(a) An Example of a Restraint System for Double-Wall Insulated Metal Vent Panels.

10.4.4.2 Forged eyebolts shall should be used required. Alternatively, a “U” bolt shall can be permitted to be substituted for the forged eyebolt.

10.4.4.3* A shock absorber device with a fail-safe tether shall should be provided.

Renumber A.�0.4.2 to A.�0.3.2, eliminate annex item to �0.4.2.A.10.3.2 Specially designed fasteners that fail, under low mechanical

stress, to release a vent closure are commercially available, and some have been tested by listing or approval agencies.

Existing Annex items to A.10.4.3, A.10.4.4 and A.10.4.4.4 become annex to A.10.4

A.10.4.3 Where large, lightweight panels are used as vent closures, it is usually necessary to restrain the vent closures so that they do not become projectile hazards. The restraining method shown in Figure A.�0.4 (b) illustrates one method that is particularly suited for conventional single-wall metal panels. The key feature of the system includes a 5 cm (2 in.) wide, �0-gauge bar washer. The length of the bar is equal to the panel width, less 5 cm (2 in.) and less any overlap between panels. The bar washer/vent panel assembly is secured to the building structural frame using at least three �0 mm (3/8 in.) diameter through-bolts.]

68-�7

Report on Comments F2006 — Copyright, NFPA NFPA 68

Bar washer (10 ga)

Vent panel

Building girt

ELEVATION showing vent panels and bar washer assemblies

+ + ++ + + ++

Bar washer

Lap

Vent panel

Girt

Girt

Girt

Girt

9.5 mmdiam through-bolt

FIGURE 10.4 (b) (previously 9.5.1) An Example of a Restraint System for Single-Wall Metal Vent Panels.

The restraining techniques shown are very specific to their application. They are intended only as examples. Each situation necessitates an individual design. Any vent restraint design should be documented by the designer. No restraint for any vent closure should result in restricting the vent area. It is possible for a closure tether to become twisted and to then bind the vent to less than the full opening area of the vent.

The stiffness of the double-wall panel is much greater than that of a single-wall panel. The formation of the plastic hinge occurs more slowly, and the rotation of the panel can be incomplete. Both factors tend to delay or impede venting during a deflagration.

The component sizes indicated in Figure �0.4(a) have been successfully tested for areas to 3.� m2 (33 ft2), and mass of up to �2.2 kg/m2 (2.5 lb/ft2). Tests employing fewer than three rope clips have, in some instances, resulted in slippage of the tether through the rope clips, thus allowing the panel to become a free projectile.

The shock absorber is a thick, L-shaped piece of steel plate to which the tether is attached. During venting, the shock absorber forms a plastic hinge at the juncture in the “L,” as the outstanding leg of the “L” rotates in an effort to follow the movement of the panel away from the structure. The rotation of the leg provides additional distance and time, over which the panel is decelerated while simultaneously dissipating some of the panel’s kinetic energy.Substantiation: The Committee is aware of users of the document who have apparently interpreted the guidance (and proposed requirement) in ROP paragraph �0.4.4.� as criteria that limited the panel to 33 sq ft and 2.5 psf and nothing larger could be used. It should be made clear that this is typical and other equivalent combinations are possible. The Committee believes moving the text to the annex still provides information useful for this application but that no specific requirement should be based upon this information.Committee Meeting Action: AcceptNumber Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.

____________________________________________________________68-36 Log #4 Final Action: Accept(A.3.3.17)____________________________________________________________Submitter: Samuel A. Rodgers, Honeywell, Inc.Comment on Proposal No: 68-3Recommendation: Clarify hydraulic diameter definition by adding a subscript “h” as follows: Dh = 4 (A/p) (�)Substantiation: Both diameter and hydraulic diameter are used in the document and the definition should clearly differentiate between them.Committee Meeting Action: AcceptNumber Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.

____________________________________________________________68-37 Log #�3 Final Action: Accept(A.7.2.2.2)____________________________________________________________Submitter: Samuel A. Rodgers, Honeywell Inc.Comment on Proposal No: 68-3Recommendation: Add a reference for A.7.2.2.2 to read: Communication from D. Herrmann to Committee on Explosion Protection Systems date May 2005.

Substantiation: The information in A.7.2.2.2 indicates an interpretation, but does not provide a source for recommendations presented. To indicate the source of this information.Committee Meeting Action: AcceptNumber Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.

____________________________________________________________68-38 Log #CC�4 Final Action: Accept(A.8.2.6.7 and A.8.2.6.8)____________________________________________________________Submitter: Technical Committee on Explosion Protection Systems Comment on Proposal No: 68-3Recommendation: Renumber existing A.8.2.6.8 as A.8.2.6.7 and add the following new Annex material for 8.2.6.8 as shown: A.8.2.6.8 Building damaging dust explosions are most often secondary dust explosions, where an initial disturbance or smaller ignition causes a high local turbulence, creating the dust cloud with immediate ignition. In order to provide enough venting to prevent building failure and additional personnel injury, the high end turbulence correction factor of �.7 is used for buildings.Substantiation: Paragraph A.8.2.6.8 as shown in the ROP draft actually applied to paragraph 8.2.6.7 and has been renumbered accordingly. The Committee added explanatory information to existing 8.2.6.8 on the need for the �.7 correction factor applied to buildings.Committee Meeting Action: AcceptNumber Eligible to Vote: 27Ballot Results: Affirmative: 23 Negative: � Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Explanation of Negative: URAL, E.: The proposed statement “Building damaging dust explosions are most often secondary dust explosions...” should not be used to justify the 70 percent vent area penalty. This explanation could mislead inexperienced users to the false sense of security that their application is covered against secondary explosions. In reality, the 70 percent penalty would not be adequate for most secondary explosions. Ideally, the justification should be based on data. Since the committee does not have the data to justify the 70 percent penalty, then perhaps we should fall back on the fact that there are a lot more uncertainties in the determinations of Pred and Pstat for building applications, and that the margins between Pred and Pstat are often too small. An alternative justification might be that buildings typically have more internal obstructions than the process vessels. Whatever the committee decides, it is essential that we have a clear definition, in Chapter 3, of what a building is.

____________________________________________________________68-39 Log #� Final Action: Accept in Principle(A.8.5)____________________________________________________________Submitter: Samuel A. Rodgers, Honeywell, Inc.Comment on Proposal No: 68-3Recommendation: This annex item does not clearly indicate the example is an iterative solution, determining Pred from a give Av6, the reverse of the equations in Section 8.5. Revise text to read as follows:

A.8.5 The flow resistance coefficient K for this correlation is defined on the static pressure drop, ∆P, from the enclosure to the duct exit at a given the average duct flow velocity, U, i.e.

212

PK

U∆

≡⋅ρ ⋅

Another convention used by some reference books is to define K on the total pressure drop or on another velocity scale. The user should ensure that the loss coefficients used in the calculations is consistent with the definition of K adopted for the vent duct calculations. See reference [��5] for additional information.

Example problem:

See FIGURE A.8.5 Example Vent Duct Installation as it appears in the ROP draft.

Example conditions:GIVEN:Enclosure volume, V (m3) 25Enclosure L/D 4Vent Diameter, Dv (m) �.5Duct Diameter, Dv (m) �.5

68-�8

Report on Comments F2006 — Copyright, NFPA NFPA 68 Av (m

2) �.77

Pstat (barg) 0.25KSt (bar-m/s) 200Pmax (bar) 8Duct length (m) �2Duct effective roughness, ε (mm) 0.26Elbows 2 × 90o

Elbow flow resistance 2 × �.2 = 2.4Rain cover flow resistance 0.75CALCULATE: Pred

While Section 8.5 provides the equations in a form to calculate the vent area based on an allowable Pred, this example shows how to determine the resulting Pred for a given vent area. In general, such calculations will be iterative. These input parameters are provided for demonstration purposes. Refer to [��5] for additional discussion on how they were selected.

Solution:

(�) Compute the Friction factor for the problem

For practically all vent ducts, the Reynolds number is so large that fully turbulent flow regime will be applicable. In this regime, the friction factor is only a function of the ratio of the internal duct surface effective roughness (ε) to duct diameter. Duct friction factor can thus be calculated using a simplified form of the Colebrook equation:

2

10

1

1.14 2logD

h

f

D

= ε −

(A.8.5)

The effective roughness for smooth pipes and clean steel pipes are typically 0.00�5 mm, and 0.046 mm, respectively. Recognizing that the pipes used repeatedly in combustion events could be corroded, a value of ε = 0.26 mm is assumed.

From Equation A.8.5, fD = 0.0�3

( ) ( )0.107

( )D

h

f LD⋅

=

Kinlet = �.5

Kelbows = 2.4

Kexit = 0.75

K = 4.757

(2) Assume a Pred value = � barg. The solution is iterative, where the assumed value of Pred is replaced with the calculated value of Pred until the two values substantially match. A � percent error is typically considered acceptable convergence.

(3) From Equation 8.�,4 4 /3 3 /4

0

81.10 1 1.54 (0.25) 200 (25) 1v

red

AP

− = ⋅ + ⋅ ⋅ ⋅ ⋅ − Av0 = 0.735 m2

(4) From Equation 8.2, 0.75 2

1 0.735 1 0.6 (4 2) exp(-0.95 )v redA P = ⋅ + ⋅ − ⋅ ⋅ Av1 = �.02 m2

(5) From Equation 8.�0a, and using the intended vent area of �.77 m2,

1

(1.77) (12)(25)

E⋅

=

E� = 0.85

(6) From Equation 8.�0b, and using the installed vent area of �.77 m2,

( )4

2 4 /3 3 /4

10 (1.77)1 1.54 (0.25) (200) (25)

E⋅

=+ ⋅ ⋅ ⋅

E2 = 6.37

(7) From Equation 8.�0, with Av2 equal to Av�, assuming no increase for turbulence

( )0.8 0.46

4.757(1.02) 1 1.18 (0.85) (6.37) 1.5vA = ⋅ + ⋅ ⋅

Av6 = 5.77 m2

(8) Since the calculated value of Av6 is not equal to the installed vent area, go back to step 2 above, and change Pred, until the Av6 calculated in step 7 is equal to the specified vent area of �.77 m2.

A trial and error process (or the goal seek button in Excel) satisfies the requirement in step 8 when Pred = 3.52 barg.

(9) Equations 8.�� show that there is no DDT propensity for this particular application.

10,000 (1.5) 11,000min ,

(200) (200)effL ⋅

≤

Leff ≤ min [75,55]

(25)(8 3.52)

(1.77)dustyL = − ⋅

Ldusty = 63 m

Since Lduct = �2 m,

Leff = min(�2,63) = �2 m ≤ 55 m, therefore DDT is not expected.

Substantiation: The annex item does not clearly indicate that the example is not a straight application of Section 8.5, but an iterative solution. The friction factor is not clearly indicated as the D’Arcy form.Committee Meeting Action: Accept in Principle Revise Annex A.8.5 text as shown:

A.8.5 The flow resistance coefficient K for this correlation is defined on the static pressure drop, ∆P, from the enclosure to the duct exit at a given average duct flow velocity, U, i.e.

2�2

∆≡

⋅ ⋅PKUρ

Another convention used by some reference books is to define K on the total pressure drop or on another velocity scale. The user should ensure that the loss coefficients used in the calculations is consistent with the definition of K adopted for the vent duct calculations. See reference [��5] for additional information.

The equations are non-linear and, under certain combinations of input values, result in two possible solutions for vent area for a given Pred. The lower value of vent area is the meaningful solution and the upper value is an artifact of the form of the equation set. There are certain combinations of Pred and vent duct length where no vent area is large enough and no solution is obtainable. When this occurs it could be possible to vary Pred or vent duct length to converge to a solution. If that solution is not satisfactory, see NFPA-654 for alternatives.

There is a minimum value for Pred as vent area increases, beyond which solutions are not meaningful. This occurs approximately when the volume of the duct exceeds a fraction of the volume of the vessel. When solving the equations, constraining Av6 as below will typically isolate the smaller root.

�6 ≤⋅

VLAv

For the following input values, Figure A.8.5 (a) illustrates the potential solutions.

V = 500 m3

Pmax = 8.5 barKSt = �50 bar-m/sec

68-�9

Report on Comments F2006 — Copyright, NFPA NFPA 68 Pstat = 0.05 barPred = 0.5 barVessel L/D = 4ε  = 0.26 mmStraight duct, no elbows, fittings, or rain hats.

10,000

Duct length (m)

Meaningful

Artifact

1,000

100

10

00 2 4 6 8

Ven

t are

a (m

2 )

FIGURE A.8.5 (a) Av vs. Duct Length

Example problem:See FIGURE A.8.5 (b) Example Vent Duct Installation as it appears

in the ROP draft.

Example conditions:

GIVEN:Enclosure volume, V (m3) 25Enclosure L/D 4Vent Diameter, Dv (m) �.5Duct Diameter, Dh (m) �.5Av (m

2) �.77

Pstat (barg) 0.25

KSt (bar-m/s) 200Pmax (bar) 8Duct length (m) �2Duct effective roughness, ε (mm) 0.26Elbows 2 × 90o

Elbow flow resistance 2 × �.2 = 2.4Rain cover flow resistance 0.75CALCULATE: Pred

While Section 8.5 provides the equations in a form to calculate the vent area based on an allowable Pred, this example shows how to determine the resulting Pred for a given vent area. In general, such calculations will be iterative. These input parameters are provided for demonstration purposes. Refer to [��5] for additional discussion on how they were selected. Solution:(�) Compute the Friction factor for the problem For practically all vent ducts, the Reynolds number is so large that

fully turbulent flow regime will be applicable. In this regime, the friction factor is only a function of the ratio of the internal duct surface effective roughness (ε) to duct diameter. Duct friction factor can thus be calculated using a simplified form of the Colebrook equation:

2

�0

�

�.�4 2log

= −

D

h

f

Dε

The effective roughness for smooth pipes and clean steel pipes are typically 0.00�5 mm, and 0.046 mm, respectively. Recognizing that the pipes used repeatedly in combustion events could be corroded, a value of ε = 0.26 mm is assumed.

From Equation A.8.5, fD = 0.0�3

( ) ( ) 0.�07

( )⋅

=D

h

f LD

Kinlet = �.5

Kelbows = 2.4

Kexit = 0.75

K = 4.757

(2) Assume a Pred value = � barg. The solution is iterative, where the assumed value of Pred is replaced with the calculated value of Pred until the two values substantially match. A � percent difference between iterations is typically considered acceptable convergence.

(3) From Equation 8.�,

4 4 / 3 3 / 40

8�.�0 � �.54 (0.25) 200 (25) �− = ⋅ + ⋅ ⋅ ⋅ ⋅ − vred

AP

Av0 = 0.735 m2

(4) From Equation 8.2, 0.75 2

� 0.735 � 0.6 (4 2) exp(-0.95 ) = ⋅ + ⋅ − ⋅ ⋅ v redA P

Av1 = �.02 m2

(5) From Equation 8.�0a, and using the intended vent area of �.77 m2, �

(�.77) (�2)(25)

⋅=E

E� = 0.85

(6) From Equation 8.�0b, and using the installed vent area of �.77 m2,

( )4

2 4 / 3 3 / 4

�0 (�.77)� �.54 (0.25) (200) (25)

⋅=

+ ⋅ ⋅ ⋅E

E2 = 6.37

(7) From Equation 8.�0, with Av2 equal to Av�, assuming no increase for turbulence

( )0.8 0.4

64.757(�.02) � �.�8 (0.85) (6.37) �.5= ⋅ + ⋅ ⋅vA

Av6 = 5.77 m2

(8) Since the calculated value of Av6 is not equal to the installed vent area, go back to step 2 above, and change Pred, until the Av6 calculated in step 7 is equal to the specified vent area of �.77 m2.

A trial and error process (or the goal seek button in Excel) satisfies the requirement in step 8 when Pred = 3.52 barg.

(A.8.5)

68-20

Report on Comments F2006 — Copyright, NFPA NFPA 68 (9) Equations 8.�� show that there is no DDT propensity for this

particular application.

�0,000 (�.5) ��,000min ,

(200) (200) ⋅

≤

effL

Leff ≤ min [75,55]

(25)(8 3.52)

(�.77)= − ⋅dustyL

Ldusty = 63 m

Since Lduct = �2 m,

Leff = min(�2,63) = �2 m ≤ 55 m, therefore DDT is not expected.Committee Statement: The changes shown to the recommended text are only editorial.Number Eligible to Vote: 27Ballot Results: Affirmative: 23 Abstain: �Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Explanation of Abstention: ZALOSH, R.: I can’t support an extremely complicated calculation procedure that can lead to multiple solutions in some cases and no valid solutions in other cases. On the other hand, the procedure itself is a fait a complete after the ROP ballot.

____________________________________________________________68-40 Log #CC9 Final Action: Accept(A.8.8.3)____________________________________________________________Submitter: Technical Committee on Explosion Protection Systems Comment on Proposal No: 68-3Recommendation: Add the following new annex material: A.8.8.3 Estimates of external pressure effects for gas venting have been made using validated computational fluid dynamics models. A simpler methodology to estimate downstream external pressures for other situations and other locations is described in T. Forcier, R. Zalosh,”External Pressures Generated by Vented Gas and Dust Explosions,” J. of Loss Prevention in the Process Industries, v. �3, pp. 4��-4�7, 2000. Substantiation: The added commentary provides information on additional methodologies for addressing external pressure effects.Committee Meeting Action: AcceptNumber Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.

____________________________________________________________68-4� Log #CC7 Final Action: Accept(Figure C.1)____________________________________________________________Submitter: Technical Committee on Explosion Protection Systems Comment on Proposal No: 68-3Recommendation: Correct caption and fix scale in Figure C.� (labeled as Figure A.��.4 in the ROP draft) as described below. The x-axis label for Figure C.� should read “Test Vessel Volume (m3)” Revise the scale for the x-axis values to go from 0.00� to �000 m3 so the values would be 0.00�, 0.0�, 0.�, �, �0, �00, �000. Revise the caption to read: “Effect of Test Volume on KG Measured in Spherical Vessels” (same as Figure B.� in 2002 edition) Figure C.� is shown below.Substantiation: The x-axis value for the tests should be presented in m3 and not liters and the scale does not correctly match the units as expressed in the label for the x-axis. This is an editorial correction to make the label and units of the graph consistent.Committee Meeting Action: AcceptNumber Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Comment on Affirmative: URAL, E.: This Annex has useful information but needs to be editorially reorganized to improve readability. Remove bits left from other sections (e.g. the 2nd paragraph that starts with ASTM E20�9). Make references to the equations in the main body of the standard rather than to the charts in Annex H. Emphasize that Figure C.� is provided as a guidance for the test labs, and that it is inappropriate to use the information provided in Figure C.� in conjunction with the vent sizing equations or vent sizing charts. There is additional guidance on “New” Kg testing within Annex E and on translating it to Bartknecht data with a linear transformation. (It may make sense to pull this into Annex C). Incidentally, Barthknecht gave propane Kg=75 Bar-m/s in his �980 English edition book. The following conclusions can be inferred from Figure C.�. A) The cubic law is not valid for gas explosions. Therefore, the gas venting equations in Chapter 7 (both high and low strength) do not have proper volume scaling since they are based on the cubic law assumption (per analysis similar to that given in Ural (200�) article in Process Safety Progress). B) Kg tests should not be done in enclosures smaller than 20 L volume. Because, the linear transformation suggested in Annex E is not valid.

KG (

bar-

m/s

ec)

800

0

100

0.001 1.00.10.01

200

300

400

600

500

700

Propane

Pentane

Hydrogen

Test vessel volume (m³)

C1

C3

Cs

H2

Methane

10 100 1000

FIGURE C.1 Effect of Test Volume on KG Measured in Spherical Vessels.

68-2�

Report on Comments F2006 — Copyright, NFPA NFPA 68 ____________________________________________________________68-42 Log #CC6 Final Action: Accept(Table F.1(e))____________________________________________________________Submitter: Technical Committee on Explosion Protection Systems Comment on Proposal No: 68-3Recommendation: Add a new entry for phenolic resin to Table F.�(e) as shown (in addition to the existing data point):

Material Median Minimum Mass Diameter Flammable Pmax KSt Dust Concentration Hazard Class Phenolic 55 – 7.9 269 2 resin

Substantiation: These additional test data come from personal communication to the Committee on Explosion Protection Systems from Henry Febo, FM Global.Committee Meeting Action: AcceptNumber Eligible to Vote: 27Ballot Results: Affirmative: 24 Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.

____________________________________________________________68-43 Log #CC�3 Final Action: Accept(Annex J)____________________________________________________________Submitter: Technical Committee on Explosion Protection Systems Comment on Proposal No: 68-3Recommendation: Revise Annex J by adding a new Equation J.9 and edit existing Equation J.9 as shown:Annex J Effect of Partial Volumes on Buildings – Example Problem

This annex is not a part of the requirements of this NFPA document but is included for informational purposes only.

J.1 NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, applies the layer thickness criteria over 5 percent of the floor area. This guide has chosen to apply the layer thickness criteria of �/32 inch over �00 percent of the floor area and other surfaces defined in Step 3 to be more conservative.

J.2 Building Example. Thin layers of coal dust are known to form on the floor of a coal-fired powerhouse with a 20 m × 30 m floor area, and a 4 m ceiling height. Deflagration vents for an end wall roof installation are to be designed for a Pred of � psi gauge pressure, and a Pstat of 0.50 psi gauge pressure.

J.3 Four samples from measured 4 ft2 (0.37 m2) areas are collected and weighed, with an average mass of �48 g.

J.4 Inspection of the other exposed surfaces in the powerhouse reveals that there are deposits on the top surface of ceiling beams. Two samples taken from measured 4 ft2 areas have an average mass of �00 g. The beam top flange surface area is 20 m2.

J.5 The mass of coal dust in the coal conveyors is estimated to be 20 kg (� percent of the total mass of coal). Although there is also a coal bunker in the powerhouse, it is assumed not to contribute to any building deflagration because it is vented through the building roof.

J.6 Testing of the samples resulted in a worst-case Pmax of 9�.7 psi gauge pressure, a worst-case KSt of 80 bar-m/sec, and a worst-case cw of 500 g/m 3.

J.7 Using the Pred of � psi gauge pressure = 0.0689 bar, and Pmax of 91.7 psi gauge pressure, Π= 0.011. Using a vent panel with a  Pstat of 0.50 psi gauge pressure = 0.0345 bar:

( ) ( )( )( ) 3 /44 /340

11 10 1 1.54 0.0345 80 30 20 4 1

0.011vA − = ⋅ ⋅ + ⋅ ⋅ ⋅ ⋅ −

20 26 m , for a single ventvA =

J.8 The building shape is generally elongated. The cross section is 20 m × 4 m, resulting in an effective area of 80 m2. The hydraulic diameter as determined in Chapter 6 is:

( )( )

4 4 20 46.67 m

20 20 4 4eff

he

AD

p

⋅ ⋅ ⋅= = =

+ + +

If all vent area is provided as a single vent, the position of the vent along the 30 m length of the building changes the effective L/D of the enclosure. If L/D is greater than 2, then additional vent area is needed. Assuming venting on one end wall, the L/D is:

30/ 4.5

6.67L D = =

An alternate approach, described in chapter 6, would be to distribute the vents along the 30 meter length of the building, determine the effective volume, Veff, and maximum flame length, H, for each section, then size the vents for each section independently.

Adjusting the vent area for L/D greater than 2:

( ) ( )( )0.75 21 26 1 0.6 4.5 2 exp 0.95 0.0689vA = ⋅ + ⋅ − ⋅ − ⋅ 2

1 57 mvA =

J.9 For buildings, the vent area is increased by a factor of �.7:

( ) ( )2 57 1.7vA = ⋅

22 97 mvA =

J.�0 If vent panels are too heavy, an inertia correction would be applied. Panel density is assumed to be 8 lb/ft2 = 39.� kg/m2 for a wall panel with pull-through fasteners. This panel density is compared to a limit of 40 kg/m2 and the threshold value as determined in Chapter 8:

( ) ( ) ( )( )( )1.670.2 0.3

0.5

6.67 0.0689 1 30 20 4

80TM ⋅ ⋅ ⋅ =

2110,700 kg/mTM =

Since the threshold value exceeds 40 kg/m2 and the assumed panel density is less than 40 kg/m2, no inertia correction is required:

23 97 mvA =

J.�� The building partial volume is determined:

( )( )( )( )( )

( )( )( )( )

( )( )100 20 20 1000148

0.37 500 4 0.37 500 2400 500 24000.20 0.0045 0.01670.22

rX = + +

= + +=

J.�2 The final vent area is reduced by the partial volume correction:

( ) ( ) ( )( )4

0.22 0.01197 0.22 0.333

1 0.011vA−

= ⋅ − ⋅−

24 74 mvA =

This area is less than the area of the end wall, matching the assumption that all venting could be on one end wall.

The designer should be aware that wall area obstructed by structural members is not available for venting.

Substantiation: The example in Annex J needs to include new methodology for buildings that incorporates �.7 factor.Committee Meeting Action: AcceptNumber Eligible to Vote: 27Ballot Results: Affirmative: 23 Abstain: �Ballot Not Returned: 3 Cashdollar, K., Guaricci, D., von Rosen, B.Explanation of Abstention: URAL, E.: This member disagrees with across the board application of the 70% vent area penalty for buildings until data to substantiate it becomes available. Such a penalty makes even less sense for Locally Dusty Enclosures (i.e. Partial Volume Deflagrations).

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I

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II

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III