Process Plant Piping System Design Document

7/27/2019 Process Plant Piping System Design Document

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OVERVIEW OF PROCESS PLANT PIPING SYSTEM DESIGN

Carmagen Engineering, Inc.



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I. INTRODUCTION

This course provides an overview of process plant piping system design. Itdiscusses requirements contained in ASME B31.3, Process Piping, plusadditional requirements and guidelines based on common industry practice. Theinformation contained in this course is readily applicable to on-the-jobapplications, and prepares participants to take more extensive courses if appropriate.

II. GENERAL

A. What is a piping system

A piping system conveys fluid from one location to another. Withina process plant, the locations are typically one or more equipmentitems (e.g., pumps, pressure vessels, heat exchangers, processheaters, etc.), or individual process plants that are within theboundary of a process facility.

A piping system consists of:

• Pipe sections

• Fittings (e.g., elbows, reducers, branch connections, etc.)

• Flanges, gaskets, and bolting

• Valves

• Pipe supports and restraints

Each individual component plus the overall system must bedesigned for the specified design conditions.

B. Scope of ASME B31.3

ASME B31.3 specifies the design, materials, fabrication, erection,inspection, and testing requirements for process plant pipingsystems. Process plants include petroleum refineries; chemical,pharmaceutical, textile, paper, semiconductor, and cryogenicplants; and related process plants and terminals.



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ASME B31.3 applies to piping and piping components that are usedfor all fluid services, not just hydrocarbon services. These includethe following:

• Raw, intermediate, and finished chemicals.

• Petroleum products.

• Gas, steam, air, and water.

• Fluidized solids.

• Refrigerants.

• Cryogenic fluids.

The scope also includes piping that interconnects pieces or stageswithin a packaged-equipment assembly.

The following are excluded from the scope of ASME B31.3:

• Piping systems for internal gauge pressures at or above zerobut less than 15 psi, provided that the fluid is nonflammable,nontoxic, and not damaging to human tissue, and its designtemperature is from -20°F through 366°F.

• Power boilers that are designed in accordance with the ASMEBoiler and Pressure Vessel Code Section I and external boilerpiping that must conform to ASME B31.1.

• Tubes, tube headers, crossovers, and manifolds that arelocated inside a fired heater enclosure.

• Pressure vessels, heat exchangers, pumps, compressors, andother fluid-handling or processing equipment. This includesboth internal piping and connections for external piping.



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III. MATERIAL SELECTION CONSIDERATIONS

Piping system material selection considerations are discussed below.

A. Strength

A material's strength is defined by its yield, tensile, creep, andfatigue strengths. Alloy content, material grain size, and the steelproduction process are factors that affect material strength.

1.0 Yield and Tensile Strength

A stress-strain diagram that is produced from a standard

tensile test (Figure 3.1) illustrates the yield and tensilestrengths. As the stress in a material increases, itsdeformation also increases. The yield strength is the stressthat is required to produce permanent deformation in thematerial (Point A in Figure 3.1).

If the stress is further increased, the permanent deformationcontinues to increase until the material fails. The maximumstress that the material attains is the tensile strength (Point Bin Figure 3.1). If a large amount of strain occurs in goingfrom Point A to Point C, the rupture point, the material is said

to be ductile. Steel is an example of a ductile material. If thestrain in going from Point A to Point C is small, the materialis brittle. Gray cast iron is an example of a brittle material.

SA

B

C

E

Typical Stress-Strain Diagram for Steel

Figure 3.1



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2.0 Creep Strength

Below about 750°F for a given stress, the strain in mostmaterials remains constant with time. Above thistemperature, even with constant stress, the strain in the

material will increase with time. This behavior is known ascreep. The creep strength, like the yield and tensilestrengths, varies with temperature. For a particulartemperature, the creep strength of a material is the minimumstress that will rupture the material during a specified periodof time.

The temperature at which creep strength begins to be afactor is a function of material chemistry. For alloy materials(i.e., not carbon steel) creep strength becomes aconsideration at temperatures higher than 750°F.

3.0 Fatigue Strength

The term “fatigue” refers to the situation where a specimenbreaks under a load that it has previously withstood for alength of time, or breaks during a load cycle that it haspreviously withstood several times. The first type of fatigueis called “static,” and the second type is called “cyclic.”Examples of static fatigue are: creep fracture and stresscorrosion cracking. Static fatigue will not be discussedfurther in this course.

One analogy to cyclic fatigue is the bending of a paper clip. The initial bending beyond a certain point causes the paperclip to yield (i.e., permanently deform) but not break. Theclip could be bent back and forth several more times and stillnot break. However after a sufficient number of bending(i.e., load) cycles, the paper clip will break under thisrepetitive loading. Purely elastic deformation (i.e., withoutyielding) cannot cause a cyclic fatigue failure.

The fatigue strength of a material under cyclic loading canthen be defined as the ability to withstand repetitive loadingwithout failure. The number of cycles to failure of a materialdecreases as the stress resulting from the applied loadincreases.



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B. Corrosion Resistance

Corrosion of materials involves deterioration of the metal bychemical or electrochemical attack. Corrosion resistance is usuallythe single most important factor that influences pipe material

selection. Table 3.1 summarizes the typical types of piping systemcorrosion.

General or UniformCorrosion

Characterized by uniform metal loss over entire surface of material.May be combined with erosion if material is exposed to high-velocityfluids, or moving fluids that contain abrasive materials.

PittingCorrosion

Form of localized metal loss randomly located on material surface.Occurs most often in stagnant areas or areas of low-flow velocity.

Galvanic Corrosion Occurs when two dissimilar metals contact each other in corrosiveelectrolytic environment. The anodic metal develops deep pits orgrooves as a current flows from it to the cathodic metal.

Crevice Corrosion Localized corrosion similar to pitting. Occurs at places such asgaskets, lap joints, and bolts, where a crevice can exist.

Concentration CellCorrosion

Occurs when different concentration of either corrosive fluid ordissolved oxygen contacts areas of same metal. Usually associatedwith stagnant fluid.

Graphitic Corrosion Occurs in cast iron exposed to salt water or weak acids. Reducesiron in the cast iron and leaves the graphite in place. Result isextremely soft material with no metal loss.

Typical Types of Piping System Corrosion

Table 3.1

For process plant piping systems in corrosive service, corrosionprotection is usually achieved by using alloys that resist corrosion. The most common alloys used for this purpose are chromium andnickel. Low-alloy steels with a chromium content of 1¼% to 9%and stainless steels are used in corrosive environments.

C. Material Fracture Toughness

One way to characterize the fracture behavior of a material is theamount of energy necessary to initiate and propagate a crack at agiven temperature. This is the material's fracture toughness, which



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decreases as the temperature decreases. Tough materials requirea relatively large amount of energy to initiate and propagate acrack. The impact energy required to fracture a material sample ata given temperature can be measured by standard Charpy V-notchtests.

Various factors other than temperature affect the fracturetoughness of a material. These include the following:

• Chemical composition or alloying elements.

• Heat treatment.

• Grain size.

The major chemical elements that affect a material's fracturetoughness are carbon, manganese, nickel, oxygen, sulfur, andmolybdenum. High carbon content, or excessive amounts of oxygen, sulfur, or molybdenum, hurts fracture toughness. Theaddition of manganese or nickel improves fracture toughness.

D. Fabricability

A material must be available in the shapes or forms that arerequired, and it typically must be weldable. In piping systems,some common shapes and forms include the following:

• Seamless pipe.

• Plate that is used for welded pipe.

• Wrought or forged elbows, tees, reducers, and crosses.

• Forged flanges, couplings, and valves.

• Cast valves.

E. Availability and Cost

The last factors that affect piping material selection are availabilityand cost. Where there is more than one technically acceptablematerial, the final selection must consider what is readily availableand what are the relative costs of the acceptable options. Forexample, the use of carbon steel with a large corrosion allowancecould be more expensive than using a low-alloy material with asmaller corrosion allowance.



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IV. PIPING COMPONENTS

A. Fittings, Flanges, and Gaskets

1.0 Pipe Fittings

Fittings are used to make some change in the geometry of apiping system. This change could include:

• Modifying the flow direction.

• Bringing two or more pipes together.

• Altering the pipe diameter.

• Terminating a pipe.

The most common types of fittings are elbows, tees,reducers, welding outlets, pipe caps, and lap joint stub ends. These are illustrated in Figures 4.1 through 4.6. Fittings maybe attached to pipe by threading, socket welding, or buttwelding.

An elbow or return (Figure 4.1) changes the direction of apipe run. Standard elbows change the direction by either45° or 90°. Returns change the direction by 180°.

90° 45°

180° Return

Elbow and Return

Figure 4.1



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A tee (Figure 4.2) provides for the intersection of threesections of pipe.

• A straight tee has equal diameters for both the run andbranch pipe connections.

• A reducing-outlet tee has a branch diameter which issmaller in size than the run diameter.

• A cross permits the intersection of four sections of pipeand is rarely seen in process plants.

Tee

Figure 4.2

A reducer (illustrated in Figure 4.3) changes the diameter ina straight section of pipe. The centerlines of the large andsmall diameter ends coincide in a concentric reducer,whereas they are offset in an eccentric type.

Concentric Eccentric

Reducer

Figure 4.3

A welding outlet fitting, or integrally reinforced branchconnection (Figure 4.4) has all the reinforcement required tostrengthen the opening contained within the fitting itself.



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Typical Integrally Reinforced Branch Connection

Figure 4.4

A pipe cap (Figure 4.5) closes off the end of a pipe section. The wall thickness of a butt-welded pipe cap will typically beidentical to that of the adjacent pipe section.

CapFigure 4.5

A lap-joint stub end (Figure 4.6) is used in conjunction withlap-joint flanges.

Note square corner

R

R

Enlarged Sectionof Lap

Lap-Joint Stub End

Figure 4.6



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2.0 Flanges

A flange connects a pipe section to a piece of equipment,valve, or another pipe such that relatively simpledisassembly is possible. Disassembly may be required for

maintenance, inspection, or operational reasons. Figure 4.7shows a typical flange assembly. Flanges are normally usedfor pipe sizes above NPS 1½.

Flange

Bolting

Gasket

Typical Flange Assembly

Figure 4.7

A flange type is specified by stating the type of attachmentand the type of face. The type of attachment defines howthe flange is connected to a pipe section or piece of



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equipment (e.g., welded). The type of flange face or facingdefines the geometry of the flange surface that contacts thegasket. Table 4.1 summarizes the types of flangeattachments and faces. Figure 4.8 illustrates flange facingtypes.

Flange Attachment Types Flange Facing Types

Threaded Flanges Flat Faced

Socket-Welded Flanges

Blind Flanges Raised Face

Slip-On Flanges

Lapped Flanges Ring J oint

Weld Neck Flanges

Types of Flange Attachment and Facing

Table 4.1



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Flange Facing Types

Figure 4.8



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3.0 Gaskets

A gasket is a resilient material that is inserted between theflanges and seated against the portion of the flanges calledthe “face” or “facing”. The gasket provides the seal between

the fluid in the pipe and the outside, and thus preventsleakage. Bolts compress the gasket to achieve the seal andhold the flanges together against pressure and otherloadings.

The three gasket types typically used in pipe flanges forprocess plant applications are:

• Sheet.

• Spiral wound.

• Solid metal ring.

B. Flange Rating

ASME B16.5, Pipe Flanges and Flanged Fittings, provides steelflange dimensional details for standard pipe sizes through NPS 24.Specification of an ASME B16.5 flange involves selection of thecorrect material and flange "Class." The paragraphs that followdiscuss the flange class specification process in general terms.

Flange material specifications are listed in Table 1A in ASME B16.5(excerpted in Table 4.2). The material specifications are groupedwithin Material Group Numbers. For example, if the piping isfabricated from carbon steel, the ASTM A105 material specificationis often used. ASTM A105 material is in Material Group No. 1.1.Refer to ASME B16.5 for additional acceptable materialspecifications and corresponding Material Group Numbers.



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ASME B16.5, Table 1A, Material Specification List (Excerpt)Table 4.2

After the Material Group has been determined, the next step is toselect the appropriate Class. The Class is determined by usingpressure/temperature rating tables, the Material Group, designmetal temperature, and design pressure. Selecting the Class setsall the detailed dimensions for flanges and flanged fittings. Theobjective is to select the lowest Class that is appropriate for thespecified design conditions.

Table 2 of ASME B16.5 provides the information that is necessaryto select the appropriate flange Class for the specified designconditions. ASME B16.5 has seven classes: Class 150, 300, 400,600, 900, 1,500, and 2,500. Each Class specifies the designpressure and temperature combinations that are acceptable for aflange with that designation. As the number of the Class increases,the strength of the flange increases for a given Material Group. Ahigher flange Class can withstand higher pressure and temperaturecombinations. Table 4.3 is an excerpt from Table 2 of ASME B16.5and shows some of the temperature and pressure ratings for

several Material Groups. Material and design temperaturecombinations that do not have a pressure indicated are notacceptable.

Specifying the flange size, material, and class completes most of what is necessary for selecting an ASME B16.5 flange. The flangetype, facing, bolting material, and gasket type and material must be



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added to complete the flange selection process. Discussion of these other factors is beyond the scope of this course.

Material Group

No. 1.8 1.9 1.10Classes 150 300 400 150 300 400 150 300 400

Temp., °F

-20 to 100 235 620 825 290 750 1000 290 750 1000200 220 570 765 260 750 1000 260 750 1000300 215 555 745 230 720 965 230 730 970400 200 555 740 200 695 885 200 705 940

500 170 555 740 170 695 805 170 665 885600 140 555 740 140 605 785 140 605 805

650 125 555 740 125 590 785 125 590 785700 110 545 725 110 570 710 110 570 755

750 95 515 685 95 530 675 95 530 710800 80 510 675 80 510 650 80 510 675850 65 485 650 65 485 600 65 485 650900 50 450 600 50 450 425 50 450 600

950 35 320 425 35 320 290 35 375 505

1000 20 215 290 20 215 190 20 260 345

ASME B16.5, Pressure-Temperature Ratings (Excerpt)

Table 4.3



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SAMPLE PROBLEM 1 - DETERMINE FLANGE RATING

A new piping system will be installed at an existing plant. It is necessary todetermine the ASME class that is required for the flanges. The following designinformation is provided:

• Pipe Material: 1¼ Cr – ½ Mo.

• Design Temperature: 700°F.

• Design Pressure: 500 psig.

SOLUTION

Determine the Material Group Number for the flanges by referring to ASME Table1A (excerpted in Table 4.2). Find the 1¼ Cr – ½ Mo material in the NominalDesignation Steel column. The material specification for forged flanges would beA182 Gr. F11, and the corresponding material Group Number is 1.9.

Refer to Table 2 for Class 150 (excerpted in Table 4.3). Read the allowabledesign pressure at the intersection of the 700°F design temperature and MaterialGroup 1.9. This is only 110 psig and is not enough for this service.

Now check Class 300 and do the same thing. The allowable pressure in thiscase is 570 psig, which is acceptable.

The required flange Class is 300.



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V. VALVES

A. Valve Functions

The possible valve functions must be known before being able toselect the appropriate valve type for a particular application. Fluidflows through a pipe, and valves are used to control the flow. Avalve may be used to block flow, throttle flow, or prevent flowreversal.

1.0 Blocking Flow

The block-flow function provides completely on or completelyoff flow control of a fluid, generally without throttling or

variable control capability. It might be necessary to blockflow to take equipment out of service for maintenance whilethe rest of the unit remains in operation, or to separate twoportions of a single system to accommodate variousoperating scenarios.

2.0 Throttling Flow

Throttling may increase or decrease the amount of fluidflowing in the system and can also help control pressure

within the system. It might be necessary to throttle flow toregulate the filling rate of a pressure vessel, or to control unitoperating pressure levels.

3.0 Preventing Flow Reversal

It might be necessary to automatically prevent fluid fromreversing its direction during sudden pressure changes orsystem upsets. Preventing reverse flow might be necessaryto avoid damage to a pump or a compressor, or toautomatically prevent backflow into the upstream part of the

system due to process reasons.



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B. Primary Valve Types

1.0 Gate Valve

Most valves in process plants function as block valves.

About 75% of all valves in process plants are gate valves. The gate valve is an optimum engineering and economicchoice for on or off service. The gate valve is not suitable tothrottle flow because it will pass the maximum possible flowwhile it is only partially open. Figure 5.1 illustrates a typicalfull-port gate valve.



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1. Handwheel Nut

2. Handwheel

3. Stem Nut

4. Yoke

5. Yoke Bolting

6. Stem

7. Gland Flange

8. Gland

9. Gland Bolts orGland-Eye Boltsand Nuts

10. Gland Lug Boltsand Nuts

11. Stem Packing

12. Plug

13. Lantern Ring

14. Backseat Bushing

15. Bonnet

16. Bonnet Gasket

17. Bonnet Bolts andNuts

18. Gate

19. Seat Ring

20. Body

21. One-Piece Gland(Alternate)

22. Valve Port

Full-Port Gate Valve

Figure 5.1

2.0 Globe Valve

The globe valve is the type most commonly used to throttleflow in a process plant. In the smaller sizes, they are



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typically used as hand-control valves. In larger sizes,applications are limited primarily to bypasses at control valvestations. They provide relatively tight shutoff in control valvebypasses during normal operations; they serve as temporaryflow controllers when control valves must be taken out of

service.

Because all globe valve patterns involve a change in flowdirection, they are not suitable for piping systems thatrequire scraping or rodding. Globe valves are rarely used forstrictly on/off block valve operations because conventionalgate valves adequately serve that function at a lower costand a much lower pressure drop.

3.0 Check Valve

Check valves prevent flow reversal. Typical check valveapplications are in pump and compressor discharge pipingand other systems that require protection against backflow.Valves which contain a disc or discs that swing out of theflow passage area usually create a lower pressure drop inthe system than those which contain a ball or pistonelement. These latter elements remain in the flowstreamand the port configurations frequently include an angularchange in flow direction. For all process designs, theintended purpose of check valves is to prevent gross flowreversal, not to effect complete leakage-free, pressure-tight

shutoff of reverse flow.

The selection of a particular check valve type generallydepends on size, cost, availability, and service. Ball and liftcheck valves are usually the choice for sizes NPS 2 andsmaller, while swing check and plate check valves are usedin the larger sizes.

3.1 Swing Check Valve

The main components of a swing check valve (Figure5.2) are the body, disc, cap, seat ring, disc hinge, andpin. The disc is hinged at the top and closes againsta seat in the valve body opening. It swings freely inan arc from the fully closed position to one thatprovides unobstructed flow. The valve is kept openby the flow, and disc seating is accomplished bygravity and/or flow reversal.



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Cap

Hinge

DiscBody

Pin

SeatRing

FlowDirection

Swing Check Valve

Figure 5.2

3.2 Ball Check Valve

The ball check valve utilizes a ball to prevent flowreversal (Figure 5.3). The basic types are thestraight-through- and globe-type (90° change indirection, similar to a typical globe valve body). Ballcheck valves are available in sizes NPS ½ through 2in all ratings and materials used in process plants. Their low cost usually makes them the first choice forvalves sized NPS 2 and smaller, provided thepressure drop is not a concern.



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Ball Check Valve

Figure 5.3

3.3 Lift Check Valve

A lift check valve (Figure 5.4) usually depends ongravity for operation. Under forward flow, a piston ordisc is lifted off the seat by the fluid while beingretained in the valve by guides. On reverse flow, thepiston or disc is forced against the seat to blockfurther flow. Some lift check valves utilize springloading to assure positive seating.

Lift check valves employing the disc- or piston-typemechanism are available in sizes from NPS ½through 2 in all ratings and materials used in processplants. They are most commonly used in the higherASME B16.5 ratings (Class 300 and greater), andwhere tighter shutoff is required. Valves of this typeshould only be used in clean services.



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SeatRing

PistonFlow

Direction

Lift Check ValveFigure 5.4

3.4 Wafer Check Valve

The wafer body or flangeless valve is a valve bodywithout flanges (Figure 5.5). Valves of this type areplaced between pipe flanges and held in place by thecompressive force between the flanges andtransmitted through the gaskets. The lug-wafer (orsingle-flanged) valve is also shown in Figure 5.5.Valves of this type are mounted between pipe flangesand are held in place by cap screws, machine bolts,or stud bolts which thread into the valve body.



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Figure 5.5

3.5 Ball Valve

Ball valves (Figure 5.6) usually function as blockvalves. Ball valves are well suited for conditionswhere quick on/off and/or bubble-tight shut-off isrequired. The pressure/temperature ratings for ballvalve soft seats above ambient temperatures areusually lower than the ASME ratings for steel valves. This is because of the lower physical properties of thesoft-seat materials. Soft-sealed ball valves are not

normally used for throttling service because the soft-seats are subject to erosion or distortion/displacementcaused by fluid flow when the valve is in the partiallyopen position.



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No. Part Names

1 Body

2 Body Cap

3 Ball

4 Body Seal Gasket

5 Seat

6 Stem

7 Gland Flange8 Stem Packing

9 Gland Follower

10 Thrust Bearing

11 Thrust Washer

12 Indicator Stop

13 Snap Ring

14 Gland Bolt

15 Stem Bearing

16 Body Stud Bolt & Nuts

17 Gland Cover

18 Gland Cover Bolts

19 Handle

Ball Valve

Figure 5.6

3.6 Plug Valve

Plug valves (Figure 5.7) usually function as blockvalves. They are well suited for conditions wherequick on/off and/or bubble-tight shutoff is required. The soft-seal-types may have lowertemperature/pressure ratings than the ASME ratingsfor steel valves because of the lesser physicalproperties of the soft-seat materials. Soft-seal plug

valves are not normally used for throttling servicesince the soft seals are subject to erosion ordistortion/displacement caused by fluid flow when thevalve is partially open.



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Wedge

Molded-In Resilient Seal

Sealing Slip

Plug Valve

Figure 5.7

C. Valve Selection Process

The steps that follow provide a general procedure for selectingvalves and valve components.

1. Identify the necessary design information. This includes designpressure and temperature, valve function, material, etc.

2. Identify potentially appropriate valve types (i.e., ball, butterfly,check, etc.) and components based on application and function(i.e., block, throttle, or reverse flow prevention).

3. Determine valve application requirements (i.e., design or servicelimitations).

4. Finalize valve selection. Check which factors need consideration if two or more valves are suitable.

5. Provide a full technical description. This is done by specifying thevalve type, material, flange rating, etc.



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Exercise 1 – Determine Required Flange Rating

For the piping system described below, determine the required flange rating (orClass) in accordance with ASME B16.5.

Pipe: 1¼ Cr – ½ Mo

Flanges: A - 182 Gr. F11

Design Temperature: 900°F

Design Pressure: 375 psig



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VI. DESIGN

A. Design Conditions

1.0 General

Normal operating conditions are those expected to occurduring normal operation, excluding failure of any operatingdevice, operator error, and the occasional, short-termvariations stated in the applicable code. Startup andcontrolled shutdown of plants and similar foreseeableevents are included within normal operation.

Design conditions are those which govern the design and

selection of piping components, and are based on the mostsevere conditions expected to occur in service. A suitablemargin is used between the normal operating and designconditions to account for normal operating variations.ASME B31.3 does not specify what margins should be usedbetween operating and design conditions; suitable marginsare determined by the user based on his experience.

2.0 Determining Design Pressure and Temperature

The design pressure and temperature are used to calculatethe required thickness of pipe and other design details. Thedesign temperature is used to determine the material basicallowable stress and other design requirements. The valuesfor design pressure and temperature are based on processrequirements.

Piping system design conditions generally are determinedbased on the design conditions of the equipment to whichthe piping is attached. Determining the piping designconditions consists of:

1. Identifying the equipment to which the piping system isattached.

2. Determining the design pressure and design temperaturefor the equipment.



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3. Considering contingent design conditions, such as upsetsnot protected by pressure-relieving devices.

4. Considering the direction of flow between the equipment.

5. Verifying the values with the process engineer.

B. Loads and Stresses

1.0 Classification of Loading Conditions

Pipe loads are classified into three principal types: sustainedloads, thermal expansion loads, and occasional loads.

Sustained loads are those that act on the piping systemduring all or most of its operating time. Sustained loadsconsist of two main categories: pressure and weight. Thepressure load (caused by the design pressure) usually refersto internal pressure, although some piping systems may alsobe designed for external pressure. Design pressure isdefined as the maximum sustained pressure that a pipingsystem must contain without exceeding its allowable stresslimits. Design pressure is normally the governing factor indetermining the minimum required pipe wall thickness.

As shown in Figure 6.1, internal pressure produces bothcircumferential (i.e., hoop) stress and longitudinal stress inthe pipe wall.



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Sl

t

P

Sc

Sc

Sl

t

P

=

=

=

=

Longitudinal Stress

Circumferential (Hoop) Stress

Wall Thickness

Internal Pressure

Stresses Produced By Internal Pressure

Figure 6.1

The weight refers to the total design weight load. The totalweight load includes the weight of the pipe, the fluid in thepipe, fittings, insulation, internal lining, valves, valveoperators, flanges, supports and any other concentratedloads. The weight loads produce a longitudinal stress in thepipe wall.

A piping system will expand or contract due to changes in itsoperating temperature. Thermal expansion loads are

created when the free expansion and contraction of thepiping is prevented at its end points by connectedequipment, or prevented at intermediate points by supportsand/or restraints that are installed. The resulting loadscause thermal stresses in the pipe. Increasing the restraintin a system increases the loading and results in higherthermal expansion stresses. Another cause of pipe thermalloads can be from the thermal expansion of equipment at



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pipe-to-equipment nozzle attachment points, causingdisplacements in the piping system.

The third type of loading comes from occasional loads.Occasional loads act during a small percentage of the

system’s operating time. Occasional loads involve seismicand/or dynamic loading. The degree of seismic loading thatmust be considered varies with geographic location and isdefined by a seismic zone (Ref. ANSI/ASCE 7). Dynamicloads may be caused by safety-relief valve discharges, valveoperation (both opening and closing), steam/water hammer,surge due to pump start-up and shutdown, and wind loads.

2.0 Stress Categorization

To evaluate the stresses in a piping system, it is necessary

to distinguish among primary, secondary, and peak stresses.

• Primary stresses are the direct, shear, or bendingstresses generated by the loading.

• Secondary stresses are those acting across the pipe wallthickness due to a differential radial deflection of the pipewall. Secondary stresses cause local yielding and minordistortions. Secondary stresses, unlike primary stresses,are not a source of direct failure from a single loadapplication.

• Peak stresses are more localized stresses which dieaway rapidly within a short distance from their origin.Peak stresses occur in areas such as welds, fittings,branch connections, and other piping components wherestress concentrations and possible fatigue failure mightoccur. Peak stresses are considered equivalent insignificance to secondary stresses, but they do not causeany significant distortion.

3.0 Allowable Stresses

The basic allowable stress is a function of materialproperties, temperature, and safety factors. The basicallowable stress provides an upper limit for the actualstresses.



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• Allowable stresses for sustained loads are established toprevent general collapse or excessive distortion of thepiping system.

• Allowable stresses for thermal expansion loads areestablished to prevent a localized fatigue failure.

• Allowable stresses for occasional loads are establishedto prevent wind and earthquake type loads fromcollapsing or distorting the piping system.

Actual stresses are calculated for the following load cases:

• Sustained loads

• Occasional loads

• Stress range due to differential thermal expansion

The piping system is designed such that the calculatedstresses are no larger than the appropriate allowablestresses.

Table 6.1 (excerpted from ASME B31.3 Table A-1) listsbasic allowable stresses in tension versus temperature forseveral materials.



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Basic Allowable Stress S, ksi. At Metal Temperature, °°°°F.

Material Spec. No/Grade 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500

Carbon Steel A 106 B 20.0 20.0 20.0 20.0 18.9 17.3 16.5 10.8 6.5 2.5 1.0

C - ½Mo A 335 P1 18.3 18.3 17.5 16.9 16.3 15.7 15.1 13.5 12.7 4. 2.4

1¼ - ½Mo A 335 P11 20.0 18.7 18.0 17.5 17.2 16.7 15.6 15.0 12.8 6.3 2.8 1.2

18Cr - 8Ni pipe A 312 TP304 20.0 20.0 20.0 18.7 17.5 16.4 16.0 15.2 14.6 13.8 9.7 6.0 3.7 2.3 1.4

16Cr - 12Ni-2Mopipe

A 312 TP316 20.0 20.0 20.0 19.3 17.9 17.0 16.3 15.9 15.5 15.3 12.4 7.4 4.1 2.3 1.3

ASME B31.3, Table A-1 (Excerpt),Basic Allowable Stresses in Tension for Metal

Table 6.1

C. Pressure Design of Components

1.0 General

Two different types of pressure may be imposed on a pipingsystem: external or internal. Most piping systems need onlybe designed for internal pressure. Some piping systemsmay be subject to a negative pressure or vacuum conditionduring operation (e.g., process vacuum conditions, steam-out, underwater lines, etc.) and must be designed forexternal pressure. This section only discusses the internalpressure design of straight sections of pipe. Refer to ASME

B31.3 for design requirements for external pressure.

2.0 Required Wall Thickness for Internal Pressure of Straight Pipe

The required wall thickness for internal pressure iscalculated using the following equation:

)PYSE(2

PDt

+=

Where:

t = Required thickness for internal pressure, in.

P = Internal design pressure, psig



106

S = Allowable stress in tension (Table 6.1), psi

E = Longitudinal-joint quality factor (Table 6.2)

Y = Wall thickness correction factor (Table 6.3)

The longitudinal-joint quality factor is based on:

• Whether the pipe is seamless or has a weldedlongitudinal seam

• The pipe material and welding process (if welded pipe)

The wall thickness correction factor is based on the type of steel and the design temperature.



107

Spec.No.

Class (or Type) Description E j

Carbon Steel

API5L

. . .

. . .

. . .

Seamless pipeElectric resistance welded pipe

Electric fusion welded pipe, double butt, straight orspiral seam

Furnace butt welded

1.000.850.95

A 53 Type S Type E Type F


Furnace butt welded pipe

1.000.850.60

A 106 . . . Seamless pipe 1.00

Low and Intermediate Alloy Steel

A 333 . . .. . .


1.000.85

A 335 . . . Seamless pipe 1.00

Stainless Steel

A 312 . . .. . .. . .

Seamless pipeElectric fusion welded pipe, double butt seamElectric fusion welded pipe, single butt seam

1.000.850.80

A 358 1, 3, 4

52

Electric fusion welded pipe, 100% radiographed

Electric fusion welded pipe, spot radiographedElectric fusion welded pipe, double butt seam

1.00

0.900.85

Nickel and Nickel Alloy

B 161 . . . Seamless pipe and tube 1.00

B 514 . . . Welded pipe 0.80

B 675 All Welded pipe 0.80

ASME B31.3, Table A-1B (Excerpt),

Basic Quality Factors for Longitudinal Weld Joints, E j

Table 6.2



108

Temperature, °°°°F

Materials 900 & lower 950 1000 1050 1100 1150 & up

FerriticSteels 0.4 0.5 0.7 0.7 0.7 0.7

AusteniticSteels

0.4 0.4 0.4 0.4 0.5 0.7

Other DuctileMetals

0.4 0.4 0.4 0.4 0.4 0.4

Cast iron 0.0 . . . . . . . . . . . . . . .

ASME B31.3, Table 304.1.1 (Excerpt),Values of Coefficient Y

Table 6.3

Two additional thickness allowances must be considered todetermine the final required pipe wall thickness: corrosionallowance and mill tolerance.

Corrosion allowance (CA) is an additional thickness that isadded to account for wall thinning and wear that can occur inservice. The corrosion allowance is based on experienceand data for the particular pipe material and fluid service. Thus:

tm = t + CA

Where:

tm = Total minimum required wall thickness, in.

Mill tolerance accounts for the difference between the actualmanufactured pipe wall thickness and the “nominal” wall

thickness specified in the relevant pipe dimensionalstandard. The typical pipe mill tolerance is 12.5%. Thismeans that the as-supplied pipe wall thickness can be up to12.5% thinner than the nominal thickness and still meet itsspecification requirements. Use the following equation todetermine the minimum required nominal thickness to order:



109

875.0

tt m

nom =

Where:

tnom = Minimum required nominal pipe wall thickness, in.

Each pipe size has several standard nominal thicknessesthat are available. The nominal pipe thickness that isspecified for a system must be selected from those readilyavailable and be at least equal to tnom.

3.0 Curved and Mitered Pipe Segments

The minimum required thickness of curved pipe (elbows or

bends) is the same as that required for straight pipesections. A mitered bed is fabricated by welding straightpipe sections together to produce the direction change. Amitered bend is generally less expensive than a wroughtelbow for large pipe sizes (over ~ NPS 24). The minimumrequired thickness for a miter may be greater that that of theconnected straight pipe sections, depending on the numberof miter welds, design conditions, size, etc. Refer to ASMEB31.3 for thickness calculation requirements.



110

SAMPLE PROBLEM 2 - DETERMINE PIPE WALL THICKNESS

A piping system must be modified to add a new, spare heat exchanger. Youhave been assigned the responsibility to determine the required wall thicknessfor the pipe from the heat exchanger to several pumps. The piping system will

have a design temperature of 650°F. The design pressure is 1,380 psig. Thepipe outside diameter is 14 in. The material is ASTM A335, Gr. P11 (1¼ Cr – ½Mo), seamless. Corrosion allowance is 0.0625 in.

What is the minimum required thickness for this pipe?

SOLUTION

The following equation applies:

( )PYSE2

PDt

+=

Based on the given information:

P = 1,380 psig.

D = 14 in.

For the A335, Gr. P 11 material:

S = 16,150 psi. [Table A-1 of ASME B31.3 at 650°F

E = 1.0 [Table A-1B of ASME B31.3]

Y = 0.4 [Table 304.1.1 of ASME B31.3, since thematerial is ferritic and the temperature is below

900oF.

Since all the required parameters have now been determined, the requiredinternal pressure thickness may be calculated as follows:



111

( ) ( )[ ]

.in577.0t

4.0380,11150,162

14380,1t

=

×+×

×=

In this case, a 0.0625 in. corrosion allowance has been specified.

Therefore:

tm = t + c = 0.577 + 0.0625

tm = 0.6395 in.

.in0.7310.875

0.6395tnom ==

4.0 Branch Reinforcement Requirements

A pipe with a branch connection is weakened by the requiredopening. Unless the wall thickness of the pipe is sufficientlygreater than that required to sustain the pressure, additionalreinforcement must be provided.

ASME B31.3 contain rules for determining the required

reinforcement for both welded and extruded outlet-typebranch connections. Branch connections can also be madeusing forged or wrought fittings (i.e., tees, laterals, crosses,couplings, or half-couplings), or an integrally reinforcedbranch connection. Reinforcement calculations are notrequired for forged or wrought type branch connectionsbecause they have adequate inherent reinforcement andhave been designed and tested to meet ASME B31.3requirements. This section discusses only branchconnections that are fabricated by welding a branch pipe tothe run pipe.



112

4.1 Area Removed By Branch Connection

A volume of metal is removed from a pipe wall when ahole is cut in it for a branch connection. However, a

simplification is made when evaluating branchreinforcement requirements.

An imaginary plane is passed through the branch andrun pipes, and the intersection is viewed in cross-section. The removed volume of pipe wall is thenlooked at as an area (see Figure 6.2).

Db

Tb

cNom.

Thk.

Nom. Thk.

Dh

Th

th

Tr

c

tb

Mill Tol.

Mill Tol.

d1

d2

d2

L4

β

ReinforcementZone LimitsReinforcement

Zone Limits

A1

A3

A4

A4

A2

A2

A3

Pipe C

Welded Branch Connection

Figure 6.2

4.2 Limits of Reinforcement Zone

The reinforcing zone is the region where credit maybe taken for any reinforcement that is present. Thebranch connection must have adequate reinforcementto compensate for the weakening caused by cutting ahole in the run pipe. This reinforcement:



113

• Must be located reasonably close to the openingto provide any practical benefit.

• May be located in the branch pipe, the run pipe, orboth.

Additional material located outside of this zone is noteffective for reinforcement.

4.3 Branch Connection Reinforcement

Branch connection reinforcement located within thereinforcement zone may come from one or more of the following sources.

• Excess thickness available in the branch orheader pipe.

• Additional reinforcement added in the form of apad, ring, saddle, or weld metal.

If excess thicknesses in the branch and header pipesdo not provide enough reinforcement, additional metalmay be added.

4.4 Reinforcement Area

The required reinforcement area is based on themetal area removed. This is calculated using:

β

−−=

sin

)c T (2Dd bb

1

Where:

d1 = Effective length removed from the run pipe,

in.

Db = Branch outside diameter, in.

Tb = Minimum branch thickness, in.

c = Corrosion allowance, in.

β = Acute angle between branch and header



114

The required reinforcement area, A1, is thencalculated using :

)sin2(dtA 1h1 β−=

Where:

th = Minimum required header thickness, in.

4.5 Reinforcement Pad

Additional branch reinforcement is needed when therequired area exceeds the available area, and may beprovided by locally increasing the thickness of eitherthe header or branch pipe. However, it is usuallymore economical to provide a reinforcement pad tosupply the additional reinforcement.

There are three variables to select in designing thereinforcement pad:

• Material

• Outside diameter

• Wall thickness

To calculate the area of the reinforcement pad, A4, thefollowing equation is used:

r

bp

4 T sin

)DD(A

öççè

æ

β

−=

Where:

D01p = Outside diameter of the pad, in.

Db = Outside diameter of the branch, in.

Tr = Pad thickness, in.

β = The acute angle between the branch andheader pipes.



115

The pad must be large enough to provide theadditional reinforcement needed and be within thereinforcement zone. The pad material is generallyequivalent to that of the pipe.

The following Sample Problem illustrates the branchreinforcement calculation procedure.



116

SAMPLE PROBLEM 3

A new steam turbine is being installed within a process plant. This will require anew NPS 16 steam supply line to be connected to an existing NPS 24 distributionheader. The following design information has been determined:

• Pipe material - Seamless, A 106/Gr. B for both the branch and header.

• Design temperature - 700°F

• Design pressure - 550 psig

• Allowable stress - 16,500 psi.

• Corrosion allowance - 0.0625 in.

• Mill tolerance - 12.5%

• Nominal Pipe - Header: 0.562 in. Thicknesses Branch: 0.375 in.

• Required Pipe - Header: 0.395 in. Thicknesses for Pressure Branch: 0.263 in.

• The branch connection is made on top of the header at a 90° angle, and doesnot penetrate a header weld.

Determine if additional reinforcement is required for this branch connection. If it

is, size the reinforcing pad, neglecting the area of any welds. Assume that thepad material is equal to the header material, and that its thickness equals theheader thickness.

SOLUTION

See Figure 6.2 for the relevant nomenclature.

• The required thicknesses for pressure were given.

• Next, the value for the effective length removed from the run pipe, d1, must becalculated. This equals the corroded inside diameter of the branchconnection after accounting for mill tolerance (i.e., the actual pipe wallthickness may be up to 12.5% less than the nominal thickness).

β

−−=

sin

)c T (2Dd bb

1



117

( )

in.15.469d

90sin

0.06250.8750.375216d

1

1

=

°

−×−=

• Now the required reinforcement area, A1, may be calculated.

2in.6.11A

)sin90(215.4690.395A

sinβi(2dtA

1

1

1h1

=

°−×=

−=

The available reinforcement areas in the header and branch pipe are nowcalculated. This is determined using any “excess” thickness available in theheader and branch that is not necessary to withstand the pressure (or other)loads. Disregard any contribution from nozzle attachment welds since this isminimal.

• Calculate the excess area available in the header, A2.

( ) ( )ct T d2dA hh122 −−−=

First determine d2 which is the greater of d1, or,

( ) ( )2

dc T c T

1

hb +−+−, but less than the header diameter, Dh

( ) ( ) in.8.432

15.4690.06250.5620.8750.06250.3750.875 =+−×+−×

∴ d2 = d1 = 15.469 in., which is less than the header diameter of 24 in.

A2 = (2 x 15.469 - 15.469) (0.875 x 0.562 - 0.395 - 0.0625)

A2 = 0.53 in.2

• Calculate the excess area available in the branch, A3.



118

( )

βsin

ct T 2L A bb4

3

−−=

First determine L4.

( ) ( ) .smalleriswhichever, Tc T2.5orc T2.5L rbh4 +−−=

Since Tr = 0 (i.e., no reinforcing pad initially) and Th is greater than Tb, L4 is

based on the second equation.

L4 = 2.5 (0.875 x 0.375 - 0.0625)

L4 = 0.664 in.

( )

23

3

in.0.003A

90sin

0.06250.2630.3750.8750.6642A

=

°

−−××=

• Calculate other excess area that may be available, A4.

There is no reinforcing pad and the area contribution from the branch weld isbeing disregarded. Therefore, A4 = 0.

• Total Available Area:

The total available reinforcement area, A T, is calculated by adding the

contributions from each source.

A T = A2 + A3 + A4

A T = 0.53 + 0.003 + 0

A T = 0.533 in.2 available reinforcement.

The available total reinforcement of 0.533 in.2 is obviously much less than therequired reinforcement area of 6.11 in.2. Therefore, a reinforcing pad isrequired. The reinforcement pad will now be sized.



119

A106, Gr. B material will be used for the reinforcement pad. Its thickness isset to be equal to the header nominal thickness of 0.562 in.

• Recalculate Available Reinforcement:

Now that a reinforcing pad is being used, the available reinforcement in thebranch must be recalculated since the height of the reinforcement zone in thebranch pipe will change slightly.

L41= 2.5 (Th - c)

L41= 2.5 (0.875 × 0.562 - 0.0625)

L41= 1.073 in.

L42= 2.5 (Tb - c) + Tr

L42= 2.5 (0.875 × 0.375 - 0.0625) + 0.562 (0.875)

L42= 1.16 in.

Therefore, L4 = 1.073 in.

βsin

c)t(T 2L A bb4

3

−−=

o390sin

0.0625)0.2630.375(0.8751.0732A

−−××=

entreinforcemavailablein.0.535A

00.0050.53A

AAAA

)calculated previously in.0.003the (vs. in.0.005A

2

T

T

432 T

223

=

++=

++=

=

• Calculate additional reinforcement required and the pad dimensions:

The required reinforcement area is 6.11in.2, and the available area is 0.535in.2. Therefore, the additional reinforcement area to be provided in the pad,A4, is:



120

A4 = 6.11 - 0.535

A4 = 5.575 in.2

• Determine the diameter of the pad, Dp.

Tr = 0.562 (0.875) = 0.492 in.

Db = 16 in.

in.27.3D

160.492

5.575D

βsin

D

T

AD

p

p

b

r

4p

=

+=

+=

The pad diameter must be at least 27.3 in. to provide adequatereinforcement. Since 2d2 = 30.938 in., this pad diameter is within the

reinforcement zone along the header and is acceptable.

The following approach of calculating the required pad width, Lr, may be usedas an alternative to calculating the pad diameter.

in. 5.66L

0.492

5.5750.5L

T

A0.5L

r

r

r

4

r

=

×=

=



121

EXERCISE 2: DETERMINE REQUIRED PIPE WALLTHICKNESS

A new project is being considered to transport 48° API crude oil in a carbon steelpipe between two areas within a tank farm. The fluid being transported will have

a design temperature of 260°F. The system design pressure is 150 psig, thepipe outside diameter is 30 in., and the pipe being used is A 106, Gr. B seamlesspipe. A corrosion allowance of 1/8 in. has been specified for the pipe. All pipingwithin the tank farm is designed in accordance with ASME B31.3. Assume thereis a 12.5% mill tolerance.

a. What is the thickness required for internal pressure?

b. What is the minimum required nominal wall thickness?

Use Table 6.1 along with Tables 6.2 and 6.3 for the necessary information.



122

VII. SYSTEM DESIGN

A. Layout Considerations

Operational, maintenance, and safety considerations influence thelayout of a piping system. These factors must be recognized whendesigning the layout and spacing of piping and equipment. Thissection discusses how these factors influence piping layout.

1.0 Operations Requirements

Operating and control points (e.g., valves, flanges,instruments, sample points, drains, and vents) should belocated so that they can be used safely and easily. For

example, valves must be located so that they can bereached.

There must be enough clearance above and below the pipeto perform basic operations on valves and flanges.

There must also be enough lateral space to access valves,sample points, vessel flanges, and other equipment that mayrequire operator attention.

2.0 Maintenance Requirements

The piping system must be laid out so that its componentscan be inspected, repaired, or replaced with minimumdifficulty. There must be ample clearance for maintenanceequipment (e.g., cranes) and for vehicles (e.g., trucks).Access must be provided so supports can be maintained. There must be enough space to access and remove largepieces of equipment if they require maintenance.

• Access near rotating equipment is important because

cranes must reach the equipment when removal orrealignment is required.

• Heat exchanger bundles must be pulled out for cleaning.

• Large valves must be removed to repair or replace theirseats.



123

• Rotating equipment requires frequent monitoring andmaintenance.

3.0 Safety Considerations

Piping layout must consider the safety of personnel near thepipe. This specifically includes access for fire fightingequipment and fire prevention. Fire fighting equipmentneeds clearance to access major pieces of equipment (e.g.,heat exchangers, vessels, and tankage). Pipeways must berouted and designed to provide the necessary clearances. There must be enough space beneath pipeways for peopleto walk and work. Firewater piping must be routed so that itwould not be damaged by piping containing hazardous fluidsthat could rupture.

B. Pipe Supports and Restraints

A piping system needs supports and restraints because of thevarious loads that are imposed upon it. Supports absorb systemweight and reduce longitudinal pipe stress, pipe sag, and end pointreaction loads. Restraints control or direct the thermal movementof a piping system. The control of thermal movement may benecessary either to keep pipe thermal expansion stresses withinallowable limits, or to limit the loads that are imposed on connectedequipment.

Selection of a specific type of support or restraint to use in aparticular situation depends on such factors as:

• Load to be supported or absorbed.

• Clearance available for attachment to pipe.

• Availability of nearby structural steel that is already there.

• Direction of loads to be absorbed or movement to berestrained.

• Design temperature.

• Need to permit vertical thermal movement at a support.



124

1.0 Rigid Supports

Rigid supports are used in situations where weight support isneeded and no provision to permit vertical thermaldisplacement is required. A rigid support always will prevent

vertical movement downward, will sometimes preventvertical thermal movement upward, and will permit lateralmovement and rotation. See Figure 7.1.

Shoe Saddle Base AdjustableSupport

Dummy Support Trunnion

Rigid Supports

Figure 7.1

Hangers are a type of rigid support. They support pipe fromstructural steel or other facilities that are located above the pipeand carry piping weight loads in tension. Pipe hangers are typically



125

one or more structural steel rods bolted to a pipe attachment and tothe overhead member. A hanger rod is designed to move freelyboth parallel and perpendicular to the pipe axis, and not restrictthermal expansion in these directions. A hanger will preventmovement both down and up. See Figure 7.2.

Hangers

Figure 7.2

2.0 Flexible Supports

Flexible or resilient supports allow the piping system to movein all three directions while still supporting the requiredweight load. Weight is supported by the use of a coil springhaving an appropriate stiffness to carry the applied weight



126

load. Since the spring is resilient, it permits vertical thermalmovement while still carrying the weight. This type of support is used in situations where support must be providedat a particular location, and vertical thermal expansion mustalso be permitted.

There are two basic types of flexible supports: variable loadand constant-load-type. In the variable-load type flexiblesupport, the amount of vertical load exerted by the supportchanges as a result of the pipe thermal movement (whichcompresses or extends the spring). The amount of verticalload exerted by a constant-load type support does notchange throughout its movement range. See Figure 7.3.

Load and DeflectionScale

Typical Variable-LoadSpring Support

Small Change inEffective Lever Arm

Large Change inEffective Lever Arm

RelativelyConstant

Load

Typical Constant-LoadSpring Support Mechanism

Flexible Supports

Figure 7.3

3.0 Typical Restraints and Anchors

3.1 Restraints

Restraints have two primary purposes in a pipingsystem.



127

• Restraints control, limit, or redirect the unrestrictedthermal movement of a pipe. They are used toeither reduce the thermal stress in the pipe or theloads exerted by the pipe on equipmentconnections.

• Restraints absorb loads imposed on the pipe byother conditions such as wind, earthquake, slugflow, water hammer, or flow-induced vibration.Excessive loads could result in high pipe stress orequipment reaction loads, or cause flangeleakage.

There are several different types of restraints thatmay be used. The selection of which type to use andits specific design details depends primarily on the

direction of pipe movement that must be restrained,the location of the restraint point, and the magnitudeof the load that must be absorbed. It is also possibleto restrain more than one direction at one location in apiping system, or to combine a restraint with asupport.

3.2 Anchors

An anchor is a special type of restraint that stops

movement in all three directions. Anchors provide fullfixation of the pipe, permitting very limited, if any,translation or rotation. An anchor is used in situationswhere it is necessary to totally isolate one section of apiping system from another from the standpoint of load and deflection. A total anchor that eliminates alltranslation and rotation at one location is not used ascommonly as one or more restraints that act at asingle location. A directional anchor which restrainsthe line only in its axial direction is more commonlyused. Figure 7.4 provides several examples of

anchors.



128

Anchor Anchor Partial Ancho

Restraints/Anchors

Figure 7.4

3.3 Guides

A guide is a particular type of restraint that permitsmovement along the pipe axis while preventing lateralmovement. Depending on the particular guide detailsemployed, pipe rotation may or may not be restricted.Common situations where guides are used are in longpipe runs on a pipe rack to control thermal movementand prevent buckling, and in straight pipe runs down

the side of a tower to prevent wind-inducedmovement and control thermal expansion. SeeFigure 7.5.



129

Guide Guide

Vertical Guide

x

Guide

Examples of Guides

Figure 7.5

C. Piping Flexibility

Piping must have sufficient flexibility to accommodate thermal

expansion (or contraction) effects. Piping systems must bedesigned to ensure that they do not fail because of thermalstresses or produce excessive forces and moments at connectedequipment. If a system does not provide adequate flexibility, theresults can be leaky flanges, fatigue failure of the pipe, excessivemaintenance, operations problems, and damaged equipment.



130

A structure that is subject to a change in temperature will change indimensions. If these thermal movements are allowed to occurwithout any restraint whatsoever, no pipe stresses or reaction loadsresult. However, in real systems, stresses are developed in thepipe and moments and forces are imposed on the connected

equipment and at supports and restraints installed in the system. The basic problem is to determine the internal pipe stresses andthe external loads, and then decide if they are acceptable. Athermal flexibility analysis is done to ensure that the piping systemis laid out, supported, and restrained such that the thermal stressesin the pipe and the loads on the end points are within allowablelimits.

1.0 Rationale for Piping Flexibility and Support Design

Support and flexibility design is a combination of art and

science with multiple factors to consider and usually morethan one way to design the system. It requires knowledge of how the operating and design conditions of a piping systeminfluence its overall design, and the supports and restraintsrequired for the system.

A piping system can be described as an irregular structuralframe in space because of its relatively slender proportionswhen compared to structural steel systems. Elevated designtemperatures or various operating scenarios may causesufficient pipe thermal stress or reduce material strength

such that supplementary structural assistance to support thepiping system is required. It is also often necessary to limitthe pipe movement at specific locations to protect sensitiveequipment, control vibration, or to resist external forces (e.g.,wind, earthquake, or shock loading).

Attention must also be paid to pipe support/restraint designdetails to ensure that localized stresses in the pipe wall arekept within allowable limits. In those situations, designdetails that spread the applied load over a wider portion of the pipe surface are used.

Planning for pipe supports and restraints should be donesimultaneously with establishing possible layoutconfigurations to achieve the most cost-effective design.



131

2.0 Approaches to Design

Due to the complexity of the piping flexibility and supportdesign process, there is no single procedure or designmethod applicable for all situations. The following is one

way to approach the problem.

• Examine the layout and operation of the piping system toidentify:

- Layout geometry.

- Pipe diameter and thickness, and locations of anychanges in these parameters.

- Piping component design details such as branchconnection details and type of elbows used (i.e., long

radius or short radius).- Design temperature and pressure.

- Fluid service, including its potential danger.

- End-point movements.

- Type of connected equipment (i.e., rotating or fixed).

- Locations of existing structural steel.

- Relevant operating scenarios.

- Special design considerations (e.g., wind, vibration-prone services, orientation of loads).

• Determine the potential effects of those conditions (e.g.,thermal movements, loads, and stresses).

• Determine the types of support or restraint required andtheir approximate locations.

• Determine if the situation warrants a detailed computeranalysis.

• If required, identify which conditions apply for theanalysis and utilize an appropriate computer program.

• Interpret the results of the analysis.



132

D. Required Design Information for Piping Stress Analysis

Detailed piping stress analysis is done using a computer programsuch as Caesar II, Simflex, or Triflex. Such programs have thecapability to consider any combination of pipe geometry, support,

restraint, and load conditions. However, several things must beconsidered:

• Applicable design conditions and operating scenarios for thepiping system.

• Allowable stresses from ASME B31.3.

• Load limitations, if any, on connected equipment.

• Extent of analysis required to identify most severe case.

Design conditions that must be known to perform a detailed pipestress analysis are listed below:

• Layout geometry of the piping system.

• Pipe diameter and wall thickness.

• Design temperature and pressure.

• Fluid service, including whether it is dangerous.

• End-point movements.

• Type of connected equipment.

• Structural steel located in the vicinity.

• Special design considerations and load cases.

Another consideration is the number of cycles that the system willundergo during its design life. This influences piping flexibilitydesign because the allowable flexibility stress is based on fatiguefailure. All ASME B31.3 piping systems are designed for aminimum of 7000 cycles. Systems that will undergo more than

7000 operating cycles during their design life are designed using areduced allowable stress basis.

E. Criteria for Allowable Equipment Nozzle Loads

A poorly designed piping system can cause damage to theequipment it is connected to, whether the equipment is a rotating



133

type (e.g., pump or compressor) or stationary type (e.g., pressurevessel or heat exchanger). Rotating equipment is the moresensitive with respect to imposed piping loads because of themoving parts and small clearances involved in its design.Excessive piping loads imposed on rotating equipment can cause

damage, poor operation, and/or maintenance problems at levelswell below those that would cause pipe or equipment stressconcerns.

Loads that are imposed by the piping system on connectedequipment are determined from the results of the piping flexibilityanalysis. These loads are then compared to allowable valuesbased on industry standards for particular types of equipment todetermine if they are acceptable. The allowable values cansometimes be read from a table contained in the applicable industrystandard. Other times, the allowable loads or the equipment

stresses that they cause must be calculated. Equipment vendorswill sometimes have allowable load criteria that must beconsidered. Table 7.1 summarizes industry standards that apply toequipment nozzle load evaluations, and the parameters that areused to determine the allowable loads.

Equipment Item Industry StandardParameters Used

To DetermineAcceptable Loads

Centrifugal Pumps API-610 Nozzle size

Centrifugal Compressors API-617, 1.85 timesNEMA SM-23 allowable

Nozzle size, material

Air-Cooled Heat Exchangers API-661 Nozzle size

Pressure Vessels, Shell-and-Tube Heat ExchangerNozzles

ASME Code SectionVIII, WRC-107,WRC-297

Nozzle size, thickness, reinforcementdetails, vessel/exchanger diameter,and wall thickness. Stress analysisrequired.

Tank Nozzles API-650 Nozzle size, tank diameter, height,

shell thickness, nozzle elevation.

Steam Turbines NEMA SM-23 Nozzle size

Equipment Nozzle Load Standards and Parameters

Table 7.1



134

F. When Should A Computer Analysis Be Used

Computer programs can perform numerous analyses with manydifferent combinations of design conditions and system geometries. They can perform many functions that would be difficult for a piping

analyst to do “by hand.” Computers can also perform uniquefunctions that would be difficult or impossible to do by hand or othermethods with sufficient accuracy. Even though hand calculationscan be used in many situations, a computer program can often beused to finalize and optimize the final design.

A computer analysis should also be used when there are severaloperating combinations to be considered and other methods wouldbe inadequate or too time consuming, when greater accuracy isrequired due to the nature of the system, and for complicated pipingsystems. Computer programs are also very useful for analyzing the

stresses and loads at piping components such as valves, branches,and bends. A piping system designer should remember that acomputer program only gives quantitative guidelines, to which theymust apply common sense and judgement.

The guidelines listed in Table 7.2 may be used to help determinewhen a computer analysis should be performed:

Type Of Piping Pipe Size, NPS Maximum DifferentialFlexibility Temp.

General piping ≥ 4≥ 8

≥ 12

≥ 20

≥ 400°F≥ 300°F

≥ 200°Fany

For rotating equipment ≥ 3 Any

For air-fin heat exchangers ≥ 4 Any

For tankage ≥ 12 Any

Computer Analysis Guidelines

Table 7.2

G. Design Considerations for Piping System Stress Analysis

The following paragraphs discuss several design considerations inpiping system stress analysis.



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1.0 Piping Flexibility Temperature

Flexibility analysis should be made for the largesttemperature difference that may be imposed on the pipe bynormal and abnormal operating conditions. This results in

the largest pipe stress range to be considered in fatiguefailure evaluation, and the largest reaction loads imposed onequipment end connections, supports, and restraints. Tables 7.3 and 7.4 provide guidelines to determine thetemperatures to consider in a flexibility analysis. Note thatmore than one of these items might require consideration ina particular system and lead to the need for multiplecomputer calculations to identify the case that governs thesystem design.

StableOperation

Gives the temperature range expected for most of the time a plant is inoperation. Some margin above equipment operating temperature (i.e.,use of the design temperature rather than operating temperature)allows for process flexibility.

Startup andShutdown

Must be examined to determine if the heating or cooling cycles poseflexibility problems. For example, if a tower is heated while someattached piping remains cold, the piping flexibility should be checkedfor that case.

Regeneration

and DecokingPiping

Must be designed for normal operation, regeneration, or decoking, and

switching from one service to the other. An example is the decoking of furnaces.

SparedEquipment

Requires multiple analyses to determine if the piping is adequate forthe expected variations of temperature, for no flow in some of thepiping, and for switching from one piece of equipment to another. Acommon example is the piping for two or more pumps with one or morespares.

Normal Temperature Conditions To Consider Table 7.3



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Loss of CoolingMedium Flow

Temperature changes due to a loss of cooling medium flowshould be considered. This includes pipe that is normally atambient temperature but can be blocked in, while subject tosolar radiation.

Steamout for Air or Gas Freeing

Most on-site equipment and lines, and many off-site lines, arefreed of gas or air by the use of steam. For 125 psig steam,300°F is typically used for the metal temperature. Pipingconnected to equipment which will be steamed out, especiallypiping connected to the upper parts of towers, should bechecked for the tower at 300°F and the piping at ambient plus50°F. This situation may govern the flexibility of lines connectedto towers that operate at less than 300°F or that have a smallertemperature variation from top to bottom.

No Process FlowWhile Heating

Continues

If process flow can be stopped while heat is still being applied,the piping flexibility should be checked for the maximum metaltemperature. Such situations can occur with steam tracing andsteam jacketing.

Abnormal Temperature Conditions To Consider

Table 7.4

Metal temperatures that govern the flexibility design of a pipingsystem are not necessarily the ones associated with the mostsevere coincident pressure and temperature which govern the wall

thickness of the pipe. Piping flexibility depends only on thetemperature. Therefore, a condition of high temperature and lowpressure may govern the piping flexibility design while the wallthickness is based on a higher pressure but a lower temperature.

Pipe thermal movement is caused by a temperature change fromthe piping installation temperature (i.e., the ambient temperature).Piping analysis computer programs typically include a “default”ambient temperature (commonly 70°F). Then, all thermalmovements and resulting thermal stresses are calculated based onthe difference between the specified pipe design temperature and

the default ambient temperature. A realistic ambient installationtemperature (typically lower than 70°F) must be used for thespecific plant site to accurately calculate the maximum thermalstress range and reaction loads.



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2.0 Extent of Analysis

The extent of a piping system analysis depends on thesituation. The overall purpose of the analysis is to provideenough flexibility for the system. The engineer must analyze

the right combination of operating conditions to determinewhere, and if, additional flexibility is needed to reduce pipestresses or loads at end points. The engineer must alsodecide if it is desirable and acceptable to not include portionsof a large, complex system in the analysis to simplify themodeling. For example, including an NPS 4 branch run inthe model of a NPS 24 main system may not be necessary. J udicious installation of anchors or other restraints in a largesystem could also help simplify the modeling by separatingthe system into sections.

Use the following steps to develop the piping design:

• Define line size, wall thickness, material, number of temperature cycles, layout, maximum differentialtemperature, and any alternative operating scenarios.

• Determine conditions of end-point restraint andmovements.

• Locate intermediate points of restraint and define anylimitations that they impose on piping movement.

• Select a suitable analysis method and calculate the loadsand stresses.

• Compare the results with the allowable stress range forthermal expansion stresses, the allowable stress atdesign temperature for weight-plus-pressure stresses,and the applicable load criteria for connected equipment.

3.0 Modifying System Design

The initial piping system layout may not be satisfactory for

thermal flexibility stresses or loads on connected equipment. The following guidelines may help the situation.

• Provide more offsets or bends, or use more expansionloops within the same space. These make the systemmore flexible and reduce the thermal stresses.



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• Install expansion joints. However, this approach shouldbe the exception rather than the rule. Expansion jointsrepresent a "weak link" in a piping system. They mayaffect the life of the system since they are moresusceptible to damage than pipe, and can create

maintenance and operational problems. Thus, the use of expansion joints should only be considered as a lastresort. One situation where expansion joints must beused is where pressure drop or other processrequirements dictate the use of relatively straight piperuns (e.g., fluidized solids transfer lines).

• Strategically locate restraints to minimize thermal andfriction loads at equipment. Restraints could also beused to direct pipe thermal expansion into a section of the system that has more inherent flexibility to absorb it.

• Use spring supports if large vertical thermal movementsare expected, or if thermal expansion causes pipe to liftoff fixed supports. Avoid fixed supports that result inlarge thermal stresses.

• Use Teflon bearing pads at supports for large-diameterpipe or other large weight loads if friction loads areexcessive on equipment connections or structuralmembers.

4.0 System Design Considerations

Each type of piping system has particular factors that mustbe considered when performing a detailed analysis. Forexample:

• Pump systems will often be installed with spared pumps. Thus, various scenarios of operating vs. spared pump(s)must be considered since portions of the system near thepumps will be hot while other portions are cold.

• Piping systems are sometimes heat traced. This mightbe done either to reduce liquid viscosity to allow thenecessary flow, or to prevent condensate accumulation. The condition with the process flow off while the heattracing remains on must also be considered since thepipe metal temperature for this case may be higher thanthe normal design temperature.

• Piping systems connected to atmospheric storage tanksmust be designed considering movement that occurs at



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the tank nozzle. When the tank is filled with liquid, theshell will bulge outward and the nozzle will rotate downdue to this shell bulging (see Figure 7.7). Over a periodof time, the tank may also settle down into its foundationwith respect to the pipe. Because of these expected tank

movements, it is often necessary to use a flexible-typepipe support located near the tank nozzle to ensure thatthe tank nozzle is not overloaded.

Tank Nozzle

Figure 7.7

• It may be necessary to consider pipe frictional effects atsupport points. If large enough, friction loads can restrictpipe movement and cause unexpectedly high pipestresses or end point reaction loads. Typical situationswhere it may be necessary to consider friction loads arefor long horizontal pipe runs, or where large concentratedweight loads are supported near equipment nozzles.

• The most common configuration for air-cooled heatexchanger piping uses short, straight sections of pipe toconnect the manifold to the exchanger nozzles. Themanifold is located directly above or below the exchanger

header box. The heat exchanger tube bundle is allowedto move laterally to accommodate the thermal expansionof the pipe manifold. The flexibility analysis shouldinclude the restraining effect of friction from movement of the exchanger bundle, which will resist lateral movementof the bundle.

NOZZLESHELL

BOTTOM



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VIII. FABRICATION, ASSEMBLY, AND ERECTION

Individual sections of pipe must be fabricated into convenient sections (i.e., spoolpieces). Individual spool pieces are then assembled and erected in the field.

A. Welding and Heat Treatment

Welding is one of the primary ways of joining pipe. Welded jointsrepresent the ultimate in safety and reliability. All design codes callfor welding to be carried out using a qualified procedure andwelders. Included in the welding procedure are: base-metalspecification, electrode type and material, joint preparation (i.e.,geometry), weld position (e.g., vertical, overhead, etc.), weldingprocess (including whether it is manual or automatic), techniques,

electrical details, preheat and interpass temperatures, and post-weld heat treatment (PWHT) requirements.

1.0 Butt-Welds

Butt-welds are made between two components whose edgesare in close proximity. Butt-welded joints in piping systemsare primarily of the single-V configuration and are weldedfrom the pipe outside surface. The joint preparation and theprocedure that is used ensure that there is complete fusionbetween the edges of the components being joined. J ointdesigns shown in Figure 8.1 are typically used for ends of equal thickness. The transition between ends of unequalthickness may be accomplished by taper grinding the thickerpipe to match the thinner, or by using weld metal to providea smooth transition as shown in Figure 8.2. Butt-welds arealways used to weld pipe ends together, to weld butt-weld-type flanges or fittings to pipe ends, or to weld the edges of formed plate together when plate is used to manufacturepipe.



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(a) Standard End Preparation

of Pipe(b) Standard End Preparationof Butt-Welding Fittings and

Optional End Preparation of Pipe 7/8 in. and Thinner

(c) Suggested End Preparation,Pipe and Fittings Over 7/8 in.

Thickness

Butt-Welded Joint DesignsEqual Thickness

Figure 8.1

(b)

(d)

(c)

3/32 in. max.(a)

Butt-Welded Joint DesignUnequal Thickness

Figure 8.2

2.0 Fillet Weld

The fillet weld generally requires no special joint preparation.It is an angular weld bead that joins components normallypositioned at a 90° angle to each other. The size of a filletweld is stated as a leg length of the largest inscribed rightisosceles triangle. In piping systems, fillet welds are onlyused for slip-on flanges, socket welds, and for welding



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attachments to piping components (e.g., reinforcing pads,supports, etc.). See Figure 8.3.

Fillet Welds

Figure 8.3



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3.0 Welding Preparation Steps

The following outlines the overall steps that are required forwelding.

• The individuals and equipment executing the weldingprocedure must be confirmed to be qualified to produceacceptable results.

• Internal and external surfaces to be welded shall beclean and free from paint, oil, rust, scale, or othermaterial that would be detrimental to either the weld orbase metal when heat is applied.

• The ends of the components to be welded must be set tothe correct geometric shape suitable for the materials,

wall thickness, and welding process involved.

4.0 Preheating

Preheating is used, along with heat treatment, to minimizethe detrimental effects of high temperature and severethermal gradients that are inherent in welding. The followingidentifies the benefits of preheating:

• Dries the metal and removes surface moisture whichcould result in weld porosity.

• Reduces the temperature difference between the basemetal and the weld to reduce the cooling rate of theweldment. This lowers the weld hardness and reducescooling/shrinkage stresses.

• Helps maintain the weld pool molten longer to permitmaximum separation of impurities.

• Helps drive off absorbed gases (e.g., hydrogen) whichcould contribute to weld porosity.

5.0 Postweld Heat Treatment (PWHT)

PWHT averts or relieves the detrimental effects of hightemperature and severe temperature gradients that areinherent in welding, and relieves residual stresses that arecreated by bending and forming. Specific heat treatmenttemperature and procedure requirements are specified in



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ASME B31.3 based on the pipe material and wall thicknessbeing joined.

The following summarizes the principal reasons for PWHT:

• Stress relief is the most common reason for specifyingPWHT. This is the only consideration for the PWHTrequirements specified in ASME B31.3. Other reasons forPWHT (e.g., due to process considerations) must bespecified by the user or contractor. Residual stresses willremain in the pipe and result from shrinkage as the weldand adjacent pipe metal cool down from elevated weldingtemperatures. Residual stresses will also remain afterbending or forming processes. If these residual stressesare too high, they can lead to premature failure of thepipe.

• After welding the normal grades of stainless steels (i.e.,those that are not stabilized with alloy additions), thematerial must be heat treated to restore its maximumcorrosion resistance.

• PWHT is required to prevent caustic embrittlement of welded carbon steel pipe that handles alkaline solutions.Caustic embrittlement is a form of stress corrosion wherethe residual stresses due to welding are sufficient tocause failure.

• PWHT is sometimes necessary to reduce weld hardnessin certain materials. Minimizing weld hardness reducesthe tendency to crack, especially in certain processenvironments (e.g., wet H2S).

B. Assembly and Erection

Additional piping fabrication requirements must be considered.Several of these are discussed below.

1.0 Storage and Handling

Improper handling and storage of pipe materials and weldingfiller metals can cause damage and result in poorconstruction quality and failures during operation.



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• Pipe should not be stored directly on the ground to helpprevent rainwater accumulation around the pipe, whichcould result in corrosion.

• Pipe should not be stacked so high that pipes or theircoatings may be damaged.

• Fittings and valves should be stored in shipping crates oron racks to provide protection until used.

• End protectors should be firmly attached to preventdamage to weld bevels, flange faces, threads, or socket-weld ends.

• Lined and coated pipes and fittings should be lifted withwide fabric or rubber-covered slings and padding toprevent damage.

2.0 Pipe Fitup and Tolerances

Good joint fitup is essential to making a sound weld andminimizing the loads imposed on the piping system andconnected equipment. Depending on the welding processused, a slight mismatch may be permissible.

• Pipe fitup for welded joints shall be as required by thewelding procedure.

• The tolerance for axial dimensions, face-to-face, center-

to-face, and location of attachments should be ±1/8 in.maximum.

• Flattening of bends, measured as the difference betweenthe largest and smallest outside diameter at any cross-section, should not exceed 5% of the nominal diameter of the pipe (3% at the ends).

• Lateral translation of branches and connections fromcenterline of run should not exceed ±1/16 in.

• Flange bolt holes shall straddle the centerlines. Rotation

of flanges, measured as the offset between elevation of bolt holes on opposite sides of a flange centerline, shouldnot exceed ±1/16 in.

• The tilt of flanges measured at the periphery across anydiameter should not exceed 1/32 in. from the squareposition. Use of a 1/64 in. tolerance is often necessaryfor flanges at load-sensitive equipment.



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3.0 Alignment of Pipe Attached to Load-SensitiveEquipment

Special care must be taken for load-sensitive equipment,especially rotating equipment. Specifically, in attaching pipe

to rotating equipment, the installation should avoid puttingexcessive forces and moments on the machinery nozzleswhich could result in misalignment.

• Installation of piping that is connected to rotatingequipment should preferably start at the machine nozzleflange. This will reduce the possibility of having a largemismatch between the pipe and machine flanges if pipeinstallation is begun from the opposite end of the system.

• Bolt on succeeding pipe sections as appropriate up to thefirst support. Adjust this support as required to just

contact the pipe at its bearing point. Proceed to anyother adjacent supports which should be similarlyadjusted.

• One or more field welds are typically used to join thepiping nearest to the machine with the rest of the system. The number and location of these field welds aredetermined such that they will permit final positionadjustments to achieve acceptable flange alignment atthe machine nozzle.

• Spring supports should be locked in their cold positionduring pipe installation.

• All spring supports will be adjusted in the locked position just until they contact their respective support points. If spring-support adjustment is insufficient, modifications toassociated structural members or shimming will berequired.

• Final bolt tensioning of component flanges close to themachinery should be done after initial alignment of nozzleflanges.

• Piping that requires any sections to be removed forflushing after completing field welds should have finalnozzle alignment and component flange boltupcompleted after replacing flushed sections.

• For piping over NPS 3 connected to machinery, flangealignment must be within more stringent limits than is



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specified for general piping systems. More stringentlimits are required to minimize the loads that are imposedby flange boltup.

• Precautions should be taken to prevent ingress of debrisinto machine internals during construction of connectingpipework.

4.0 Flange Joint Assembly

Flange joint assembly procedures directly affect the ability of the flange to be leak-tight in service. In many low-pressure,low-temperature, and/or nonflammable services, many rulesof good flanged joint design and makeup can and have beenviolated with no adverse consequences. However, it isdangerous to break these rules in critical, high-temperatureservices since the results can be serious leakage problemswith consequent fires. The primary factors for successfullymaking up a flanged joint and controlling leakage are thefollowing:

• Proper selection and design of the flanged joint.

• Proper preparation, inspection, and installation of theflanged joint.

• Identifying and controlling the causes of leakage.

Flanged joint assembly and leakage control are discussedbelow.

5.0 Flange Preparation, Inspection, and Installation

The following discusses the primary steps that are requiredto achieve a properly assembled flanged joint.

• Redo Damaged Surfaces. Warped or badly corrodedflanges should be replaced or refaced. Reface flangeswith tool marks or scratches across the gasket seating

surface.

• Clean Faces. All gasket and flange surfaces should beclean. Remove all burrs, rust, and dirt from flange faceswith scrapers or wire brushes.



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• Align flanges. Flanges at rest should be within thealignment tolerances previously discussed, with theflanges practically mating before the bolts are installed.

Bringing the flanges into alignment should not leave any

residual stresses in the piping system. Residual stressescould lead to flange leakage in service or overloadproblems in systems that are connected to load-sensitiveequipment. This becomes more important withincreasing pipe diameter, as the residual stress increaseswith increasing diameter for the same amount of misalignment.

• Lubricate Threads and Nuts. Lubricate the bolt threadsand the nut faces where they will contact the flange.Lubrication helps increase the amount of bolt load that

goes into tightening the flange rather than intoovercoming friction.

• Place Gasket Properly. The gasket must be centered onthe flange faces to achieve a reliable joint, but holding thegasket in place can be a problem. If something must beused to hold the gasket, a high-temperature grease maybe used sparingly in systems that operate at less than200°F. No grease, paste, or adhesive should be used tohold gaskets for systems operating at 200°F or more. The high temperature causes these materials to burn off,which could damage the gasket and cause leakage.

Thin cellophane tape may be used on the outside edgesof a gasket, but never on the seating surfaces. Tape onthe seating surfaces will deform the gasket during jointassembly, burn out at operating temperature, and thusprovide a leakage path. Centering rings on spiral-woundgaskets help by allowing the gasket to be supported inthe proper position by a few bolts while the other boltsare inserted. Sheet gaskets should be cut so that theiroutside diameter corresponds to the bolt position, againto help centering.

• Use Proper Flange Boltup Procedure. Flanges may bemade up using a wrench and hammer, an impact wrench,a torque wrench, or a stud tensioner. The most importantaspects of a proper boltup procedure, regardless of method, are to:



149

- Use a "criss-cross" pattern bolt-tightening sequence,as is used when bolting a wheel onto a car. Thisapproach helps to achieve a uniform bolt load aroundthe flange. See Figure 8.4.

- Use at least three rounds of tightening around the

flange, increasing the applied load in each round,with two rounds at the maximum load. Thisapproach helps achieve uniform bolt load around theflange circumference.

- For the most critical high-temperature or high-pressure flanges, use a method that permitsmeasuring the applied load (i.e., torque wrench orstud tensioner). In this way, there is greaterassurance that uniform bolt load is achieved. Forsuch applications, a maximum stud stress during

boltup of 40-50,000 psi is the normal target.

6.0 Causes of Flange Leakage

Most of the primary causes of flange leakage are directlyrelated to poor inspection or installation. These aresummarized below:

• Uneven Bolt Stress. An incorrect boltup procedure orlimited working space near one side of a flange can leavesome bolts loose while others crush the gasket. This is

especially troublesome in high-temperature services,when the heavily loaded bolts relax during operation.

• Improper Flange Alignment. Improper flange alignment,especially nonparallel faces, causes uneven gasketcompression, local crushing, and subsequent leakage.

• Improper Gasket Centering. If a gasket is off-center, itwill be unevenly compressed and more prone to leakage.

• Dirty or Damaged Flange Faces. Dirt, scale, scratches,protrusions, or weld spatter on gasket seating surfaces

provide leakage paths or can cause uneven gasketcompression that results in leakage.

• Excessive Loads in the Piping System at FlangeLocations. Excessive piping system forces and momentsat flanges can distort them and cause leaks. Commoncauses of this are inadequate flexibility, using excessive



150

force to align flanges, and improper location of supportsor restraints.

• Thermal Shock. Rapid temperature fluctuations cancause flanges to deform temporarily and leak.

• Improper Gasket Size or Material. Using the wronggasket size or material can cause leakage.

• Improper Flange Facing. A rougher flange-surface finishthan specified for spiral-wound gaskets can result inleakage.

Typical "Criss-Cross" Bolt-Tightening Sequence

Figure 8.4



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IX. QUALITY CONTROL

A. Inspection

Prior to initial operation, each piping installation, including individualcomponents and overall workmanship, shall be examined. Thefollowing requirements are based on ASME B31.3.

Defects must be identified before a piping system can be tested orgo into operation. Defect identification is especially important inwelded areas. A good weld starts with a proper design and isexecuted using a qualified procedure and welder. However, thequality that is achieved in a particular instance may not beacceptable for a variety of reasons. The method of weld

examination needed to ensure that welds of acceptable quality areachieved must be specified. Not all welds are inspected in thesame manner. Determining the proper type of weld inspection is afunction of technique, weld type, anticipated type of defect, locationof weld, and pipe material.

The following are common weld defects (illustrated in Figure 9.1):

• Lack of fusion between adjacent weld passes.

• Lack of fusion between weld bead and base metal.

• Incomplete penetration due to internal misalignment.

• Incomplete penetration of weld groove.

• Concave root surface.

• Undercut.

• Excess external reinforcement.

• Cracks.

Table 9.1 summarizes the primary weld inspection methods, wherethey are typically used, and the types of defects they can locate.



152

Typical Weld Imperfections

Figure 9.1



153

Type of Inspection Situation/Weld Type Defect

Visual All welds • Minor structural welds

• Cracks

• Slag inclusionsRadiography • Butt welds

• Girth welds

• Miter groove welds

• Gas pockets

• Slag inclusions

• Incomplete penetration

Magnetic Particle • Ferromagnetic materials

• For flaws up to ¼ in.beneath the surface

• Cracks

• Porosity

• Lack of fusion

Liquid Penetrant • Ferrous and nonferrous

materials• Intermediate weld passes

• Weld root pass

• Simple and inexpensive

• Cracks

• Seams

• Porosity

• Folds

• Inclusions

• Shrinkage

• Surface defects

Ultrasonic Confirms high weld quality inpressure-containing joints

• Laminations

• Slag inclusions in thick plates

• Subsurface flaws

Guidelines for Weld Inspection

Table 9.1

The following inspection guidelines also apply:

• ASME B31.3 specifies weld examination requirements andacceptance criteria based on fluid service category (i.e.,Normal, Severe Cyclic Conditions, and Category D fluidservices).

• For P-Nos. 3, 4, and 5 materials, examination shall beperformed after heat treatment. Thus, any defects caused byheat treatment will be present.

• For a welded branch connection, the examination of and anynecessary repairs to the pressure-containing weld shall becompleted before any reinforcing pad or saddle is added. Thus, the reinforcement will not prevent inspection and repair.



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• At least 5% of all fabrication shall be visually examined.

• 100% of fabrication for longitudinal welds, except incomponents made in accordance with a listed specification,shall be visually inspected.

• Random visual examination of the assembly of threaded,bolted, and other joints.

• Random visual examination during the erection of piping.

• Not less than 5% of circumferential butt- and miter-groovewelds shall be examined fully by random radiography orrandom ultrasonic examination.

• Not less than 5% of all brazed joints shall be examined, by in-process examination.

• Piping in severe cyclical service requires additionalexamination.

B. Testing

The piping system must be pressure tested after it has beencompletely fabricated, erected, and inspected. The pressure testdemonstrates the mechanical integrity of the system before it isplaced into operation. The following highlights several testrequirements.

• A hydrostatic test must be used unless otherwise approved forspecial situations.

• The hydrostatic test pressure at any point in a metallic pipingsystem shall be as follows:

a) Not less than 1½ times the design pressure.

b) For design temperatures that are above the testtemperature, the minimum test pressure shall be calculatedas follows, except that the value of S T/S shall not exceed

6.5:

S

PS5.1P T

T =



155

Where:

P T = Minimum hydrostatic test pressure, psig

P = Internal design pressure, psig

S T = Allowable stress at test temperature,

psi

S = Allowable stress at design temperature,psi

c) If the test pressure as defined above would produce astress in excess of the yield strength at testtemperature, the test pressure may be reduced to themaximum pressure that will not exceed the yieldstrength at test temperature.

• Pneumatic strength tests, when approved, shall be conducted at110% of the design pressure.

• Instrument take-off piping and sampling system piping, up tothe first block valve, shall be strength tested with the piping orequipment to which it is connected.



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X. OTHER CONSIDERATIONS

A. Nonmetallic Piping

The following highlights several aspects of nonmetallic pipingdesign. Refer to ASME B31.3 for additional details.

Examples of nonmetallic piping include:

• Thermoplastic Piping. Piping fabricated from a plastic whichis capable of being repeatedly softened by an increase of temperature and hardened by a decrease of temperature.

• Reinforced Thermosetting Resin Piping (RTR). Piping

fabricated from a resin capable of being changed into asubstantially infusible or insoluble product when cured at roomtemperature, or by application of heat, or by chemical means.

Some differences in the design of nonmetallic piping vs. metallicpiping in normal fluid service include:

• Allowances for variations of pressure or temperature, or both,above design conditions are not permitted. The most severeconditions of coincident pressure and temperature will be usedto determine design conditions.

• Piping systems shall be designed to prevent thermal expansionor contraction, pressure expansion, or movement of pipingsupports and terminals from causing:

- Failure of piping supports from overstrain or fatigue.

- Leakage at joints.

- Detrimental stresses or distortions in connected equipment.

• The stress-strain behavior of most nonmetals differsconsiderably from that of metals. Therefore, the assumptions

that stresses throughout the piping system can be predictedfrom strains, or that displacement strains will produceproportional stress because of fully elastic behavior of the pipingmaterials, are generally not valid.

• In addition to the requirements of flexibility and support formetallic piping in normal fluid service:



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- Nonmetallic piping shall be supported, guided, andanchored to prevent damage to the piping.

- Point loads and narrow areas of contact between piping andsupports shall be avoided.

- Suitable padding shall be placed between piping andsupports where piping damage may occur.

- Valves and equipment that would transmit excessive loadsto the piping shall be independently supported.

• Thermoplastics shall not be used in flammable fluid serviceabove ground and shall be safeguarded when used in most fluidservices.

• Nonmetallic piping is joined by bonding. Bonding can beachieved through many methods including adhesive, wrapping,

heat fusion, hot gas welding, and solvent cementing.

B. Category M Fluid Service

The following highlights several aspects of Category M fluidservice. Refer to ASME B31.3 for additional details.

Category M defines a fluid service in which the potential forpersonnel exposure is judged to be significant, and a singleexposure to a very small quantity of the toxic fluid can cause

irreversible harm to breathing or points of bodily contact. Thefollowing highlights several provisions, in addition to those specifiedfor normal fluid service, that apply to Category M Fluid Service.

• Design, layout, and operation of piping shall be conducted tominimize impact and shock loads.

• Conditions which could lead to detrimental vibration, pulsation,or resonance effects should be avoided or minimized.

• No allowances may be made for pressure-temperaturevariations. The coincident pressure-temperature conditions

requiring the greatest wall thickness or the highest componentrating will determine design temperature and pressure.

• All fabrication, as well as all threaded, bolted, and othermechanical joints, shall be visually examined.

• A sensitive leak test in addition to the required leak test mustbe included.



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• The following may not be used:

- Miter bends not designated as fittings, fabricated laps, andnonmetallic fabricated branch connections

- Nonmetallic valves and specialty components

- Threaded nonmetallic flanges

- Expanded, threaded, and caulked joints

C. High Pressure Piping

The following highlights several aspects of high pressure pipingdesign. Refer to ASME B31.3 for additional details.

Design Conditions and Criteria

Piping is generally considered to be high pressure if it has apressure over that allowed by Class 2500 for the specificdesign temperature and Material Group. However, there areno specific pressure limitations for the application of therules for high pressure piping.

In most cases, the design pressure of each component in ahigh pressure piping system must be at least equal to thepressure at the most severe condition of coincident internalor external pressure and temperature expected duringservice. The design temperature of each component in apiping system is the temperature at which, under thecoincident pressure, the greatest thickness or highestcomponent rating is required.

Consideration must be given to the ambient effects on apiping system.

• The cooling of a gas or vapor may reduce the pressuresufficiently to create a vacuum.

• The heating of a static fluid in a piping component causesa pressure increase.

• Moisture condensation can result in atmospheric icingwhen piping system design minimum temperature is lessthan 32°F.



159

In any case, the design must allow the system to eitherwithstand or provide some type of relief from the ambienteffects.

Other effects to consider include:

• Dynamic Effects (e.g., impact, wind, earthquake,vibration, discharge reactions).

• Weight Effects (e.g., live loads, dead loads).

• Thermal Expansion and Contraction Effects.

• Effects of Support, Anchor, and Terminal Movements.

• Allowable stresses.

• Wall thickness calculation requirements.

• No allowance for pressure above the design pressurepermitted.

• Particular fabrication details not permitted (e.g., miters).

2.0 Examination

While the examination of High Pressure Piping is very similarto that of piping in normal fluid service, it must be moreextensive. For example, in normal fluid service, a sample

selected at random per the inspector's judgement issufficient to make a determination as to the acceptability of the material. In high pressure piping, 100% of the materialand components must be examined. Also, only 5% of thefabrication must be examined for normal fluid service,whereas 100% of fabrication must be examined in highpressure piping.

3.0 Testing

Prior to initial operation, each piping system shall be either

hydrostatically or pneumatically leak tested. Each weld andeach piping component (except bolting and individualgaskets to be used during final assembly) shall be tested. If the testing is done on the equipment prior to installation, anadditional test of the installed piping system shall beconducted at a pressure not less than 110% of the designpressure. If the initial testing is done on the installed piping,then the additional test is not necessary.



XI. SUMMARY

A process plant piping system includes much more than just straight sections of pipe. It also includes fittings, flange assemblies, valves, pipe supports, and

restraints. ASME B31.3 specifies the design, materials, fabrication, erection,inspection, and testing requirements for process plant piping systems. Thiscourse provided an overview of process plant piping system requirements,including items that are not explicitly included in B31.3 (e.g., valve selection anddesign, flexibility analysis guidelines, equipment nozzle load requirements, etc.).Participants can use this information on their jobs, and are prepared to take moreextensive courses if appropriate.

Process Plant Piping System Design Document

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