------------c-H-A-p-T-E-R-llPipe Racks

A pipe rack is the main artery of a process unit. ItconnectS all equipment with lines that cannot runthrough adjacent areas, Because it is located in themiddle of most plants, the pipe rack must be erectedfirst, before it becomes obstructed by rows of equip­n\':'nt, The corresponding piping drawings are also re­quired early for the same reason, Pipe racks ~an"y

process and utility piping and may also include instru­ment and electrical cable trays as well as equipmentmounted over ,,: of these, This vital area requiresconsiderable planning and coordination with othertechnology groups regarding construction becausecosts are so high,

This chapter explains what is required to finalizethe pipe rack width, number of levels and elevations,and bent spacing and addresses pipe flexibility andaccess and maintenance concerns for each item lo­cated within the pipe rack area,

The primary data required for the detailed develop­ment of a pipe rack includes the follOWing

• Plot plan,• Piping and instrumentation diagrams,

• Plant layout specification,

• Client specification,

• Construction materials,

• Fireproofing requirements,

ESTABLISHING WIDTH, BENT SPACING,AND ELEVATIONS

The first step in the development of any pipe rack isthe generation of a line-routing diagram, shown inExhibit 11-1, Aline-routing diagram is a schematic rep­resentation of all process piping systems drawn on acopy of the plot plan, Although it disregards exactlocations, elevations, or interferences, it locates themost congested piping bent in the pipe rack. Csually,the information available on early piping and instru-

mentation diagram issues covers only commodity, linenumber, and preliminary sizes,

Process flow diagrams prOVide insight to operatingtemperatures and identify the need for insulation,Once the routing diagram is complete, the develop­ment of rack Width, bent spaCing, and numbers oflevels and elevations may proceed,

Bent Spacing

A pipe bent consists of a vertical column or columnsand a horizontal structural member or members thatcarry piping systems, usually above headroom, Theline sizes that are installed in the rack establish thebent spaCing. Exhibit 11-2 is a typical pipe span chartand shows how far a particular line can span on thebasis of size, schedule, liqUid or vapor, and insulatedor bare pipe, Pipe racks are tailored to a specific plant;pipe sizes in chemical plants are smaller than thosefound in refinery units If a plant requires a 16-ft(4,900-mm) spacing, the variation in Exhibit 11-3 al­lows for a 32-ft (9,700-mm) spaCing by adding inter­mediate bents supported from spandrels Spandrelsare horizontal structural members located along thelongitudinal centerline that are used for structural sta­bility, pipe suppOrt, or intermediate pipe bents, Dou­bling the column spacing as shown in 2A of Exhibit11-3 may be reqUired to cross roadways or avoid un­derground obstructions, The civil and structural engi­neers should be consulted to review the economics ofthe approach

Setting the width of the pipe rack may then pro­ceed, With the routing diagram, a dimensioned crosssection is developed at the bent that will carry themost piping, which is bent No 12 in the exampleshown in Exhibit 11-4, Usually, pipe racks carry pro­cess lines on the lower level or levels, and the utilitylines on the top level Instrument and electrical traysare integrated on the utility level if space permits oron a separate level above all pipe levels, Any pipe rackdesign should prOVide for 20% future growth When

261

262

EXHIBIT 11-1 Line-Routing Diagram,.....------ - --.....,.--,,~r-----­ -------

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Process Plant Layout and Piping Design

263

EXHIBIT 11-2 Basic Pipe Span Table

Vapor Line Liquid LineInsulation (Span) Insulation (Span) Bare Pipe (Span)

Size Schedule Corrosion 351 0 to 601 0 to 351 0 to 601 0 to Empty Water Filled Size(inches) (inches) Allowance To 3500 F 600 0 F 750' F To 350' F 6000 F 750' F (to 350' F) (to 3500 F) (inches)

3/4 40 005 12 11 8 12 10 14 13 3/41 40 005 14 13 10 14 12 9 16 14 1

1 1/2 40 005 18 14 14 1~ 15 12 19 Ie 1 1/22 40 010 18 16 II I ~ I'; 11 21 18 2

21/2 40 010 23 19 16 19 18 15 2'; 2] 2 li2

3 40 010 24 21 18 21 19 16 26 22 34 40 010 r 25 22 24 23 19 29 25 4-,6 40 010 33 31 28 29 r 25 34 29 68 40 0.10 39 36 33 33 32 29 40 33 8

10 40 010 44 42 39 3- 35 34 46 38 10

12 3/8 w 010 4~ 45 42 39 38 36 49 40 1214 318 w 0.10 49 47 44 40 39 37 52 41 1416 318 w 010 53 50 4"'7 42 4 I 39 55 43 1618 318 w 010 56 54 50 44 43 40 59 45 1820 318 w 0.10 59 57 53 46 45 41 62 46 20

24 3/8 w 010 65 62 58 48 47 43 68 49 243/4 80 010 12 10 11 10 6 14 13 3/4

1 80 0.10 14 12 10 13 12 9 16 14 II 1/2 80 0.10 17 16 lq 16 15 13 10 17 I 112

2 80 010 19 17 14 18 16 13 2I 19

2 1/2 80 010 22 20 18 20 19 1- 23 21 2 112

3 80 010 24 22 20 22 21 19 2'; 23 380 010 2-:" 26 23 25 24 22 29 26

6 80 010 34 32 30 31 29 28 35 31 68 112 "'. 010 39 r 35 35 33 32 40 36 8

10 1/2 w 010 44 42 39 38 37 35 45 39 1012 1/2 w 010 47 45 43 41 40 38 49 42 1214 112 w 0.10 50 48 4" 42 41 40 ';1 44 1416 112 w 010 53 "1 49 44 43 42 55 46 1618 1/2 w 010 57 55 52 47 46 44 59 48 18

20 112 w 010 59 "7 "" 49 47 46 62 49 2024 1/2 w 0.10 65 63 60 52 50 49 68 52 24

1 XXS 025 14 13 10 13 12 10 15 14 11 1/2 XXS 025 17 16 14 16 15 14 18 17 1 112

2 XXS 0.25 20 18 15 18 ]7 15 21 19 2

21/2 160 025 21 19 17 19 18 16 23 21 21/23 160 025 24 22 20 22 21 19 25 23 34 120 025 27 26 23 25 24 22 29 26 46 80 0.25 33 31 28 28 27 26 35 29 68 112 w 025 38 34 33 32 30 40 34 8 8

10 1/2 w 025 43 41 38 36 35 33 45 37 1012 1/2 w 025 47 45 41 38 3- 35 49 39 1214 1/2 w 025 49 47 44 39 38 37 52 40 1416 1/2 w 025 52 50 48 41 40 39 55 42 1618 1/2 w 025 56 53 50 43 42 40 59 47 18

20 1/2 w 025 59 56 53 45 44 42 62 45 2024 1/2 w 025 64 60 58 4- 46 45 68 48 24

PIpe Racks

264EXHIBIT 11-3Pipe RacJc ColumnSpacing

A

Process Plant Layout and Piping Design

EXHmIT 11-4Pipe Rack Cross Section(at Column 12)

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265

EXHIBIT 11-5Pipe Rack Composite

the electrical conduit trays are located on the tOp leveland a row of motOr-driven pumps is located beneaththe rack at grade, a 6-in (lSO-mm) slot should be pro­vided to allow the conduit to run in the most directmanner and avoid running to the outside of the rackand back to the pumps. This feature is illustrated inExhibit l1-S Once future rack growth and conduithave been allowed for in the plan, the spaCing may beset with the line spacing chart in Exhibit 11-6.

When flanges or flanged valves are reqUired on rwoadjacent lines, the flanges are staggered as depicted inExhibit 11-7. Thermal expanSion or contraction mustbe accommodated, as shown in Exhibit 11-8. When allthe distances have been established berween all lineson each level, including allowances for future growthand conduit, the only remaining dimension to be set isthe distance from the first line in the rack to the verti­cal column centerline. Column sizes are furnished bythe civil/structural engineers.

The last step is to add up all the dimensions andround off to the next whole number-for example, 20

ft (6,100 mm) rather than 19 ft 3 in (5,850 mm). Toillustrate, if the pipe requires a much greater area inthe rack, the designer would work with the structuralengineers to determine whether the pipe rack shouldbe rwo 30-ft (9,150-mm) wide levels or three 20-ft(6,l00-mm) wide levels This decision affects the costof the structure and pipe and mu:;t be made carefully.

After the bent spacing, rack Width, and number oflevels are established, the elevation of the levels mustbe set. As discussed in Chapter 2, the plant layoutdesigner must know the minimum clearances to setthe elevations. Plant roads, type of mobile equipment,and equipment located beneath the pipe rack can in­fluence the pipe rack elevation. Usually, space is al­lowed below the pipe rack for equipment, with a mini­mum clearance of 10 ft (3,050 mm)

The next factor to consider is the dimension be­rween the bottom of a line in the rack and the bottomof a branch as it leaves the rack. For example, if areview of the largest lines in the entire pipe rackindicates that there are rwo or three large-diameter

ptpeRacks

266EXHmIT 11-6 line Spacing Chan

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EXHmIT 11-7line Spacing

Process Plant Layout and Piping Design

267

EXHIBIT 11-8Planning for Line Growth

eND VIEW

EXHIBIT 11-9Large-Diameter Lines

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~De VIEW

Pipe Racks

268

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Process Pla----:-:---ntLa"o ~-" ut and p'p .I mg Design

269

EXHmIT 11·11Pipe Rack Meter Runs

lines (e.g., 18,20, or 24 in) and the remaining lines are12 in, the exit level above and below the rack can be 3ft (915 mm). The dimension from the end of a 90°bun-wdd elbow to the centerline of a 12-in line is 18in (457 mm). Exhibit 11-9 illustrates how to handle thelarge-diameter lines by using a 45° elbow or trimmingan elbow to a more shallow angle. If the instrumentand electrical conduit are installed on a separate level,the estimated dimensions of each tray must be ob­tained from the instrument and electrical engineers toensure that adequate space is prOVided.

The design of the pipe rack is now complete withthe exception of installing equipment over the rack.

SElTING LINE, VALVE, ANDINSTRUMENT LOCATIONS

Many factors must be conSidered when locating eachline, valve, and instrument in a pipe rack. Exhibit 11-10

is an example of a typical layout.A common arrangement of a standard process unit

pipe rack is one in which the process lines are on thelower level or levels. The utility piping is on the toplevel, which carries piping Electrical and instrumentcable trays are located on the top level with the utilitypiping or on a separate level above the utility piping,depending on the extent of cable tray area required.The plant layout designer must consult the electricaland instrument engineers early in the pipe rack layoutto establish these requirements.

When locating lines in the rack, the plant layoutdesigner should run the largest lines near the outsidewhere possible to reduce the overall load on the sup­porting beams Meter runs should be installed directlynext to the columns so that access is available by porta­ble ladder or mobile platform, as shown in Exhibit11-11. Meter runs are located in the pipe rack onlywhen absolutely necessary.

Many relief headers must be located above the top

270

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EXHIBIT 11-12Relief Header Location

EXHIBIT 11-13 Alternative Pipe Rack Expansion

level of the rack to allow the line to drain to the blow­dOV'lTl drum. The designer should avoid locating theline over the centerline of the column for support sothat the columns can be extended for future rack ex­pansion. Exhibit 11-12 shows a suggested location forthe relief header that does not impede future expan­sion.

Shut-off valves at utility headers are located insidethe rack area in the horizontal position, directly abovethe header if room permits. Operating valves must beaccessible from platforms or by chain operators. Thelocation of the valve must also permit the chain to fallfree of obstructions that would hamper operation. Anadditional 20% of space must be allowed for futurepipe rack growth. An alternative approach to such ex­pansion is shown in Exhibit 11-13

Because space in the pipe rack is limited once thedesign is set, it is important to route lines to avoid

Process Phlnt Layout and Piping Design

EXHIBIT 11-14 Hose Station at a Pipe Rack Column

f---+-h--+-----:l

dead spaces. The designer can minimize runs in therack by consulting with the adjacent-area designers toidentify which lines can run within the areas. Becausethe development of a pipe rack often includes swap­ping lines, it is advised that the designer draw the lineslightly until satisfied with the design. Once the layoutis optimized, the line definition carl be finalized.

A pipe rack composite is shown in Exhibit 11-5.This view highlights features mentioned previouslyand clarifies additional considerations. For example,the width of the access way is determined by the spaceneeded to maintain the equipment located at gradebelow the pipe rack. For process or COSt reasons, shelland tube exchangers may be located under a pipe rackin certain process units. Allowances must be made to

maintain such units (e,g" providing a hitch pOint overthe channel end to facilitate its removal), Once again,it is extremely important to know exact Iv what kind ofmobile handling equipment the plant will use,

The vertical drop of lines outside the rack, althoughusually 2 IT (610 mm), is once again set by the averageline size in the unit. If the average line size is 2 in, a 12­in (300-mm) drop may be sufficient. This view alsoshows how the electrical conduit can be run directlyto the pump starter switch,

Exhibit 11-14 shows a typical arrangement for ahose station, Battery limit valving for a single-levelpipe rack is shown in Exhibit 11-15, The valves arestaggered on either side of the catwalk, and hand­wheel extension stems are furnished when necessaryto facilitate operation, Exhibit 11-16 also displays asingle-level rack. Here, however, an elevation changeis required between the process unit and the off-sitepipe rack. This design has the block valves installed inthe vertical portion of the line, which allows for rela­tive ease of operation,

Exhibit 11-17 illustrates a two-level process unitpipe rack; the elevation change to the off-site area iseither above or below the process unit pipe rack,

PIPE FlEXIBILIlY AND SUPPORTS

Although conducting the final stress analysis is the re­sponsibility of the mechanical or stress engineer, thepipe rack designer makes preliminary calculations us­ing relevant books and nomograms to ensure that thedesign will not require major rework during the for­mal Stress check. Exhibit 11-18 highlights the stepsinvolved in making a preliminary fleXibility check,which are discussed in the following seaions,

Establishing potential flexibility problems The linesthat would most likely require expansion loopsshould be defined, Steam headers in the top level ofthe pipe rack are such examples,

271

EXHIBIT 11·15 Battery Limit Valving: Single-Level Rack(ProceSS/Off-Site Common Elevation)

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Pipe Racks

272

EXHIBIT 11-16Banery limit Valving:Single-Level Rack(Process/Off-SiteElevation Change)

Process Plant Layout and Piping Design

273

EXHIBIT 11-17Battery limit Valving:Two-Level Rack( Process/Off-SiteElevation Change)

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Pipe Racks

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274

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Process Plant Layout and Piptng Design

EXHmIT 11-18Flexibility Check Steps

275

EXHIBIT 11·19Pipe Rack Anchor Bent

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Determining line growth The gWl.vth of such utilityheaders should be determined by multiplying the co­efficient of expansion by the length of the line. Thecoefficient of expansion is based on a particular mate­rial operating at a specific temperature. Upset temper­atures take precedence over operating temperatures.

Determining whether one anchor point will sufficeAssuming that an anchor is located in the center of theheader, the designer should calculate the grmvth of.arious branches to determine whether they haveenough flexibility to absorb the header gro\V-th. If not,

two anchor points approximately one-quarter of thedistance from each end of the header should be tried.Using the nomograms, the designer can calculate theamount of expansion leg required to satisfy all flexibil­ity requirements.

Arranging lines in proper sequence The line thatrequires the largest leg must be located on the outsideof the loop. Placing the headers along one side of thepipe rack allows the expansion loops to sit with aslight overhang along the adjacent side of the piperack Exhibit 11·19 shows such an arrangement. As a

Pipe Racks

276EXHIBIT 11-20Steam line Dr~p Legs

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EXHmIT 11-21Proper Line Support

EXHmIT 11-22 Intermediate Pipe Support

result of imposing stop loads on a particular bent,bracing may be required to grade, prohibiting the lo­cation of any equipment in that particular bay. Ameans of removing condensate bUild-up must be pro­vided on either side of the expansion loop. The mostcommon way to accomplish this is to add drip legs andtraps, as shown in Exhibit 11-20.

Header growth causes another problem that is of­ten not as obvious. The line spacing chart may havebeen used to set distances between lines, or lines mayhave been set close to a column Exhibit 11-8 revealsthat the movement of a line must not be restricted by

Process Plant Layout and Piping Design

an adjacent line or column, because it will aa as a lint'stop and could cause a problem. Enough space mustbe proVided for the line to move its maximumdistance and still have an ample clearance of 3 in(75 mm).

Exhibit 11-21 shows the correct way to support aline that has exceeded its allowable span. A commonmistake is to extend the 10-in process line over therack bent and cap it, when the line should have beenrun as if a suPPOrt problem did not exist. A smallerpiece of pipe or dummy leg could then be welded tothe elbow for support (a hole should not be cut in theprocess line).

Exhibit 11-22 shows how larger lines in a pipe rackare used to suppOrt a group of smaller lines that maynOt be adequately supported because of the bent spac­ing. The uninsulated line is U-bolted to the supportingsteel; the insulated line has its shoe welded to thesteel. The smaller lines then rest on the steel. When aninsulated line is used for support, the growth of th(line at the proposed suPPOrt pOint must be checked.Its growth could become restriaed by this type of

277

EXHIBIT 11-23Pipe Rack SpandrelLocation

EXHIBIT 11-24Pipe Rack SpandrelVariations

suPPOrt, and it may be better to use anOther line forthis application.

STRUCTURAL CONSIDERATIONS

Most lines require support when leaving or entering apipe rack Structural members called spandrels are themost common means of satisfying this requirementAfter all the lines have been run in the pipe rack, theplant layout designer must begin to locate the span­drels necessary to support all of these lines. Exhibit11-23 shows how the requirement can be handled. Ifthe structural engineers require additional spandrelsfor stability of the pipe rack, they should bring thisrequirement to the attention of the plant layout de­signer. Exhibit 11-24 shows some variations of span·drel design

The plant layout designer should be aware that pre­cast concrete pipe racks require structural membersthat are much larger than most designs Exhibit 11-25illustrates a precast column with an embedded steel

member for the spandrel support. The spandrel alsohas an embedded steel member that is bolted to thecolumn and eventually grouted in. An installation se­quence for a precast pipe rack is also shown in Exhibit11-25

Such equipment as drums and deaerators is oftenlocated above pipe rack columns. To avoid wastingvaluable rack space with the large support columns,the inside face of the pipe rack column must line upwith the inside face of the equipment suPPOrt column,as shown in Exhibit 11-26. This approach also allowsthe feed-water outlet piping to run vertically down to

the pump before the expansion loop is added.Fireproofing of pipe rack columns is shown in Ex­

hibit 11-27 If hydrocarbons are prevalent, it is com­mon to fireproof the columns to a level just below thelower rack support beam. If air coolers or otherequipment is located above a pipe rack, the fireproof­ing is extended to the equipment support beam. Thisissue must be reviewed with the client; allowancesmay need to be made for as much as 4 in (100 mm) offireproofing.

PtpeRacks

278

EXHmIT 11·25Precast Concrete PipeRack Considerations

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a. Precast Concrete Pipe Rack

b. Installation Sequence

Process Plant Layout and Piping Design

EXHIBIT 11-26Equipment SupportColumn Location

EXHIBIT 11-27FireproofingRequirements

279

ptpeRacks

280

EXHIBIT 11-2890° Pipe Rack Turns

I

EXHIBIT 11·29Rack Intersection

a. Rack Intersection Layout

OTHER CONSIDERATIONS

Occasionally, a situation arises in which a flat-turnpipe rack may be employed. This often happens near adead-end area where the potential for problems isminimal. As shown in Exhibit 11-28, the line sequence

Process Plant Layout and Piping Design

on the left side of the rack must remain constant aslong as flat turns are used. A different elevation mustbe used at a 90° turn in the rack if the sequence mustchange, as shown on the right side of the diagram.This approach must be well thought out before it isused.

281

EXHIBIT 11-29lUck Intersection (Com)

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EXHIBIT 11-30The secondary pipe rack intersection is shown in

Exhibit 11-29. There is a right and a wrong way to setthis location. Although it may seem more uniform to

set the secondary rack directly south of the mainnorth/south rack during the plot plan developmentstage, Exhibit 11-29 also clearly shows why this shouldbe avoided The lines heading north off the main east!west rack restrict the lines from the south from en­tering this common area. Therefore, the secondaryrack should be shifted east one bay to eliminate theproblem,

Alternative pipe rack eX'Pansion of the individuallevels can be accomplished by adding a cantileverbeam as required on the outside of the column. Theonly problem with this approach is that, if not plannedfor, the venical risers commonly found outside thepipe rack use a considerable amount of the space ofthe extension, as shown in Exhibit 11-13-

Pipe rack additions are shown in Exhibit 11-30.Area A shows a standard two-level pipe rack, asplanned. Because it is always possible for the pipe rackto be expanded in the future, the area over thecolumns must be kept free of piping and conduit. Thefuture expanSion may include another new level (B),an air cooler (C), or a series of shell and tube exchang­erS (D). The preferred location of a relief headerabove a pipe rack is illustrated in Exhibit 11-12.

Lighting panels and welding receptacles also must

PtpeRaas

282

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EXHIBIT 11-31lighting Panels andWelding Receptacles

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Process Plant Layout and Piping Design

283

EXHIBIT 11-32Operator Access

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be planned for during the early stages of a pipe racklayout. They are mounted direaly on pipe rackcolumns; their location must be recorded on the ap­plicable documents. Electrical engineers designatewhere the regular and emergency panels are located;construction personnel selea the preferred locationfor the welding receptacles (see Exhibit 11-31). Whenlocating piping manifolds, control stations, instru­ments, and pull boxes along the pipe rack columns,

the designer should avoid blocking access from underthe pipe rack to adjacent equipment areas by leavingclear space, as illustrated in Exhibit 11-32.

Overall pipe rack design must meet the currentneeds of a client as well as any expansion plans with·out making major modifications to existing facilities.Available space in the pipe rack must be consideredvaluable and used to the utmost advantage of presentand future needs.

PipeRadls