11

API 12F 750 Bbl TankAPI 12F 750 Bbl Tank

Finite Element Analysis- Shop Welded TankFinite Element Analysis- Shop Welded TankPerformed by – Kieran ClaffeyPerformed by – Kieran Claffey

Baker Consulting GroupBaker Consulting Group

22

BackgroundBackgroundKieran ClaffeyKieran Claffey

Bsc Mechanical EngineeringBsc Mechanical Engineering Vacuum Chamber Design for Vacuum Chamber Design for

Semiconductor Industry, Semiconductor Industry, Chemical Vapor Deposition Reactor Chemical Vapor Deposition Reactor

Design, Design, Damage Mechanics, Fatigue, Impact Damage Mechanics, Fatigue, Impact

Analysis, R&DAnalysis, R&D Stress Analysis and FE for Above Ground Stress Analysis and FE for Above Ground

Storage TanksStorage Tanks

33

Objective of AnalysisObjective of Analysis

1.1. Validate the current API 12F standard Validate the current API 12F standard requirements for recommended sizes requirements for recommended sizes and pressure and vacuum limits.and pressure and vacuum limits.

2.2. Determine if the pressure rating of API Determine if the pressure rating of API 12F shop welded tanks can be 12F shop welded tanks can be increased. increased.

44

SummarySummary1.1. Modeling of 750 bbl tank sitting on medium density sand.Modeling of 750 bbl tank sitting on medium density sand.2.2. Results for vapor space pressure of 10 psi, 4.5 psi, 2 psi, 1 psi and Results for vapor space pressure of 10 psi, 4.5 psi, 2 psi, 1 psi and

0.5 with a full tank of water.0.5 with a full tank of water.3.3. Results of hard vacuum analysis.Results of hard vacuum analysis.4.4. Results with 0.5 oz/in^2 vacuum in vapor space.Results with 0.5 oz/in^2 vacuum in vapor space.5.5. Results for double fillet corner weld model.Results for double fillet corner weld model.6.6. Results for hybrid joint comprising of fillet on inside corner weld and Results for hybrid joint comprising of fillet on inside corner weld and

bevel on outside.bevel on outside.7.7. Points of interest - stress concentration at flush clean-out, bottom to Points of interest - stress concentration at flush clean-out, bottom to

shell joint, shell to roof joint, dome/nozzle to roof joint, etc.shell joint, shell to roof joint, dome/nozzle to roof joint, etc.8.8. Buckling instability under vacuum conditions.Buckling instability under vacuum conditions.9.9. Tank DeflectionTank Deflection10.10. Validation of model.Validation of model.11.11. Recommendations from FE study.Recommendations from FE study.12.12. Summarize results Summarize results

55

Selection of Stress Intensity (Tresca Criterion) Selection of Stress Intensity (Tresca Criterion) for API Stress Analysisfor API Stress Analysis

Reference from API 579 B.2.1.2bReference from API 579 B.2.1.2b The American Petroleum Institute recommends the use of Von The American Petroleum Institute recommends the use of Von

Mises and/or Tresca (Stress Intensity) stress to the tank Mises and/or Tresca (Stress Intensity) stress to the tank engineer/stress analyst when analyzing above ground storage engineer/stress analyst when analyzing above ground storage tanks. tanks.

Solid Works Simulation software allows the stress analyst to view Solid Works Simulation software allows the stress analyst to view the effective stress experienced at a point in a material, independent the effective stress experienced at a point in a material, independent of the type of loading or failure mechanism whether it be pure of the type of loading or failure mechanism whether it be pure compression, pure tension, pure shear, torsion, bending or buckling. compression, pure tension, pure shear, torsion, bending or buckling. This effective stress can be calculated within the finite element This effective stress can be calculated within the finite element software in the form of Tresca stress (stress intensity P1-P3). software in the form of Tresca stress (stress intensity P1-P3). Tresca criterion calculates principal stress in 3D space, (σ1, σ2, Tresca criterion calculates principal stress in 3D space, (σ1, σ2, σ3); thus providing an assessment of stress through out the σ3); thus providing an assessment of stress through out the thickness of the steel plate. Tresca analysis tends to highlight stress thickness of the steel plate. Tresca analysis tends to highlight stress concentrations better than equivalent Von Mises Stress. concentrations better than equivalent Von Mises Stress.

66

Model ParametersModel Parameters The tank is modeled in Solid Works FEA Simulation software as a The tank is modeled in Solid Works FEA Simulation software as a

series of events where there is pressure differential between the series of events where there is pressure differential between the inside and outside of tank. inside and outside of tank.

No fixed boundary conditions were close to the shell to bottom weld No fixed boundary conditions were close to the shell to bottom weld or close to the shell to roof weld. This was done in order to allow full or close to the shell to roof weld. This was done in order to allow full motion of these joints as would occur in reality as most 12F tanks sit motion of these joints as would occur in reality as most 12F tanks sit on soils or sand foundations that are allowed to move and bend with on soils or sand foundations that are allowed to move and bend with the foundation. the foundation.

The natural frequency and mass was used to calculate the rigidity-The natural frequency and mass was used to calculate the rigidity-stiffness of the bottom and roof plate. stiffness of the bottom and roof plate.

The modulus of sub-grade reaction for medium density sand was The modulus of sub-grade reaction for medium density sand was used to calculate the rigidity-stiffness of the sand underneath tank. used to calculate the rigidity-stiffness of the sand underneath tank.

These stiffness values were added to the model in order to simulate These stiffness values were added to the model in order to simulate how the materials deflect in real world situations. how the materials deflect in real world situations.

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Model DescriptionModel Description Tank size - 15 ft 6” inches diameter and 24 ft high. Tank size - 15 ft 6” inches diameter and 24 ft high. Three shell courses (8ft high ea), flat bottom tank. Three shell courses (8ft high ea), flat bottom tank. Gravity effects are taken into consideration in all of the Gravity effects are taken into consideration in all of the

FE models. FE models. All material in the tank was modeled as A 283 Grade C All material in the tank was modeled as A 283 Grade C

steel which is the lowest strength material in API 12F.steel which is the lowest strength material in API 12F.(Yield Strength = 30,000 psi)(Yield Strength = 30,000 psi)

Minimum thickness shell plate (courses 1 through 3) Minimum thickness shell plate (courses 1 through 3) per API 12F. per API 12F.

Minimum thickness flat bottom plate per API 12F. Minimum thickness flat bottom plate per API 12F. Sloped deck (1:12 gradient per API 12F). Sloped deck (1:12 gradient per API 12F).

88

ConstraintsConstraints Bottom is free to deflect into typical sand Bottom is free to deflect into typical sand

foundation. foundation. Bottom of 1m thick sand is given a fixed Bottom of 1m thick sand is given a fixed

constraint. constraint. Top of sand surface assigned a stiffness Top of sand surface assigned a stiffness

value.value. Tank bottom assigned a stiffness value.Tank bottom assigned a stiffness value. Roof assigned a stiffness value.Roof assigned a stiffness value.

99

Configurations StudiedConfigurations Studied Double fillet weld at shell to bottom joint.Double fillet weld at shell to bottom joint. Hybrid bevel/fillet weld at shell to bottom Hybrid bevel/fillet weld at shell to bottom

joint.joint. Shell to roof joint – fillet welded.Shell to roof joint – fillet welded. Top nozzle/dome - ANSI 20 nozzle at top Top nozzle/dome - ANSI 20 nozzle at top

of deck/dome. of deck/dome. Flush Clean Out (36” high x 24” wide) Flush Clean Out (36” high x 24” wide)

with rectangular corners as per API 12F. with rectangular corners as per API 12F. Horizontal and vertical weld joints.Horizontal and vertical weld joints.

1010

Conditions AnalyzedConditions Analyzed Extreme positive pressure; Tank full of water, with 10 psi Extreme positive pressure; Tank full of water, with 10 psi

pressure in vapor space above water level. A sudden build up pressure in vapor space above water level. A sudden build up of pressure in the vapor space is added to the liquid pressure of pressure in the vapor space is added to the liquid pressure which varies with the height of the tank. When the tank is full which varies with the height of the tank. When the tank is full of liquid there is an uneven pressure distribution, with the of liquid there is an uneven pressure distribution, with the maximum pressure at the bottom of tank and the minimum maximum pressure at the bottom of tank and the minimum pressure in the vapor space. 10 psi is considered extreme pressure in the vapor space. 10 psi is considered extreme positive pressure condition.positive pressure condition.

Tank full of water, with 4.5, 2.0, 1.0 and 0.5 psi pressure in Tank full of water, with 4.5, 2.0, 1.0 and 0.5 psi pressure in vapor space above water level.vapor space above water level.

Tank full of water with 0.5 oz/in2 vacuum in vapor space.Tank full of water with 0.5 oz/in2 vacuum in vapor space. Extreme negative pressure; Tank empty of product, under Extreme negative pressure; Tank empty of product, under

full/hard vacuum (400 inches of water), considered extreme full/hard vacuum (400 inches of water), considered extreme negative pressure condition.negative pressure condition.

1111

ConfigurationsConfigurations Fig. 2Fig. 2 Cross-section of hybrid bottom to shell weld with fillet weld on Cross-section of hybrid bottom to shell weld with fillet weld on

inside and bevel weld on outside of tank.inside and bevel weld on outside of tank.

1212

Fig. 3Fig. 3 Exaggerated view of bottom deforming into sand foundation Exaggerated view of bottom deforming into sand foundation which was modeled as 1m thick.which was modeled as 1m thick.

1313

ResultsResults Tank DeflectionTank Deflection

Fig.4 Maximum deflection at tank dome when there is 1 psi in Fig.4 Maximum deflection at tank dome when there is 1 psi in vapor space and tank is full. Result = 0.096” at top nozzle.vapor space and tank is full. Result = 0.096” at top nozzle.

1414

Fig. 5Fig. 5 Displacement of clean out with 1 psi in vapor space and tank full. Displacement of clean out with 1 psi in vapor space and tank full. Result =0.032” (approx. 1/32”).Result =0.032” (approx. 1/32”).

1515

Vacuum AnalysisVacuum Analysis

Fig. 6 Negative pressure can be created in tank due to wind from Fig. 6 Negative pressure can be created in tank due to wind from outside AND/OR from process conditions within tank.outside AND/OR from process conditions within tank.

1616

Fig. 7Fig. 7 400” water vacuum - displacement plot; Roof is more likely to 400” water vacuum - displacement plot; Roof is more likely to deflect before shell to roof joint fails.deflect before shell to roof joint fails.

The tank was modeled in The tank was modeled in the extreme condition of the extreme condition of hard vacuum – 400” of hard vacuum – 400” of water. This was water. This was performed in order to see performed in order to see where the tank is weakest where the tank is weakest under extreme and under extreme and improbable conditions in improbable conditions in order to find the weakest order to find the weakest point in the design for point in the design for vacuum conditions. vacuum conditions.

1717

Fig. 8Fig. 8 400 inches of water vacuum stress plot – Result – 155,000 psi at 400 inches of water vacuum stress plot – Result – 155,000 psi at deck to shell weld. Plastic non-linear analysis was not conducted deck to shell weld. Plastic non-linear analysis was not conducted

however the stresses are so far beyond the yield point of steel that it however the stresses are so far beyond the yield point of steel that it can be said that plastic instability is likely to occur at this joint under can be said that plastic instability is likely to occur at this joint under

hard vacuum.hard vacuum.

1818

Hard Vacuum ResultHard Vacuum Result

From a vacuum perspective, the weakest From a vacuum perspective, the weakest joint is the deck to shell weld which joint is the deck to shell weld which experiences very high stresses (>155 ksi). experiences very high stresses (>155 ksi).

The roof to shell joint will likely fail due the The roof to shell joint will likely fail due the extremely high stresses encountered at extremely high stresses encountered at that point under hard vacuum conditions.that point under hard vacuum conditions.

1919

Fig. 9Fig. 9 Buckling safety factor under hard vacuum is 0.0005 which Buckling safety factor under hard vacuum is 0.0005 which means plastic instability will occur at roof joint under worst case full means plastic instability will occur at roof joint under worst case full

vacuum (unlikely event). The shell tends to hold its shape as it has a vacuum (unlikely event). The shell tends to hold its shape as it has a more rigid shape, however an initiator (initial weakness) in shell plate more rigid shape, however an initiator (initial weakness) in shell plate

could cause collapse in shell under full vacuum conditions.could cause collapse in shell under full vacuum conditions.

2020

Buckling of tank under 0.5 oz/in2Buckling of tank under 0.5 oz/in2

What is the buckling safety factor for What is the buckling safety factor for current standard?current standard?

API 12F allows for API 12F allows for 0.5 oz/in0.5 oz/in22 vacuum for vacuum for this tank.this tank.

Result – Buckling safety factor = 7.7Result – Buckling safety factor = 7.7 No elastic instability at No elastic instability at 0.5 oz/in0.5 oz/in22

Good NewsGood News

2121

Fig. 10Fig. 10 Half oz/in Half oz/in22 of vacuum in vapor space with tank almost full; of vacuum in vapor space with tank almost full; Buckling safety factor = 7.7 Note: elastic deformation of shell is highly Buckling safety factor = 7.7 Note: elastic deformation of shell is highly

exaggerated.exaggerated.

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Results cnt’dResults cnt’d API 579 B.4.2.1 type 1 buckling analysis for a API 579 B.4.2.1 type 1 buckling analysis for a

tank fitness for service evaluation recommends tank fitness for service evaluation recommends an in-service buckling safety factor of 3; an in-service buckling safety factor of 3;

Which means 0.5 oz/inWhich means 0.5 oz/in22 is within safe is within safe parameters. parameters.

Elastic deformation is likely to occur in the shell Elastic deformation is likely to occur in the shell at this low vacuum level but elastic instability is at this low vacuum level but elastic instability is unlikely; i.e. the shape of the structure is unlikely unlikely; i.e. the shape of the structure is unlikely to be altered as a result of insufficient stiffness.to be altered as a result of insufficient stiffness.

No stiffeners are required at the top of tank if No stiffeners are required at the top of tank if the pressure differential between inside and the pressure differential between inside and outside of tank is kept below 0.5 oz/inoutside of tank is kept below 0.5 oz/in22..

2323

Compressive Stress in Empty Compressive Stress in Empty tank at atmospheric pressuretank at atmospheric pressure

Fig. 11 The compressive stress at the bottom of tank shell Result = 1800 psi when tank is empty. Compressive stress is caused by the weight of the tank on itself.

2424

Fig. 12Fig. 12 Magnified view of outside bottom to shell joint. There is an inherent Magnified view of outside bottom to shell joint. There is an inherent stress in the weld due to the weight of the tank (compressive in the first 3” stress in the weld due to the weight of the tank (compressive in the first 3”

inches on shell). The transition in shape from circular shell to flat bottom also inches on shell). The transition in shape from circular shell to flat bottom also induces a tensile bending moment at the tank bottom when the tank is filled. induces a tensile bending moment at the tank bottom when the tank is filled. Another factor which is considered is the tank’s ability to squash into the sand Another factor which is considered is the tank’s ability to squash into the sand

foundation causing bending stress in the corner weld joint.foundation causing bending stress in the corner weld joint.

2525

Weld Configuration Comparison – 10 psi in Weld Configuration Comparison – 10 psi in Vapor Space Vapor Space

(Extreme positive pressure)(Extreme positive pressure)

Fig. 13 Maximum Stress = 9,137 psi in fillet weld cross section with 10 psi in vapor space and bottom allowed to deform into sand foundation.

2626

Hybrid Fillet/Bevel WeldHybrid Fillet/Bevel Weld

Fig. 14 Maximum Stress = 4,753 psi in hybrid weld cross section with 10 psi in vapor space and bottom allowed to deform into sand foundation. There is a reduction of 96% in stress intensity by changing from fillet weld to hybrid fillet/bevel weld. The hybrid fillet/bevel weld is a much stronger joint than traditional fillet weld.

2727

Double Fillet Configuration – 10 psi in Vapor Double Fillet Configuration – 10 psi in Vapor Space (Extreme Condition)Space (Extreme Condition)

Fig. 15 Maximum stress with 10 psi in vapor space and with corner fillet weld.

2828

Results cnt’dResults cnt’d

Stress at the flush cleanout corner joint = Stress at the flush cleanout corner joint = 107,261 psi 107,261 psi

The nozzle joint stress at the dome = The nozzle joint stress at the dome = 44,652 psi. 44,652 psi.

Clearly 10 psi is too great a pressure in Clearly 10 psi is too great a pressure in the vapor space in addition to the the vapor space in addition to the hydrostatic pressure from full tank of waterhydrostatic pressure from full tank of water

2929

Reduce Pressure Reduce Pressure Double Fillet Configuration – 4.5 psi in Vapor Double Fillet Configuration – 4.5 psi in Vapor

SpaceSpace

Fig. 16 Maximum stress with 4.5 psi in vapor space with fillet weld.

3030

Results cnt’dResults cnt’d The 90° corner at flush cleanout has the largest The 90° corner at flush cleanout has the largest

stress = 77,382 psi stress = 77,382 psi The next largest stress occurs at the nozzle = The next largest stress occurs at the nozzle =

19,887 psi. 19,887 psi. 4.5 psi is too high a vapor pressure due to the 4.5 psi is too high a vapor pressure due to the

stress at the flush cleanout. stress at the flush cleanout. It is worth noting that the stresses in the shell to It is worth noting that the stresses in the shell to

bottom joint and shell to roof joint are less than bottom joint and shell to roof joint are less than those seen in the top nozzle at 4.5 psi in vapor those seen in the top nozzle at 4.5 psi in vapor space.space.

3131

Reduce Pressure againReduce Pressure againDouble Fillet Weld Configuration – 2 psi in Double Fillet Weld Configuration – 2 psi in

Vapor SpaceVapor Space

Fig. 17 Maximum stress with 2 psi in vapor space with fillet weld.

3232

Results cnt’dResults cnt’d The 90° corner at flush cleanout has the The 90° corner at flush cleanout has the

largest stress = 63,835 psi. largest stress = 63,835 psi. 2.0 psi is too high a vapor pressure due to 2.0 psi is too high a vapor pressure due to

the stress at the flush cleanout. the stress at the flush cleanout. Note: If the flush clean-out were given a Note: If the flush clean-out were given a

stress reducing radius it may be possible stress reducing radius it may be possible to increase the pressure to 2 psi in vapor to increase the pressure to 2 psi in vapor space for this size tank (with carefully space for this size tank (with carefully made design changes in welds etc.). made design changes in welds etc.).

3333

Reduce Pressure againReduce Pressure again Double Fillet Weld Configuration – 1 psi in Double Fillet Weld Configuration – 1 psi in

Vapor SpaceVapor Space

Fig. 18 Tank stress with 1 psi in vapor space and double fillet weld at tank bottom.

3434

Results cnt’dResults cnt’d Flush cleanout general stress concentration of Flush cleanout general stress concentration of

11,432 psi exists in vicinity of cleanout with 11,432 psi exists in vicinity of cleanout with concentrated maximum stress at 90° corner of concentrated maximum stress at 90° corner of 33,356 psi. 33,356 psi.

Yield strength of A283-Gr C is 30,000 psi which Yield strength of A283-Gr C is 30,000 psi which means an increase in pressure rating of API 12F means an increase in pressure rating of API 12F tanks should not occur without applying stress tanks should not occur without applying stress reducing radius to top corners of flush cleanout. reducing radius to top corners of flush cleanout.

Increasing the height of API 12F will also Increasing the height of API 12F will also increase the pressure further and should not be increase the pressure further and should not be considered without applying stress reducing considered without applying stress reducing radius to top corners of flush cleanout. radius to top corners of flush cleanout.

3535

Strain for tank with 1 psi in Strain for tank with 1 psi in Vapor SpaceVapor Space

Fig. 19 Maximum strain for 1 psi in vapor space.

3636

Strain Results cnt’dStrain Results cnt’d Maximum Strain Result occurs at flush Maximum Strain Result occurs at flush

cleanout = 1.029 x 10cleanout = 1.029 x 10-3-3.. The allowable Hookean strain for A283-Gr The allowable Hookean strain for A283-Gr

C is 1.024 x 10C is 1.024 x 10-3-3, which means that area of , which means that area of the tank is over-strained with 1 psi in vapor the tank is over-strained with 1 psi in vapor space. space.

BCG does not recommend an increase in BCG does not recommend an increase in pressure without applying stress reducing pressure without applying stress reducing radius to top corners of flush cleanout.radius to top corners of flush cleanout.

3737

Reduce Pressure againReduce Pressure again Double Fillet Weld Configuration – 0.5 psi in Double Fillet Weld Configuration – 0.5 psi in

Vapor Space (Current API 12F spec.)Vapor Space (Current API 12F spec.)

Fig. 20 Stress concentration at corner joint in flush cleanout with 0.5 psi in vapor space. Result = 28,619 psi.

3838

Results cnt’dResults cnt’d The current API 12F standard allows for 8 oz/in2 (0.5 psi). The current API 12F standard allows for 8 oz/in2 (0.5 psi). The yield strength of A283-Gr C is 30,000 psi which means the yield The yield strength of A283-Gr C is 30,000 psi which means the yield

safety factor = 1.048 (which is too close to unity). safety factor = 1.048 (which is too close to unity). The API 650 allowable hydrostatic stress for A283-Gr C is 22,500 The API 650 allowable hydrostatic stress for A283-Gr C is 22,500

psi which means the stress in this corner joint (28,619 psi) exceeds psi which means the stress in this corner joint (28,619 psi) exceeds the API allowable stress for that material by 27%. the API allowable stress for that material by 27%.

Serious consideration should be made to providing a stress Serious consideration should be made to providing a stress reducing radius at these 2 corners. reducing radius at these 2 corners.

Brittle fracture is unlikely because the plate thicknesses are less Brittle fracture is unlikely because the plate thicknesses are less than 0.5” but leaks are likely to occur at this joint, especially if than 0.5” but leaks are likely to occur at this joint, especially if corroded. corroded.

This is the weakest point in the design of API 12F shop welded This is the weakest point in the design of API 12F shop welded

tanks.tanks.

3939

FatigueFatigue It is assumed that the pressure/vacuum valve prevents It is assumed that the pressure/vacuum valve prevents

continuous cycling due to minimal pressure differentials continuous cycling due to minimal pressure differentials and that fatigue is generally not an issue for these tanks.and that fatigue is generally not an issue for these tanks.

However, it is worth noting that fatigue starts to become However, it is worth noting that fatigue starts to become an issue when the stress is greater than the fatigue an issue when the stress is greater than the fatigue strength of lowest grade steel Se = 27,550 psi. strength of lowest grade steel Se = 27,550 psi.

The stress of 28,600 psi at corner joint in flush cleanout The stress of 28,600 psi at corner joint in flush cleanout is susceptible to fatigue failure in older tanks that have is susceptible to fatigue failure in older tanks that have experienced more than 10,000 cycles (filling, emptying experienced more than 10,000 cycles (filling, emptying causing pressure change at cleanout). causing pressure change at cleanout).

Leaks initiated by fatigue are likely to occur in older Leaks initiated by fatigue are likely to occur in older tanks with this type of 90° joint. tanks with this type of 90° joint.

4040

Shell to Bottom WeldsShell to Bottom Welds The stress in the weld at the tank bottom to shell The stress in the weld at the tank bottom to shell

joint can be complicated. joint can be complicated. An analysis of the fillet weld configuration is An analysis of the fillet weld configuration is

given here for the three Cartesian directions; X, given here for the three Cartesian directions; X, Y, Z normal stress in order to explain the stress Y, Z normal stress in order to explain the stress regime at this critical joint in ASTs. regime at this critical joint in ASTs.

The final figure shows the stress intensity The final figure shows the stress intensity (Tresca results) for the combined loading (Tresca results) for the combined loading situation which takes X,Y, and Z normal stresses situation which takes X,Y, and Z normal stresses and combines them into a useful engineering and combines them into a useful engineering stress.stress.

4141

Double Fillet Weld Stress with 1 Double Fillet Weld Stress with 1 psi in Vapor Spacepsi in Vapor Space

Fig. 21 Double fillet weld X normal stress with 1 psi in vapor space - bending in toe of internal fillet due to movement downwards on tank bottom pressing into sand is the dominant stress in the X-X direction.

4242

Fig. 22Fig. 22 Double fillet weld Y normal stress –with 1 psi in vapor space. Double fillet weld Y normal stress –with 1 psi in vapor space. Bending stress in the head of internal fillet weld and the fluid Bending stress in the head of internal fillet weld and the fluid

longitudinal stress are dominant in this stress direction (Y-Y direction).longitudinal stress are dominant in this stress direction (Y-Y direction).

4343

Fig. 23Fig. 23 Double fillet weld Z normal stress when 1 psi is in vapor space Double fillet weld Z normal stress when 1 psi is in vapor space – the membrane stress from the inside of shell is dominant causing a – the membrane stress from the inside of shell is dominant causing a tensile stress in fillet weld; along with secondary bending stress from tensile stress in fillet weld; along with secondary bending stress from the bottom pushing down into the sand are stress factors in the Z-Z the bottom pushing down into the sand are stress factors in the Z-Z

direction.direction.

4444

Result for Flexible FoundationResult for Flexible FoundationFig. 24Fig. 24 Weld stress intensity at cross section with 1 psi in vapor space Weld stress intensity at cross section with 1 psi in vapor space

double fillet weld; Result = 4,443 psi max stress occurs at center of double fillet weld; Result = 4,443 psi max stress occurs at center of internal fillet (corner weld).internal fillet (corner weld).

4545

Result for Rigid FoundationResult for Rigid FoundationFig. 25Fig. 25 Higher weld stresses due to rigid bottom compared with flexible Higher weld stresses due to rigid bottom compared with flexible

sand foundation. There is an approximate increase of 15% in the sand foundation. There is an approximate increase of 15% in the maximum stress due to the increased rigidity (4,443 psi max stress on maximum stress due to the increased rigidity (4,443 psi max stress on

flexible sand vs. 5,133psi on completely rigid foundation.flexible sand vs. 5,133psi on completely rigid foundation.

4646

Roof Stress IntensityRoof Stress Intensity

Fig. 26 Bending stress in roof plate at dome-nozzle junction with 1 psi in the vapor space.

4747

Results cnt’dResults cnt’d Stress at Dome-Nozzle at very top of Tank = Stress at Dome-Nozzle at very top of Tank =

4,550 psi4,550 psi There is a high bending stress approximately 2” There is a high bending stress approximately 2”

away from the welded joint caused by the steel away from the welded joint caused by the steel at that point attempting to move upwards due to at that point attempting to move upwards due to internal tank pressure, yet being constrained by internal tank pressure, yet being constrained by the rigidity and weight of the nozzle. the rigidity and weight of the nozzle.

Yield safety factor of 6.52 (FYield safety factor of 6.52 (Fyy = 30,000 psi) = 30,000 psi)

4848

FE Model Reality CheckFE Model Reality Check

Fig. 28 Stress along height of tank when 1 psi exists in the vapor space.

4949

Model ValidationModel Validation Model reality check. Calculated value (hand Model reality check. Calculated value (hand

calculation) of hoop stress = 5,658 psi calculation) of hoop stress = 5,658 psi Correlates well with FE result of 5,620 psi at Correlates well with FE result of 5,620 psi at

tank bottom. tank bottom. Stress concentrations exist at shell plate joints, Stress concentrations exist at shell plate joints,

nozzle and flush cleanout joints.nozzle and flush cleanout joints. Stress concentrations correlate with R.E. Stress concentrations correlate with R.E.

Peterson’s book “Stress Concentration Factors”.Peterson’s book “Stress Concentration Factors”. Hand calculations predict a stress concentration Hand calculations predict a stress concentration

of 28,158 psi at cleanout; FE predicts 28,619 of 28,158 psi at cleanout; FE predicts 28,619 psi. psi.

5050

5151

ConclusionsConclusions 1.1. There is an acceptable amount of tank deflection with current pressure limits.There is an acceptable amount of tank deflection with current pressure limits.2.2. From a vacuum perspective, the weakest area is the deck to shell weld.From a vacuum perspective, the weakest area is the deck to shell weld.3.3. From a positive pressure perspective, the weakest area is 2” away from the corner in From a positive pressure perspective, the weakest area is 2” away from the corner in

the flush cleanout.the flush cleanout.4.4. Elastic instability is unlikely to occur with current allowable vacuum of 0.5 oz/inElastic instability is unlikely to occur with current allowable vacuum of 0.5 oz/in22 i.e. i.e.

the shape of the structure is unlikely to buckle as a result of insufficient stiffness.the shape of the structure is unlikely to buckle as a result of insufficient stiffness.5.5. The hybrid fillet/bevel weld is a stronger joint than traditional fillet weld.The hybrid fillet/bevel weld is a stronger joint than traditional fillet weld.6.6. The API 650 allowable hydrostatic stress for A283-Gr C is 22,500 psi which means The API 650 allowable hydrostatic stress for A283-Gr C is 22,500 psi which means

the stress in the clean-out corner joint (28,619 psi) exceeds the API allowable stress the stress in the clean-out corner joint (28,619 psi) exceeds the API allowable stress for that material by 27%. for that material by 27%.

7.7. The allowable Hookean strain for A283-Gr C is 1.024 x 10The allowable Hookean strain for A283-Gr C is 1.024 x 10 -3-3, which means the clean , which means the clean out area of the tank is over-strained with 1 psi in vapor space.out area of the tank is over-strained with 1 psi in vapor space.

8.8. Serious consideration should be made to providing a stress reducing radius at these Serious consideration should be made to providing a stress reducing radius at these 2 corners.2 corners.

9.9. Leaks initiated by fatigue, could possibly occur in older tanks with 90° corner joint at Leaks initiated by fatigue, could possibly occur in older tanks with 90° corner joint at clean out (>10,000 cycles).clean out (>10,000 cycles).

10.10. Brittle fracture is unlikely to occur at the flush cleanout as material is thinner than 0.5”. Brittle fracture is unlikely to occur at the flush cleanout as material is thinner than 0.5”. Failure mechanism is more likely ductile cleavage mechanism and not brittle fracture. Failure mechanism is more likely ductile cleavage mechanism and not brittle fracture.

5252

Conclusions cnt’dConclusions cnt’d11.11. An increase in pressure rating of API 12F tanks should not occur without applying stress An increase in pressure rating of API 12F tanks should not occur without applying stress

reducing radius to top corners of flush cleanout.reducing radius to top corners of flush cleanout.12.12. Increasing the height of API 12F tanks will increase the pressure further and should not be Increasing the height of API 12F tanks will increase the pressure further and should not be

considered without applying stress reducing radius to top corners of flush cleanout.considered without applying stress reducing radius to top corners of flush cleanout.13.13. The allowable pressure for this tank cannot be increased from 0.5 psi to 2 psi because there The allowable pressure for this tank cannot be increased from 0.5 psi to 2 psi because there

is a large stress concentration at clean out which is greater than the tensile strength of the is a large stress concentration at clean out which is greater than the tensile strength of the shell plate. shell plate.

14.14. If the allowable pressure were to be increased from 0.5 psi (8 oz/inIf the allowable pressure were to be increased from 0.5 psi (8 oz/in22), the 90° corner would ), the 90° corner would require rounding to reduce the stress concentration.require rounding to reduce the stress concentration.

15.15. Double fillet welds are sufficiently strong when there is a full tank of water and 1 psi in vapor Double fillet welds are sufficiently strong when there is a full tank of water and 1 psi in vapor space.space.

16.16. Bending stress at top nozzle was higher than expected.Bending stress at top nozzle was higher than expected.17.17. If the flush clean-out were given a stress reducing radius it may be possible to increase the If the flush clean-out were given a stress reducing radius it may be possible to increase the

pressure to 2 psi in vapor space for this size tank (with carefully made design changes in pressure to 2 psi in vapor space for this size tank (with carefully made design changes in welds etc.).welds etc.).

18.18. An increase in allowable vacuum above 0.5 oz/inAn increase in allowable vacuum above 0.5 oz/in22 is possible. That limit value requires more is possible. That limit value requires more analysis to keep buckling safety factor above 3.analysis to keep buckling safety factor above 3.

19.19. Current pressure and vacuum allowable limits should not be changed without accompanying Current pressure and vacuum allowable limits should not be changed without accompanying design changes.design changes.