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Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco. Chapter : Piping & Valves For additional information on this subject, contact File Reference: MEX10107 K.S. Chu on 8732648 or R. Hingoraney on 873-2649 Engineering Encyclopedia Saudi Aramco DeskTop Standards Piping Layout, Support, And Flexibility

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Piping Layout, Support, And Flexibility

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Page 1: Piping Layout, Support, And Flexibility

Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramco’semployees. Any material contained in this document which is notalready in the public domain may not be copied, reproduced, sold, given,or disclosed to third parties, or otherwise used in whole, or in part,without the written permission of the Vice President, EngineeringServices, Saudi Aramco.

Chapter : Piping & Valves For additional information on this subject, contactFile Reference: MEX10107 K.S. Chu on 8732648 or R. Hingoraney on 873-2649

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Piping Layout, Support, And Flexibility

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Piping Layout Support & Flexibility

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CONTENTS PAGE

IDENTIFYING OPERATIONAL, MAINTENANCE, AND SAFETY

FACTORS THAT INFLUENCE PIPE LAYOUT ............................................................1

Operations Requirements...............................................................................................1

Clearances Above and Below Piping.............................................................................1

Location of Shutoff and Control Valves, Sample Points, Vessel Flanges .....................2

Economy........................................................................................................................4

Corrosion .......................................................................................................................4

Maximum Use of Existing Pipeways and Supports.......................................................5

Pump and Compressor Piping .......................................................................................5

Thermal Expansion Clearances .....................................................................................6

Facilitate Support and Restraint.....................................................................................6

Maintenance Requirements ...........................................................................................7

Access Clearance For Maintenance Equipment and Vehicles.......................................7

Removal of Equipment Requiring Maintenance............................................................7

Heat Exchanger Bundle Removal..................................................................................7

Safety Considerations ....................................................................................................8

Access Clearance for Firefighting Equipment ...............................................................8

Locating Pipeways to Prevent Injuries to Personnel......................................................8

Separating Hazardous Piping From Piping Vital to Safety............................................8

THE FUNCTION OF THE DIFFERENT TYPES OF SUPPORTS

AND RESTRAINTS USED IN PLANT AND TRANSPORTATION PIPING................9

Supports and Restraints ...............................................................................................10

Rigid Supports .............................................................................................................10

Hangers........................................................................................................................12

Flexible or Resilient Supports......................................................................................13

Restraints .....................................................................................................................15

Guides..........................................................................................................................15

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Anchors........................................................................................................................17

Sample Problem 1........................................................................................................18

Solution........................................................................................................................20

DETERMINING THE MAXIMUM SUPPORT SPACING BASED

ON WEIGHT AND DEFLECTION CRITERIA AND DESIGN LOADS......................21

Piping Weight Stress and Deflection Criteria ..............................................................22

Stress Criteria...............................................................................................................22

Deflection Criteria .......................................................................................................22

Determining the Maximum Allowable Span ...............................................................23

Maximum Span Tables................................................................................................24

Sample Problem 2........................................................................................................24

Solution........................................................................................................................24

Loads on Supports .......................................................................................................26

Requirements for Pads and Saddles.............................................................................27

Prevention of Wind-Induced Vibration .......................................................................27

DETERMINING THE NEED FOR A PIPING THERMAL FLEXIBILITY

WEIGHT ANALYSIS.....................................................................................................29

Rationale and Approaches for Piping Flexibility and Support Design ........................30

Approaches ..................................................................................................................32

Guidelines for Whether to Perform a Thermal Flexibility and Weight Analysis.........33

Saudi Aramco Flexibility Requirements......................................................................36

DETERMINING THE REQUIRED DESIGN CONDITIONS

FOR A THERMAL FLEXIBILITY/WEIGHT ANALYSIS ...........................................37

Design Conditions .......................................................................................................37

Saudi Aramco Requirements .......................................................................................38

Piping Flexibility Temperature....................................................................................41

Number of Cycles to be Considered ............................................................................43

Load Limitations On Equipment .................................................................................44

Analysis Considerations for Rotating Equipment........................................................45

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Considerations for Stationary Equipment....................................................................46

Extent of Analysis........................................................................................................47

Providing Additional Thermal Flexibility....................................................................48

Sample Problem 3........................................................................................................49

Solution........................................................................................................................49

USE OF ANCHORS FOR BURIED PIPING SYSTEMS AND DESIGN TOOLS........51

Pipeline Forces to Be Resisted.....................................................................................51

Types of Anchors.........................................................................................................53

Forces Available to Resist Anchor Movement ............................................................54

WORK AID 1: CRITERIA FOR DETERMINING MAXIMUM

SUPPORT SPACING......................................................................................................55

GLOSSARY....................................................................................................................57

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IDENTIFYING OPERATIONAL, MAINTENANCE, AND SAFETY FACTORS THATINFLUENCE PIPE LAYOUT

Operational, maintenance, and safety considerations are the primary factors that influence thelayout of a piping system. The Saudi Aramco engineer must recognize these factors whendesigning the layout and spacing of piping and equipment. This section discusses how thesefactors influence piping layout.

Operations Requirements

Operating and control points such as valves, flanges, instruments, sample points, drains, andvents should be located so that these components can be used safely and easily. For example,when specifying the location of valves, the engineer must ensure that the valves can bereached.

Clearances Above and Below Piping

There must be enough clearance above and below the pipe to perform some basic operationson valves and flanges. Pipe needs to be elevated above grade because:

• Flanges extend from the pipe.

• Contact with the ground causes corrosion.

Clearances above the pipe are necessary:

• For opening and closing valves.

• To manually operate and/or replace equipment.

SAES-L-011, Flexibility, Support, and Anchoring of Piping, Paragraph 3.1, specifiesclearance requirements as follows:

• Above-grade piping shall be supported to provide a clearance between the bottom ofpipe and the finished grade not less than the following:

- In plant areas and where the grade under the pipe is a hard surface: 300 mm (12in.).

- Outside of plants without nearby unstabilized sand dunes: 450 mm (18 in.).

- In areas with moving sand dunes: 0.9 m (3 ft.).

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The minimum clearance between buried pipelines at crossings, and between a pipe, flange, orvalve and any structure not used to support the piping, shall be 0.3 m (1 ft) over a length up tothree pipe diameters, or a minimum of 0.6 m (2 ft) if the length is longer. Clearances betweenaboveground piping and structures shall provide reasonable access for inspection and freedomof interference in case of pipe movement.

Location of Shutoff and Control Valves, Sample Points, Vessel Flanges

There must be enough space to access valves, sample points, vessel flanges, and otherequipment that may require manual operation.

• SAES-L-020, Design of Transportation Piping Systems, Paragraphs 6.1 to 6.3, specifyspacing requirements for the following system components: mainline valves, checkvalves, vents and drains.

Mainline Valves – Based on Sections 434.15 of ASME/ANSI B31.4 and Section 846 ofASME/ANSI B31.8, the maximum spacing between mainline block valves shall be asfollows:

a) For all fluid services in Class 1 locations (SAES-B-064), 32 km (20 miles);b) For pipelines of 750 mm (30 in.) NPS or larger, in any liquid service in Class 1 or 2

locations, valves shall be provided such that not more than 14,300 m3 (90,000 barrels)of liquid can gravitate from any section between two closed block valves;

c) For all hydrocarbon and chemical fluid services in Class 2 locations, 24 km (15 miles);d) For gas, multiphase crude and liquids with an absolute vapor pressure above 450 kPa

(65 psia) in Class 3 locations, 12 km (7.5 miles);e) For hydrocarbon and chemical fluids other than in d) in Class 3 and 4 locations, 16 km

(10 miles); andf) For fluids as in d) in Class 4 locations, 8 km (5 miles).g) For sour water, seawater, brine, and other contaminated or chemically treated waters in

Class 2 locations, 24 km (15 miles): in Class 3 and 4 locations, 16 km (10 miles).h) For water services other than in g) in all locations, 32 km (20 miles).

In the event that the length of a location class is less than the maximum spacing of blockvalves, the maximum section length shall be the weighted average of the distances that arespecified for the classes involved, provided that the pipe in the entire section meets therequirements for the higher class. (Refer to ADP-L-020.)

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Check Valves – A check valve shall be installed in each branch near the intersection point,and near the upstream end of pipelines that are in hazardous service. The check valve willprevent backflow in case of an upstream line rupture or an emergency in the plant that feedsthe pipeline. If the pipeline is designed for bidirectional flow service, a check valve and aparallel block valve shall be installed. If a check valve was installed on an existing outgoingpipeline which will be converted to bidirectional flow, the block valve is left on the main line.For the design of a new pipeline in bidirectional flow, the check valve shall be installed on thebypass line.

Vents and Drains – Permanent vents and drains with plugged or blinded valves, and provisionfor blow-down of pipeline sections at selected locations, shall be installed only as required bythe operating department for emergency conditions. Temporary vent valves, if requiredduring the initial filling of the connection, are plugged and seal welded after the test.

Layout of Piping Associated with Scraper Traps

A scraper trap is a device that is used for internal cleaning, gauging, inspection, or batching ofa piping system. Operators need enough space around the scraper trap to remove and reinsertscrapers. Saudi Aramco Standard Drawing AC-036541 shows the layout for a typical scrapertrap. SAES-L-045, Paragraph 6.4, also specifies the following with regard to scraper traps forpipelines:

• A suitable work floor shall be provided with sufficient room around the traps for loadingand unloading scrapers and for access with automotive equipment. The clearance betweenthe bottom of the trap and finished grade in onshore plants shall be approximately 1 m (40in.). A surface drainage system shall be provided to collect any spill from the trap andwash water.

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Economy

Piping should follow the most economical route, subject to design and safety requirements.For example, some designers provide too much flexibility, which results in higher materialand fabrication costs.

• SAES-L-012, Design of Piping Systems Inside Plant Areas, Paragraph 5.1, specifiesrequirements for hydraulic considerations.

• The pipe size, layout and supports shall be designed such that all hydraulic requirementsare met and any problems such as may be caused by erosion, cavitation, surges,vibration, noise, slugs in two-phase flow, or undesirable flow patterns shall be avoidedas much as possible.

Corrosion

Areas in which corrosion is likely need corrosion monitoring fittings and/or drop-out spools.

• SAES-L-012, Design of Piping Systems Inside Plant Areas, Paragraph 5.2 , specifiesrequirements for corrosion considerations.

• Carbon steel piping that is in potentially corrosive fluid service shall be provided withcorrosion monitoring fittings and/or drop-out spools subject to the approval of theassigned specialist in the Engineering Services Organization for Project Proposals and/orfinal design. The piping layout shall contain no sections or branches longer than threepipe diameters which are normally or frequently stagnant (dead ends). Corrosive wateror other deposits can collect at the bottom in such nonself-draining sections and causeaccelerated corrosion unless there is adequate internal corrosion protection.

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Maximum Use of Existing Pipeways and Supports

When feasible, piping should utilize existing supports to minimize support costs. SAES-L-012, Design of Piping Systems Inside Plant Areas, Paragraph 5.3, specifies requirements forpipeways.

Pipeways – Above-grade plant piping between various items of plant equipment or betweenseparate units within a plant area shall be projected within pipeway boundaries that areindicated on Plot Plans and Piping Plans, which are laid out so as to provide the necessaryaccess to all areas for operations and maintenance. The elevations of intersecting pipewaysshall normally be at different levels to allow for the installation of additional piping in thefuture. The minimum spacing of lines that are supported on sleepers or pipe racks shall be asshown on Standard Drawing AC-036207.

Pump and Compressor Piping

There must be enough space to operate pumps and compressors. SAES-L-014, Design ofPump and Compressor Station Piping, Paragraphs 3 and 4, specify requirements for suctionand discharge piping.

Suction Piping – The suction piping shall be sized to provide the net positive suction head(NPSH) as required by the pump at maximum flow rate to prevent cavitation. The suctionpipe shall be laid out to provide a balanced flow at the entry of the pump, in particular tohorizontal, double suction and double volute-type pumps. Piping to pumps that operate inparallel shall be laid out in a symmetrical manner to ensure equal distribution of the flow toeach pump.

Suction piping to all pumps shall have a straight length of pipe of at least five times thesuction nozzle diameter immediately upstream of the nozzle.

Long taper reducers shall be used. The top of pump suction piping shall be such that gascannot collect in pockets between the top of the suction header pipe and the inlet nozzle.Eccentric reducers with the flat side on top shall be located upstream of the straight pipelength immediately upstream of the suction nozzle of the pump.

A suction screen shall be provided during the initial operation until no more debris iscollected on the screen.

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Discharge Piping – Discharge piping immediately downstream of the discharge nozzle shallhave a straight length of at least two and one-half times the discharge nozzle diameter.

The discharge piping, including any flow control station and minimum flow bypass piping,shall be designed so as to avoid excessive noise, vibration or erosion. This shall beaccomplished by proper sizing, suitable flow pattern, use of long taper reducers, dampeninganchors, etc.

Thermal Expansion Clearances

When the pipe heats up or cools down, thermal expansion or contraction of the pipe willoccur. Sufficient clearance should be provided between adjacent lines and between lines andstructures to allow for free thermal expansion of the piping without interference.

Facilitate Support and Restraint

Supports and restraints are discussed in greater detail in a later section of this module. Thefollowing considerations affect routing of the piping for favorable support.

• Piping system should support itself as much as possible.

• Piping with excessive flexibility may require restraints to avoid excessive movement orvibration.

• Piping that is prone to vibration, such as compressor suction and discharge piping,should be supported independently.

• Piping in structures should be routed beneath platforms near major structural members,at points that are favorable for added loading, to avoid increasing the size of structuralmembers.

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Maintenance Requirements

The piping system must be laid out so that its components can be inspected, repaired, orreplaced with minimum difficulty.

Access Clearance For Maintenance Equipment and Vehicles

There must be ample clearance for maintenance equipment, such as cranes, and for vehicles,such as trucks. Access must be provided so supports can be maintained.

Removal of Equipment Requiring Maintenance

There must be enough space to access and remove large pieces of equipment if they requiremaintenance.

• Access near rotating equipment is important because cranes must reach the equipmentwhen removal or realignment is required.

• Heat exchanger bundles need to be pulled out for cleaning.

• Large valves must be removed to repair or replace their seats.

• Rotating equipment requires frequent monitoring and maintenance.

Heat Exchanger Bundle Removal

Clearance must be provided at the end of shell-and-tube heat exchangers to permit theremoval of tube bundles for cleaning and alignment. These tube bundles are over 6.1 m (20ft.) long and require removal by crane. Piping layout must provide the required clearances.

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Safety Considerations

Piping layout must consider the safety of personnel near the pipe. This specifically includesaccess for firefighting equipment and fire prevention.

Access Clearance for Firefighting Equipment

Firefighting equipment needs clearance to access major pieces of equipment, such as heatexchangers, vessels, and tankage. Pipeways must be routed and designed to provide thenecessary clearances.

Locating Pipeways to Prevent Injuries to Personnel

There must be enough space beneath pipeways for people to walk and work. Typically, 2 m(6 ft.) of clearance beneath a pipeway is sufficient.

Separating Hazardous Piping From Piping Vital to Safety

Firewater piping must be routed so that it would not be damaged by piping containinghazardous fluids that could rupture.

Saudi Aramco Standards SAES-B-007A and SAES-B-007C contain the operational anddesign requirements for firewater piping.

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THE FUNCTION OF THE DIFFERENT TYPES OF SUPPORTS AND RESTRAINTSUSED IN PLANT AND TRANSPORTATION PIPING

A piping system may be supported or restrained in several different ways. The Saudi AramcoEngineer must know the different types of support and restraint in order to properly design apiping system or evaluate a design made by others. The following describes the differenttypes of support and restraint and their functions.

A piping system needs supports and restraints because of the various loads that are imposedupon it. Supports and restraints are often needed to permit the piping system to functionunder normal operating conditions without failure in the pipe itself or associated equipment.Supports are commonly needed to absorb system weight loads in order to keep the sustainedlongitudinal stress in the pipe within allowable limits, or to limit pipe sag to avoid processflow problems. Restraints are used to control or direct the thermal movement of a pipingsystem. The control of thermal movement may be necessary either to keep pipe thermalexpansion stresses within allowable limits, or to limit the loads that are imposed on connectedequipment. Restraints also may be necessary to absorb other loads imposed on a pipingsystem and thus to limit pipe deflection and the resultant stresses. Examples of such loads toconsider are wind, earthquake, slug flow, water hammer, and other dynamic loads.

There are various classes of supports and restraints that are suitable for a particularapplication. Within these classes, there are many different types, such as shoes, saddles, andvertical guides.

Selection of a particular type of support or restraint to use in a particular situation depends onsuch factors as:

• Weight load to be supported or restraint load to be absorbed.

• Clearance available for attachment to the pipe.

• Availability of nearby structural steel that is already there for other purposes.

• The direction of loads to be absorbed or movement to be restrained.

• Design temperature.

• The need to permit vertical thermal movement at a support.

Selection of specific support and restraint designs will generally require some degree ofdetailed engineering which is beyond the scope of this course. The following discusses themajor classes of supports and restraints and several specific types within each class.

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Supports and Restraints

Supports sustain a portion of the piping weight and any superimposed vertical loads. Theweight comes from the pipe itself, its contents, insulation or lining (if any), and other pipingcomponents such as valves, flanges, etc. Restraints are devices which prevent, resist, or limitthe free thermal movement of piping, or absorb other applied loads so that they do not have adetrimental impact on the pipe or connected equipment. Depending on the particularsituation, a combination of support and restraint types may be installed at one location.

There are two general classes of supports: rigid supports and flexible or resilient supports.

Rigid Supports

Rigid supports are used in situations where weight support is needed and no provision topermit vertical thermal expansion is required. A rigid support always will prevent verticalmovement downward, also will prevent vertical thermal movement upward sometimes, andwill permit lateral movement and rotation. A rigid support is the more common of the twosupport classes.

Figure 1 provides some samples of rigid supports. The use of any particular type availabledepends primarily on the magnitude of the load to be carried, the point of attachment to thepipe (i.e., horizontal or vertical run, elbow, etc.), and the distance to available supportstructure, or grade. For example:

• Two different shoe support concepts are shown. The choice between the two dependson the pipe diameter and the load to be carried. The design with the single verticalmember would be used with small diameter, lightly loaded pipe. The design with twovertical members spreads the applied load over a larger portion of the pipe wall, reducesthe local stress in the pipe wall, and would be used for larger pipe diameters and greaterloads.

• Designs that employ a trunnion arrangement must also consider the bending momentthat is imposed on the pipe resulting from the weight load being supported and appliedat the end of the moment arm. Because of this, the trunnion length must be kept as shortas possible to minimize the bending moment that must be designed for.

• Pipe supports are often made using sections of pipe to provide support, rather thanstructural members. This type is called a "dummy" support, indicating that there is noflow in the pipe section that is providing the support.

• A saddle-type support uses a reinforcing plate that is first welded to the underside of thepipe. This reinforcing plate will normally have an included angle of 90° to 120°, alongthe pipe circumference, that distributes the weight load over a wider portion of the pipeand reduces the local pipe stress.

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RIGID SUPPORTS

Shoe Saddle

Base Adjustable Support Dummy Support

Trunnion

Source: Piping Stress Handbook, Second Edition by Victor Helguero M. Copyright ©1986 by Gulf PublishingCompany, Houston, Texas. Used with permission. All rights reserved.

FIGURE 1

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Hangers

Hangers support pipe from structural steel or other facilities that are located above the pipeand carry piping weight loads in tension. Pipe hangers, as shown in Figure 2, are typicallyone or more structural steel rods bolted to a pipe attachment and to the overhead member. Ahanger rod is designed to move freely both parallel and perpendicular to the pipe axis, and notto restrict thermal expansion in these directions. A hanger will prevent movement both downand up, and therefore cannot be used to provide support at locations where any verticalthermal movement will occur. Many types and sizes of pipe supports can be found in atypical vendor's pipe hanger catalog.

HANGERS

FIGURE 2

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Flexible or Resilient Supports

Flexible or resilient supports allow the piping system to move in all three directions while stillsupporting the required weight load. Weight is supported in this application by use of a coilspring having an appropriate stiffness to carry the applied weight load. Since the spring isresilient, it will permit vertical thermal movement while still carrying the weight. This type ofsupport is used in situations where support must be provided at a particular location, andvertical thermal expansion must also be permitted. There are two basic types of flexiblesupports: variable load and constant-load-type.

• In the variable-load-type flexible support, the load exerted by the spring on the pipechanges as a result of the pipe thermal movement imparted to the spring. The amount ofthis load change equals the amount of thermal moment multiplied by the spring constantfor the spring. The spring is selected such that it provides the correct amount ofsupporting load to the pipe while considering the thermal movement to be absorbed.This is the more commonly used of the two resilient-support-types.

• In the constant-load type flexible support, the load exerted by the support on the piperemains constant throughout the entire moment range of the support. This isaccomplished by using a pivoting, lever arm mechanism. This type of support is used insituations where the load variation in a variable-load-type spring is too large to beaccommodated by the piping system, or where the thermal movement is over 75 mm (3in.).

Each type of resilient support is selected from standard available models based on designload, required movement, and installation geometry considerations. Their attachments to thepipe and support members are made similarly to other rigid supports and hangers, and may belocated under, over, or to the side of the pipe.

Figure 3 illustrates various flexible supports.

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FLEXIBLE SUPPORTS

Source: Piping Stress Handbook, Second Edition by Victor Helguero M. Copyright © 1986 by GulfPublishing Company, Houston, Texas. Used with permission. All rights reserved.

FIGURE 3

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Restraints

Restraints have two primary purposes in a piping system.

1. Control the unrestricted thermal movement (expansion or contraction) of the pipe bydirecting or limiting it.

A piping system is generally considered to be totally restrained at its end connectionpoints to equipment. Restraints control, limit, or redirect the thermal movement to eitherreduce the thermal stress in the pipe or the loads exerted on equipment connections dueto thermal movement.

2. Absorb loads imposed on the pipe by other conditions.

This includes wind, earthquake, slug flow, water hammer, or flow-induced vibrationwhich could result in excessive pipe stress, equipment reaction loads, or flange leakage.

There are several different types of restraints that may be used. The selection of which typeto use and its specific design details depends primarily on the direction of pipe movement thatmust be restrained, the location of the restraint point, and the magnitude of the load that mustbe absorbed. It is also possible to restrain more than one direction at one location in a pipingsystem, or to combine a restraint with a support. Figure 4 provides several examples ofrestraints.

Guides

A guide is a particular type of restraint. It is used in situations where movement along thepipe axis must be permitted while movement perpendicular to the pipe axis in one or bothdirections must be prevented. Depending on the particular guide details employed, piperotation may or may not be restricted. Common situations where guides are used are in longpipe runs on a pipe rack to control thermal movement and prevent buckling, and in straightpipe runs down the side of a tower to prevent wind-induced movement and control thermalexpansion. Figure 4 provides several examples of guides.

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RESTRAINTS/GUIDES

Guide Guide Guide

Vertical Guide Stop W/ Shoe Section A-A

A

A

FIGURE 4

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Anchors

An anchor is a special type of restraint that stops movement in all three directions. Anchorsprovide full fixation of the pipe, permitting very limited, if any, translation or rotation. Ananchor is used in situations where it is necessary to totally isolate one section of a pipingsystem from another from the standpoint of load and deflection. A total anchor thateliminates all translation and rotation at one location is not used as commonly as one or morerestraints that act at a single location. A directional anchor is used more commonly in plantpiping, which restrains the line only in its axial direction. Figure 5 provides several examplesof anchors.

RESTRAINTS/ANCHORS

FIGURE 5

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Sample Problem 1

Figure 6 illustrates an approximate layout of a pump suction piping system. Pumps P-101A/B take suction from Tower T-101. The pipe diameter is 300 mm (12 in.), the designpressure is 1,034 kPa (150 psig), and the design temperature is 260°C (500°F). It is carbonsteel, welded A53, Gr. B material and standard wall thickness. There are two existingstructural members at Locations 3 and 4. It is necessary to determine the general types ofsupports and restraints required for this system in order to estimate the general needs foradditional structural steel.

The following additional information is also available:

• It is unlikely that the pump nozzles can tolerate the load resulting from thermalexpansion of the 46 m (150 ft.) long North/South horizontal run from T-101.

• There is a 15.9 mm (0.625 in.) upward thermal expansion at Locations 1 and 2.

• There is a 46 mm (1.8 in.) downward thermal expansion at Location 6.

• The T-101 nozzle cannot support the weight load and bending moment from the verticalrun without being overstressed.

• The vertical run up to the T-101 nozzle is exposed to wind loading.

The following is to be determined:

a. What type of support should be used at Locations 1 and 2?

b. What type of support should be used at Location 6?

c. What type of restraint should be used to absorb the wind loading in the vertical run to T-101 at Location 7?

d. What type of restraint should be used to prevent thermal expansion from the 46 m (150ft.) long run from imposing high loads on the pump nozzles, and where might be a goodplace to try placing it?

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SCHEMATIC FOR SAMPLE PROBLEMS

FIGURE 6

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Solution

a. Note that, since Locations 1 and 2 are at the top of vertical runs directly over the pumpnozzles and there is a 15.9 mm (0.625 in.) thermal movement upward at each location,spring-type supports must be used. If a rigid support located under the pipe was used,the pipe would lift off the support due to the thermal expansion and the support wouldnot carry any weight. A hanger-rod-type support would be even less effective, since itwould restrain the upward thermal movement and result in a large compressive loadbeing imposed on the pump nozzles.

b. A spring support should be used at Location 6 also, for similar reasons as for Locations1 and 2. Note that if a rigid support was used at Location 6, it would stop the verticalmovement downward and cause a very high force and bending movement on the T-101nozzle.

c. A North/South and East/West lateral guide should be used in the vertical run at Location7. The guide will absorb the wind load while still permitting free vertical thermalexpansion. The exact location of this guide in the vertical run, or whether there shouldbe more than one, would still need to be determined. Since the guide will stopNorth/South movement, it is also acting against thermal expansion in the 46 m (150 ft.)run. Therefore, its location affects the pipe thermal stresses in this portion of the system.It is a good choice to locate the guide 3 m to 4.6 m (10 ft. to 15 ft.) below the towernozzle.

d. A North/South restraint (i.e., a stop) must be used to prevent thermal expansion in the 46m (150 ft.) long run from imposing large thermal loads on the pump nozzles. Theobjective is to direct this thermal expansion away from the pumps and into the rest of thesystem. For this purpose, the stop should be located relatively near the pumps.Location 4 would be a good first choice, since locating the stop there will isolate bothpumps from the effects of the large, North/South thermal movement.

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DETERMINING THE MAXIMUM SUPPORT SPACING BASED ON WEIGHT ANDDEFLECTION CRITERIA AND DESIGN LOADS

This section discusses criteria to determine how many supports are needed in a pipe section toensure that it does not get overstressed or sag too much. This is based on the weight of thepipe and components, type of fluid service, design pressure and temperature, pipe material,and diameter and wall thickness. Once the maximum span is determined, the engineer candetermine the number of supports that are needed and where they are needed. This will allowhim to calculate the load on each support, and therefore permit doing the detailed design ofeach support. For example, if the load is high, the support must be designed to spread it outover a larger area on the pipe to reduce localized pipe stresses. Depending on the complexityof the piping layout, additional weight loads to be applied, the nature of the fluid service andoperations, the applied loads at support points and the required support spacing may bedetermined by hand calculations, available tables, or by a detailed analysis using a pipingflexibility computer program.

The discussion here will be confined to supports in straight horizontal sections of single-diameter pipe without other weight loads imposed. More complex systems must be evaluatedby using other equations to account for differences in pipe geometry or loading, or a pipingflexibility analysis computer program. Discussion of other hand-calculation techniques isbeyond the scope of this course. The general requirements for a computer analysis arediscussed later in this module. With the general availability and ease of use of computerprograms, use of hand calculations and table solutions are generally confined to relativelysimple systems (i.e., piperack runs or offsite piping systems), or initial screening studies.

Determining the maximum spacing between two supports consists of:

• Establishing stress and deflection criteria.

• Identifying and using the applicable Saudi Aramco or industry table to determine themaximum permitted span.

• If the situation is beyond the limitations of the tables, calculating the maximumpermitted span given stress and deflection criteria, using either hand calculations or acomputer program, as appropriate.

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Piping Weight Stress and Deflection Criteria

Support spacing for horizontal pipes in open areas is governed by the strength of the pipe.Support spacing for pipes in process plants is determined more by the spacing of convenientlylocated structural steel. Spacing of the supports in a pipe rack is usually based on supportingthe weakest pipe, although larger spans are acceptable if sagging and pockets in smaller linesis not objectionable. Small lines can be supported off larger lines, bundled with other smalllines, or increased in size to be self-supporting.

Allowable spans for horizontal lines are influenced by limits on longitudinal stress ordeflection to avoid interference with the nearby pipe or structure, or to avoid excessivesagging that could be detrimental to fluid flow. The span may also be chosen to change thepipe's natural frequency to avoid a resonant-vibration condition.

Stress Criteria

Stress criteria for a particular situation is a function of material, pressure, and temperature.The value for the allowable longitudinal stress is obtained by using the applicableASME/ANSI B31 Code equation and table. The sum of the longitudinal stresses due toweight and pressure must be limited to the pipe material allowable stress.

MEX 101.03 discussed calculation of the required pipewall thickness based on designpressure considerations, and this is based on limiting the pipe circumferential stress to theallowable stress. The longitudinal stress in a pipe due to internal pressure is half thecircumferential stress. Thus, if the pipewall thickness is exactly the value that is required forinternal pressure, then half the allowable stress is still available as a limit for longitudinalweight stress.

Deflection Criteria

Deflection under weight effects is generally of secondary importance in piping just as it is instructures. In most process units, however, the deflection should be kept within reasonablelimits to minimize pocketing of liquids at low points. Appearance may also be a factor. Themaximum deflection is typically limited to the smaller of 25 mm (1 in.) or half the normalpipe diameter, unless a smaller deflection is required due to pocketing concerns.

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Determining the Maximum Allowable Span

The maximum span between two supports is based on the allowable stress and deflectioncriteria. This is determined through two calculations:

• A calculation based on stress limits:

L ≤ 0 . 8 Z f s

W

• A calculation based on deflection limits:

L = EI∆

13. 5 W 4

where: L = Length of span, ft.

fs = 1/2 x allowable stress at design temperature per applicable ASME/ANSIB31 Code. Note that this assumes that the pipewall thickness exactlymatches that required for internal pressure. The longitudinal pressure stressthus also equals half the ASME/ANSI B31 Code allowable stress. This is asimplified assumption, but is conservative for most situations.

∆ = Maximum deflection, in.

W = Weight of pipe, including commodity lining, and insulation if any, lb/ft.

E = Hot modulus of elasticity of pipe at design temperature, psi.

I = Moment of inertia of the pipe, in.4

Z = Section modulus of the pipe, in.3

The values for the weight of the pipe, W, and the section modulus, Z, are obtained from thePipe Properties Table discussed in MEX 101.03. The weight of the pipe must includeconsideration of the pipe material, contained fluid, external insulation, and internal lining, inlb/ft. Determining the weight of insulation and lining is beyond the scope of this course. Theequations used are based on a mean between a uniformly loaded beam simply supported atboth ends and one with both ends fixed.

The maximum allowable span is the lower value that results from the two calculations.

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Maximum Span Tables

Saudi Aramco Standard Drawing AC-036697 also provides maximum allowable spans forunrestrained pipelines based on pipe sizes from 350 mm to 1,500 mm (14 in. to 60 in.) ofspecified thicknesses, maximum allowable internal pressure, and specified wear pad or saddledetails to distribute the load or saddles. This is included in Work Aid 1, and may be used as aconvenience for pipelines that are within its limitations.

Sample Problem 2

Refer again to Figure 6 of Sample Problem 1. It is now necessary to determine if the 10.7 m(35 ft.) support span between Locations 3 and 4 is excessive, and estimate the number ofsupports required in the 45.7 m (150 ft.) North/South run. For this work, assume thefollowing:

• The specific gravity of the liquid in the system is equal to that of water.

• The allowable stress for the pipe material based on ASME/ANSI B31.3 requirements is130.3 MPa (18,900 psi.).

• The Modulus of elasticity at 260°C (500°F) is 188.2 x 103 MPa (27.3 x 106 psi.).

• There is 75 mm (3 in.) of calcium silicate insulation on the pipe. Its weight may beassumed to be 18.9 kg/m (12.7 lb/ft.).

Solution

This problem will be solved using Work Aid 1.

From the Pipe Properties Table in MEX 101.03, obtain the following information:

Weight of pipe = 49.6 lb/ft.Weight of fluid = 49.0 lb/ft.I = 279 in4.Z = 43.8 in3.

Then W = 49.6 + 49.0 + 12.7 = 111.3 lb/ft.

fs = 0.5 x 18,900 = 9,450 psi.

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Stress limit calculation:

L ≤ 0.8Z f s W

= 0.8 x 43.8 x 9450 111.3

L ≤ 54.5 ft

Deflection limit calculation:

L ≤ EI ∆ 13.5W

4

= 27.3 x 10 6 x 279 x 113.5 x 111.3

4

L ≤ 47.4 ft

Thus, the maximum allowable span is 14.5 m (47.4 ft.). Therefore, in the 45.7 m (150 ft.)run:

150 = 3.16 spans, rounding up to 4.47.4

Therefore, five supports are needed.

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Loads on Supports

The loads imposed on supports must be considered in the detailed support design to ensurethat they are not overstressed, and that they do not overstress the pipe, locally. The loads onthe supports will, in turn, be transmitted to other structural members and foundations whichalso must be designed considering the applied loads. The design of these elements mustensure that the support will perform its intended function in the piping system. For example,if the structure under a support is not rigid enough, it will deflect excessively under theapplied load which will let the piping system deflect as well.

The details that are used to attach the support to the pipe must consider the local stresses inthe pipe wall resulting from the applied load. In the extreme, high-weight loads at supportpoints could cause the pipe wall to locally deform. Therefore, the support attachment detailmust spread the load enough along and around the pipe wall to keep the local stresses in thepipe wall within reasonable limits.

These detailed support design considerations may, in some cases, require the support span tobe reduced even if the overall pipe stress and deflection criteria are met. This would occur ifthe support load is so high that the detailed design becomes impractical, or is more expensivethan adding an additional support location to reduce the load.

The following loads should be considered in the design of supports:

• Weight of pipe and insulation, and internal lining (if any). The weight of other pipingcomponents such as valves and fittings, must also be accounted for.

• Weight of the line contents based on water or the operating fluid, whichever is larger. Ifthe line is not hydrostatically tested, the weight of the line contents is sufficient. Springhangers are normally designed for the weight of the line contents, so additional supportmay be needed during hydrostatic tests to avoid overstress if the line contents is a gas.

• Lateral loads due to wind. Since a support acts only in the vertical direction, wind loadmust be considered to the extent that it influences the structure to which the support isattached. Structural movement deflects the support and, in turn, moves the pipe.

• Lateral loads due to movement of the pipe. Pipe movement causes a frictional load to beapplied to the support that acts opposite to the direction of pipe motion. The support andassociated structure must be designed for this frictional load.

Support loads can be calculated using the equations of statics once all support locations aredetermined. However, it is much simpler, and more accurate, to use a piping flexibilitycomputer program to find the support loads as discussed later in this module.

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Requirements for Pads and Saddles

Loads that act on or in the pipe create stresses in the pipe wall as previously discussed. Themagnitude of these stresses determines whether or not the load needs to be distributed over awider area. If the load needs to be distributed, then reinforcement pads, saddles or wider pipeshoes are typically used.

Saudi Aramco Standard Drawings AD-036253, AD-036252, and AD-036999 providestandard details for pipe shoes, pads and saddles. Note that these details are based on pipediameter. More load spreading is required as the diameter increases since the pipe wallbecomes more flexible and less able to absorb and transmit loads without being overstressed.

Prevention of Wind-Induced Vibration

• All external loads must be considered in support and flexibility design. Preventing wind-induced vibration is particularly important in support design because it can have aprofound impact, as indicated on the next page by the requirements of SAES-L-002 andSAES-L-011. Vortex shedding induced vibration caused by wind may become a problemwith piping that is more than about 10 m (30 ft.) long. This generally occurs with pipingthat runs up along the length of a vertical tower, or for long horizontal runs in exposedlocations such as a section of aboveground pipeline.

• When wind flows past a circular pipe section, the air behind the pipe is no longer smooth.There is a region of pressure instability where vortices are shed in a regular pattern,alternating from one side of the pipe to the other. These vortices cause an alternatingforce to act perpendicular to the wind direction and can make the pipe vibrate. If thefrequency of vortex shedding corresponds to a mechanical natural frequency of the pipingsystem, resonant vibration could cause pipe fatigue failure. Analyzing and solving vortexshedding vibration problems is best handled by applying certain principles that includedimensionless parameters and experimental data, which often requires using computerprograms. Further discussion regarding vortex shedding is beyond the scope of thiscourse.

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SAES-L-002, Paragraph 6.2, specifies:

• Exposed piping systems shall be designed for wind loading based on 35 m/s (78 mph)fastest mile wind speed and shall take into account the effects of wind-induced vibrationwhere applicable.

• The wind speed causes a uniform lateral load to be exerted on the pipe. This lateral loadis resisted by friction that acts at support points, as long as they are not hanger-typesupports which will allow the pipe to be moved by the wind. Thus, even thoughsupports are installed to carry weight load, their presence may also provide sufficientresistance to wind loads in many cases so that additional restraint is not required.

• The location of supports influences the mechanical natural frequency of the pipingsystem. Thus, they will affect any evaluation of wind-induced vibration since thevibration forcing frequency must be compared to the piping system mechanical naturalfrequency to determine if a problem exists.

SAES-L-011, Paragraph 3.3 specifies requirements for support spacing due to vibration:

When aboveground, cross-country pipelines with diameters larger than 450 mm (18 in.)are supported at regular intervals, every seventh span length shall be reduced by 20% tomitigate wind-induced resonant vibration of the pipelines. The basic support spacingshall be selected so that the natural frequency of the pipeline in the operating conditionis outside the range of wind-induced frequencies, plus or minus 10%, for any windspeed above 9 m/s (20 mph) which will cause vortex shedding.

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DETERMINING THE NEED FOR A PIPING THERMAL FLEXIBILITY / WEIGHTANALYSIS

Piping must have sufficient flexibility to accommodate thermal expansion (or contraction)effects. Piping systems must be designed to ensure that they do not fail because of thermalstresses or produce excessive forces and moments at connected equipment. If a system doesnot provide adequate flexibility, the results can be leaky flanges, fatigue failure of the pipe,excessive maintenance, operations problems, and damaged equipment.

A thermal flexibility analysis calculates the thermal movements of the pipe. These result instresses in the pipe, and in reaction forces and moments on end points, supports, restraints,and connected equipment. The analysis examines the interaction among pipe, equipment,piping components, and restraints.

A structure that is subject to a change in temperature will change in dimensions. If thesethermal movements are allowed to occur without any restraint whatsoever, no pipe stresses orreaction loads result. However, in real systems, stresses are developed in the pipe andmoments and forces are imposed on the connected equipment and at supports and restraintsinstalled in the system. The basic problem is to determine the internal pipe stresses and theexternal loads, and then decide if they are acceptable. A thermal flexibility analysis is done toensure that the piping system is laid out, supported, and restrained such that the thermalstresses in the pipe and the loads on the end points are within allowable limits.

A thermal flexibility analysis using a computer is not required for every piping system designproblem. Determining when a detailed analysis is needed depends on the complexity of thesystem and the design conditions of each individual situation. There is no single definition ofwhether to perform an analysis or not that applies to every situation. The following are someguidelines to help the engineer determine when a thermal flexibility analysis is required.Generally speaking, if a detailed analysis is required based on temperature considerations, aweight analysis will be done at the same time.

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Rationale and Approaches for Piping Flexibility and Support Design

Support and flexibility design is a combination of art and science with multiple factors toconsider and usually more than one way to design the system. It requires knowledge of howthe operating and design conditions of a piping system influence its overall design, and thesupports and restraints required for the system.

Consider the scenario shown in Figure 7. The supports and restraints exist in this situation forthe following reasons:

• To control movement of the pipe to reduce stress that may cause fatigue failure andloads that could damage connected equipment.

• To absorb some of the loads created by the operating or design conditions.

A piping system can be described as an irregular structural frame in space because of itsrelatively slender proportions when compared to structural steel systems. Elevated designtemperatures or various operating scenarios may cause sufficient pipe thermal stress or reducematerial strength such that supplementary structural assistance to support the piping system isrequired. It is also often necessary to limit the pipe movement at specific locations in order toprotect sensitive equipment, control vibration, or to resist external forces such as wind,earthquake, or shock loading.

• Careful attention must also be paid to pipe support/restraint design details to ensure thatlocalized stresses in the pipe wall are kept within allowable limits. This is especiallyrelevant in large-diameter piping systems with relatively thin walls (i.e., outsidediameter/thickness ratio over about 95) or in very high-temperature systems. In suchcases, support/restraint design details that spread the local loads over larger areas of thepipe wall are typically used to reduce local stresses.

• Planning for pipe supports and restraints should be done simultaneously withestablishing possible layout configurations to achieve the most cost effective design.The location and type of supports and restraints used must also consider the sometimesconflicting requirements of providing support or restraint while still permitting thermalexpansion. For example, too little support may result in high loads that must beconsidered in the detailed design of the support and associated structure, even if the pipestress itself is acceptable. Too much support is not cost effective, and may provideexcessive restraint of pipe thermal movement.

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TYPICAL SCHEMATIC OF PUMP SUCTION MANIFOLD

FIGURE 7

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Approaches

Because of the complexity of the piping flexibility and support design process, there is nosingle procedure or design method applicable for all situations. Considering this, the engineercan approach support and flexibility design in many ways. The following is a basic way ofapproaching the problem.

• Examine the layout and operation of the piping system to identify:

- Layout geometry.

- Pipe diameter and thickness, and locations of any changes.

- Piping component design details such as branch connection details and type ofelbows (long radius or short radius).

- Design temperature and pressure.

- Fluid service, including its potential danger.

- End-point movements.

- Type of connected equipment, rotating or fixed.

- Locations of existing structural steel.

- Relevant operating scenarios.

- Special design considerations, such as wind, vibration-prone services, orientationof loads.

• Determine the potential effects of those conditions, such as thermal movements, loads,and stresses.

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

• Determine if the situation warrants a detailed thermal flexibility analysis.

• If required, identify which conditions are applicable for the analysis and utilize anappropriate computer program to perform the analysis.

• Interpret the results of the analysis.

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Guidelines for Whether to Perform a Thermal Flexibility and Weight Analysis

The determination of whether or not to perform a thermal flexibility analysis depends on thecomplexity of the system and the design conditions, and must be evaluated for each situation.Generally, the need for an analysis is determined by visual examination of the layout, designtemperature, the type of equipment connected to the system, and the complexity of theprocess operations. There are no standard guidelines that will provide the engineer withspecific rules on whether or not to perform an analysis that are valid in all cases. However,the basic approach to the problem as outlined above and the parameters established by theapplicable ASME/ANSI B31 Code for allowable stresses provide guidelines that helpdetermine if an analysis is needed.

Determining the need for a thermal flexibility/weight analysis requires:

• Referring to SAES-L-011 and other applicable standards and code requirements.

• Identifying design conditions, including pipe size, temperature, and layout.

• Identifying load limitations on connected equipment, particularly load-sensitive rotatingequipment.

Guidelines contained in ASME/ANSI B31.3 may be used as an initial screening tool todetermine whether a flexibility analysis is required. Piping layouts can be compared to oneswhich have adequate flexibility, can be judged by using relatively simple hand calculation orchart methods, or can be extensively analyzed used a piping flexibility analysis computerprogram. To save time and money, the engineer can identify lines that do not require furtheranalysis by using the following criteria:

• The piping duplicates a successfully operating system.

• The piping is judged to be adequate by comparison with previously analyzed systems.

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• The piping is of uniform size, has no more than two points of fixation, has nointermediate restraints, and meets the limit in the following empirical equation:

Dy

L - U ( ) 2 ≤ K 1

where: D = Pipe outside diameter, in.

y = Resultant of total displacements to be absorbed by the piping system, in.

= ∆ x ( ) 2 + ∆ y ( ) 2 + ∆ z ( ) 2 , where ∆x, ∆y, and ∆z are the net thermalmovements to be absorbed by the piping system in the three coordinatedirections, considering any end-point movements as well.

L = Developed length of piping between anchors, ft.

U = Straight line distance between anchors, ft.

K1 = 0.03.

This formula should not be used for abnormal configurations such as unequal-leg U-bendswith L/U over 2.5 or saw-tooth patterns, for large-diameter thin-walled pipe, nor for systemshaving large end movements not along the direction that connects anchor points.

Systems that do not meet these criteria should be analyzed using simplified calculations orcomputer methods, as applicable, to confirm adequate flexibility.

• Where load-sensitive equipment is involved, an accurate flexibility analysis is usuallyadvisable since approximate approaches are apt to be particularly unreliable incalculating reaction loads.

• Accurate calculations are also advisable when the fluid service is hazardous and the pipematerial strength is significantly reduced due to high temperature.

• Accurate analyses should also be considered for unusually stiff systems due to size,thickness, or configuration; for economic use of expensive material; for cyclic services;or when approximate analyses indicate overstress.

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Because it is hard to determine when a particular system must be analyzed, the followingguidelines may be used to help determine when a detailed analysis is needed.

TYPE OF PIPING PIPE SIZEMAXIMUM

DIFFERENTIALFLEXIBILITY TEMP.

mm in.General Piping ≥ 100

≥ 200≥ 300≥ 500

≥ 4≥ 8≥ 12≥ 20

≥ 222°C (400°F)≥ 167°C (300°F)≥ 111°C (200°F)any

For rotating equipment ≥ 75 ≥ 3 anyFor air-fin heat exchangers ≥ 100 ≥ 4 anyFor tankage ≥ 300 ≥ 12 any

Note that when an "accurate" flexibility analysis is required, it should generally be done usinga recognized computer program for all but the simplest systems that are not connected toload-sensitive equipment. For aboveground, unrestrained piping systems, the maximumdifferential flexibility temperature is normally the difference between the design temperatureand a base temperature of no higher than 21°C (70°F). For underground, fully restrainedpipelines, the maximum differential flexibility temperature is taken as the difference betweendesign temperature and tie-in temperature.

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Saudi Aramco Flexibility Requirements

Saudi Aramco Engineering Standard SAES-L-011, Flexibility, Support, and Anchoring ofPiping, Paragraphs 2.1 through 2.3, contain specific requirements for the flexibility analysis.

• Formal analysis shall be made to calculate the significant stresses due to thermalexpansion and movements in all piping to show compliance with the design criteria ofthe Code (i.e., allowable stresses). The exception is for aboveground plant pipingwithout substantial axial restraint that can be readily judged to have adequate flexibilityby comparison with successfully operating existing systems.

• This requirement establishes the need for a detailed (formal) analysis, unless it can bereadily established that the piping system has adequate flexibility. The guidelinesdiscussed previously may be used to help establish the need for this formal analysis.

• The formal analysis shall be recorded as part of the design package specified in SAES-L-012 for piping systems which require a safety instruction sheet, per SAES-A-005(discussed in MEX 101.10).

• This emphasizes the importance of the formal analysis by requiring that it be made partof the permanent record for piping systems that require a safety instruction sheet. Thus,the analysis results are available for future reference should there be problems with thesystem or if changes to it are needed in the future.

• The formal analysis shall include computer calculations using a generally acceptedcomputer program for piping flexibility analysis, except when the system is consideredfully restrained. A sufficient number of calculations shall be made to establish the mostsevere combinations of load conditions which result in the highest combined pipingstresses at various locations, and the highest loads on anchors, connected equipment,guides, and stops.

This establishes the requirement that when a formal analysis is necessary, it must be doneusing a computer program. Thus, hand calculations and/or chart form solutions are notacceptable for such systems. It also indicates that multiple calculations may be needed todetermine the operating scenario that will govern the system design. For example, take thecase of a piping system with two pumps, one of which is a spare. Either pump may beoperating while the other is down, and both will be running for a short period while they arebeing switched. Except for a perfectly symmetric piping system layout, it is usually necessaryto perform calculations for all three operating scenarios to establish the one case that governsthe design. This is because different portions of the system will be hot while others are coldin each case.

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DETERMINING THE REQUIRED DESIGN CONDITIONS FOR A THERMALFLEXIBILITY/WEIGHT ANALYSIS

If a detailed piping flexibility analysis is required, it will normally be done using a recognizedcomputer program such as Caesar II, Simflex, or Triflex. Saudi Aramco engineers willtypically use the Simflex program. Such a program has the capability to consider anycombination of pipe geometry, support, restraint, and load conditions that must be considered.When such an analysis is required, the engineer must determine:

• The applicable design conditions and operating scenarios for the piping system.

• The allowable stresses from the applicable ASME/ANSI B31 Code.

• The load limitations, if any, on connected equipment.

• The extent of the analysis required to identify the most severe case.

This information is necessary to perform the analysis.

Design Conditions

The design conditions that must be considered for a thermal flexibility analysis are listedbelow. Anything that influences the thermal flexibility of the piping system can be anapplicable design condition that is needed for the analysis.

• Layout specifications.- Distances that completely describe the overall piping system geometry.- Location of connected equipment and other piping components.- Curves, such as long- or short-radius elbows, bends.- Branch connections, such as fabricated reinforced or unreinforced tee, forged tee,

integrally-reinforced fitting.• Pipe diameter and wall thickness.• Design temperature and pressure.• Fluid service, including whether it is dangerous.• End-point movements.• Type of connected equipment, rotating or fixed.• Structural steel located in the vicinity.

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• Special design considerations and load cases. (Also discussed in MEX 101.03).- Thermal analysis, to confirm that the pipe thermal stress is within allowable limits.- Weight analysis, to confirm that the pipe longitudinal stress (including thelongitudinal pressure stress) is within allowable limits.- Thermal-plus-weight analysis to confirm that operating loads imposed on connected

equipment are acceptable.- Wind.- Vibration-prone services.- Orientation of externally applied loads, slug forces.- Alternate operating scenarios that result in different portions of the system being hot

while others are cold.- Extent of analysis. For example, must an entire piping system be modeled, or may

portions of the system be deleted without affecting accuracy?

The thermal, weight, and thermal-plus-weight cases will apply to every system that isanalyzed. The applicability and impact of the other considerations depend on the particularsituation.

For example, the system to be analyzed may include any common piping components:straight runs, elbows, tees, valves, spring hangers, etc. These components may have anyorientation. Loads due to thermal expansion, wind, pipe weight, etc., may be considered.Forces, moments, and deflections may be applied and/or evaluated. Further detaileddiscussion of piping flexibility analysis is beyond the scope of this course.

Saudi Aramco Requirements

Saudi Aramco Engineering Standard SAES-L-011 contains additional detailed requirementsthat must be considered in the detailed pipe-stress analysis.

• Paragraph 2.5 provides requirements for external lateral loads.

The stress calculation shall take into account the circumferential bending stress in thepipe wall due to any loads from supports, anchors, and/or soil pressure or other externalforce, except when transmitted through a full encirclement sleeve weldedcircumferentially to the pipe.

This requirement forces consideration of the detailed design that is used for attachmentsmade to the pipe for supports and restraints. Locating a support or restraint at aparticular location may satisfy the limitations on overall pipe stress and end-pointreaction loads. However, the local reaction load at the support or restraint may besufficient to damage the pipe. This must be considered and appropriate design detailsutilized.

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• Paragraph 2.9 specifies requirements related to friction.

Friction forces from supports and guides shall be considered as external loads acting inthe direction opposite to the expected displacements, where such friction would tend toreduce the piping flexibility significantly.

Displacement calculated by the computer program must agree with the assumeddirection of the friction force. In the absence of experimental or reliable vendor's data,the following friction factors shall be used for the flexibility calculation:

MATERIALSFRICTIONFACTOR

Steel to steel 0.40Teflon to steel 0.20Teflon to Teflon 0.10Sand to pipewrap 0.25Sand to plastic coating 0.20Sand to concrete 0.40

• Friction forces will develop whenever a pipe slides across support or restraint points.These friction forces are to be considered in all cases since they would tend to reducethe flexibility of the pipe and increase reaction loads. Friction forces must be includedin the design of any associated structural members as an additional applied load.

• The extent that friction loads must be considered in any formal flexibility analysisdepends on the particular circumstances. In most cases, the magnitudes of the frictionforces are much smaller than the thermal loads developed in the system and can besafely ignored. However, in situations where the friction loads are relatively high, theyshould be directly included in the flexibility analysis to accurately evaluate their impact.Computer programs that are available today easily permit this by allowing a frictionfactor to be specified at support or restraint points.

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• Two typical situations where friction should be considered are:

- Large-diameter, long pipe runs such as on pipe racks or in the offsite area, wheresupport-point loads are relatively high. In these cases, the friction forces canbecome high enough to restrict the thermal expansion of the pipe.

- Systems that are connected to load-sensitive equipment where there are largereaction loads at supports or restraints located near the equipment nozzle. In thesecases, the friction loads can be transmitted back to the equipment and overload it.

One method of overcoming problems that are caused by high-friction loads is to usesliding surfaces with lower friction factors. Note in the previous table that using Teflon-on-Teflon bearing surfaces reduces the friction load by 75% when compared to steel-on-steel.

• Paragraph 3.5 specifies requirements for welded attachments.

If the piping is designed to operate at a hoop stress in excess of 45% of the specifiedminimum yield stress (SMYS) of the pipe material, all structural attachments whichtransfer loads to the pipe through welds shall be welded to full-encirclement sleeves orsaddle pads of at least 90° with rounded corners, unless a comprehensive stress analysisis made to prove the adequacy of the design. ASME/ANSI B31.4Paragraph 421.1(d) and ASME/ANSI B21.8 Paragraph B34.5(B), if applicable, may notbe waived. All welds to the pipe shall be continuous.

This requirement ensures that the applied external load will be distributed around thecomplete circumference of the pipe, unless a stress analysis indicates that this is notnecessary. This reduces the local stresses in the pipe wall in situations where it isalready highly stressed due to pressure, and eliminates the risk of local pipe buckling.

• Paragraph 3.6 specifies requirements for dummy supports.

The dummy supports shall not create excessive stresses at the attachment welds to therun pipe. This can be accomplished by minimizing the length of the supports and/orincreasing the size of the supports.

This requirement recognizes that a dummy support will result in an additional bendingmoment being applied to the pipe at the attachment point, and this bending momentmust be considered in its design. Minimizing the support length reduces the magnitudeof the moment to be considered. Increasing the support size, i.e. its diameter, increasesthe strength of the support itself and spreads the applied moment over a larger area ofthe pipe. Spreading the applied moment over a larger area on the pipe wall reduces thelocal pipe stress that is caused by the moment.

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Piping Flexibility Temperature

Flexibility analysis should be made for the largest temperature difference that may beimposed on the pipe by normal and abnormal operating conditions. This results in the largestpipe stress to be considered in fatigue failure evaluation, and the largest reaction loadsimposed on equipment end connections, supports, and restraints. The following tableprovides guidelines to determine the temperatures to consider in a flexibility analysis. Notethat more than one of these items might require consideration in a particular system and leadto the need for multiple computer calculations to identify the case that governs the systemdesign.

NORMAL TEMPERATURE CONDITIONS TO CONSIDER

Stable Operation Gives the temperature range expected for most of thetime a plant is in operation. Some margin aboveequipment operating temperature, i.e., use of the designtemperature, allows for process flexibility.

Startup and Shutdown Must be examined to determine if the heating or coolingcycles pose flexibility problems. For example, if atower is heated while some attached piping remainscold, the piping flexibility should be checked.

Regeneration andDecoking Piping

Must be designed for normal operation, regeneration, ordecoking, and switching from one service to the other.An example is the decoking of furnaces.

Spared Equipment Requires multiple piping flexibility analyses todetermine if the piping is adequate for the expectedvariations of temperature, for no flow in some of thepiping, and for switching from one piece of equipmentto another. A common example is the piping for two ormore pumps with one or more spares.

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ABNORMAL CONDITIONS

Loss of CoolingMedium Flow

Temperature changes due to a loss of cooling mediumflow should be considered. This includes pipe that isnormally at ambient temperature but can be blocked in,while subject to solar radiation.

Steamout for Air orGas Freeing

Most on-site equipment and lines and many off-sitelines are freed of gas or air by the use of steam. For 862kPa (125 psi.) steam, 149°C (300°F) is used for themetal temperature. Piping connected to equipmentwhich will be steamed out, especially piping connectedto upper parts of towers, should be checked for thetower at 149°C (300°F) and the piping at ambient plus28°C (50°F). This situation may govern the flexibilityof lines connected to towers that operate at less than149°C (300°F) or have a smaller temperature variationfrom top to bottom.

No Process Flow WhileHeating Continues

If process flow can be stopped while heat is still beingapplied, the piping flexibility should be checked for themaximum metal temperature. Such situations can occurwith steam tracing and steam jacketing.

Metal temperatures that govern the flexibility design of a piping system are not necessarilythe ones associated with the most severe coincident pressure and temperature which governthe wall thickness of the pipe. Piping flexibility depends only on the temperature. Therefore,a condition of high temperature and low pressure may govern the piping flexibility designwhile the wall thickness is based on a higher pressure but a lower temperature. However,note that the design pressure is considered with the pipe weight when calculating the totallongitudinal stress in the pipe during a weight analysis.

For restrained pipelines, SAES-L-011, Paragraph 2.4, specifies the following:

• A tie-in temperature range shall be established for the design and construction of allburied and aboveground fully-restrained pipelines. The design shall be based on theexpected temperature rise during operation as well as the maximum anticipated decreasein temperature after tie-in.

• This requirement results in the use of the maximum expected temperature range in thedesign of a fully-restrained pipeline. This then yields the maximum possible thermalstress in the pipe, and the largest loads that must be considered in the design ofassociated anchors.

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Number of Cycles to be Considered

The number of times that a line experiences the combination of temperature and endmovement influences piping flexibility design because the flexibility stress basis is based onfatigue failure. ASME/ANSI B31.3 includes a factor "f" in the equation for the allowablestress range to account for the number of cycles as shown below. A plant life of 20 yearsshould be used to estimate the number of cycles. One cycle a day for 20 years is about 7,000cycles. If the number of cycles exceeds 7,000, the number of cycles should be indicated inthe design specification for the affected lines.

SA = f (1.25 Sc + 0.25 Sh)

where: SA = Allowable displacement stress range, psi.

Sc = Basic allowable stress at minimum metal temperature expected during thedisplacement cycle under analysis, psi.

Sh = Basic allowable stress at maximum metal temperature expected during thedisplacement cycle under analysis, psi.

NUMBER OF CYCLES f7,000 or less 1.0Over 7,000 to 14,000 0.9Over 14,000 to 22,000 0.8Over 22,000 to 45,000 0.7Over 45,000 to 100,000 0.6Over 100,000 to 200,000 0.5Over 200,000 to 700,000 0.4Over 700,000 to 2,000,000 0.3

Note that the allowable stress range for thermal flexibility stresses does not use thelongitudinal weld-joint efficiency factor for any type of pipe. Therefore, the cold and hotstresses in the equation will be the same for seamless and welded pipes.

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Load Limitations On Equipment

A poorly designed piping system that is connected to rotating equipment can cause damage tothe equipment. For example, an excessive load on a pump can cause high vibration orbearing and seal wear problems that will lead to excessive maintenance requirements. In anextreme case, the single application of an excessive load can result in immediate damage andrequire a shutdown.

Rotating equipment, i.e., pumps, turbines, and compressors, are the most sensitive type withrespect to imposed piping loads due to the moving parts and small clearances involved in theirdesign. However, pipe loads that are imposed on stationary equipment items must not beallowed to become excessive either. This will be discussed further below.

Loads that are imposed by the piping system on connected equipment are determined fromthe results of the piping flexibility analysis. These loads are then compared to allowablevalues based on industry standards for particular types of equipment to determine if they areacceptable. For some equipment items, the allowable loads may just be read directly fromtables that are contained in the applicable industry standard. In other cases, the allowableloads must be calculated based on criteria contained in an industry standard. In still othercases, the stresses that result from the imposed loads must be calculated, and the stresses thencompared to allowable values. Equipment vendors will sometimes also have allowable loadcriteria that must be considered.

SAES-L-014, Design of Pump and Compressor Station Piping, also requires that theadditional loads caused by slight misalignment between pipe and equipment flanges beconsidered. Flanges must be aligned to within relatively small tolerances to ensure that pipeinstallation and flange boltup do not impose excessive loads on equipment nozzles.

Discussion of the actual allowable equipment loads is beyond the scope of this course.However, the following table summarizes the industry standards that apply to equipmentnozzle load evaluations, and the parameters that are used to determine the allowable loads.

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EQUIPMENT ITEM INDUSTRY CODEPARAMETERS USED

TO DETERMINEACCEPTABLE LOADS

Centrifugal Pumps API 610 Nozzle size.Centrifugal Compressors API 617, 1.85 times NEMA

SM-23 allowableNozzle size, material.

Air-Cooled HeatExchangers

API 661 Nozzle size.

Pressure Vessels, Shell-and-Tube, and HeatExchanger Nozzles

ASME Code Section VIII,WRC 107, WRC 297

Nozzle size, thickness,reinforcement details,vessel/exchanger diameter,and wall thickness.

Tank Nozzles API 650 Nozzle size, tank diameter,height, shell thickness,nozzle elevation.

Steam Turbines NEMA SM-23 Nozzle size.

Analysis Considerations for Rotating Equipment

Rotating equipment piping represents one of the more difficult systems to design for thermalflexibility. The difficulty increases as the design temperature and pipe diameter increase. Usepump piping systems as an example. The loads on the pump nozzles must be withinacceptable limits for all possible spare pump operating conditions. The overall design ofpiping systems connected to rotating equipment is generally governed by the load limitationson the equipment rather than the thermal expansion stresses in the pipe. If the equipment loadcriteria are met, the thermal expansion stresses will generally be well within their acceptancelimits. The allowable loads for rotating equipment are based on nozzle diameter, and areeither read from a table or calculated from simple equations that are contained in theappropriate industry standard.

The analysis should consider all pertinent branch runs that are connected to commonmanifolds. For a pump system, one set of operating conditions is all pumps operating. Theeffect of each pump being used as a spare or being blocked off for maintenance must also bechecked. For pumps that are on standby, unless they have warmup lines, the entire dead legfrom the manifold branch connection to the pump is assumed to be at ambient temperature.Nozzle load limits must be satisfied for combined thermal, weight, and friction loads. Springsupports are often needed near the pump nozzles to effectively reduce the weight load on thepumps while free pipe thermal expansion is still allowed to take place.

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Considerations for Stationary Equipment

Air-Cooled Heat Exchangers – The most common configuration for air-cooled heat exchangerpiping uses short, stiff, straight sections of pipe to connect the pipe manifold to the exchangernozzles. The manifold is located directly above or below the exchanger header box. The heatexchanger tube bundle is allowed to move laterally to accommodate the thermal expansion ofthe pipe manifold. The flexibility analysis should include the restraining effect of frictionfrom movement of the exchanger bundle, which will resist lateral movement of the bundle.The reaction loads at the exchanger nozzles that are obtained from the results of the flexibilityanalysis are compared to the allowable loads contained in a table in API 661 based on nozzlediameter.

Pressure Vessels and Shell-and-Tube Heat Exchangers – The need to evaluate the loads thatare imposed on pressure vessel or shell-and-tube heat exchanger nozzles is not as obvious asfor rotating equipment or air-cooled heat exchangers. The comparison basis to employ is notas simple either. These equipment items are not as "load sensitive" as the other twocategories. There is also no readily available industry standard that simply relates nozzle sizeto allowable load.

Evaluation of nozzle loads that are imposed on these items requires calculating the localstresses in the nozzle and vessel or exchanger shell resulting from these loads, combiningthese with the stress due to design pressure, and limiting these combined stresses to allowablelimits. Performing this analysis requires consideration of the nozzle design details at thevessel shell and the strength and stiffness of the vessel shell itself. Accepted procedures toperform this evaluation are contained in Welding Research Council Bulletins WRC-107 andWRC-297. Discussion of these procedures is beyond the scope of this course.

Fortunately, it is normally not necessary to perform such nozzle-load evaluations. In themajority of cases, as long as the vessel nozzle details have been adequately designed forpressure, and the piping system thermal expansion stresses are within allowable limits, theloads on the vessel nozzles will normally be acceptable and do not require separateverification. Unfortunately, there are no specific guidelines to indicate exactly when loads onthese equipment items should be checked. Evaluating nozzle loads should be considered insituations where the pipe is of relatively large diameter [over about 600 mm (24 in.)],especially if it is connected to a large-diameter/relatively thin-walled vessel (such as anatmospheric or vacuum pipestill tower).

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Tank Nozzles – The loads that are transmitted from piping to the shell nozzles of large-diameter storage tanks are a major concern of tank designers. The loads are a result of shellradial movement and nozzle rotation while filling and emptying a tank, thermal expansion ofpiping, differential settlement between the tank and the piping supports, and the weight ofpiping, valves, and other system components. As with pressure-vessel nozzles, determiningthe allowable loads on tank nozzles is not a simple "table lookup" procedure based only onnozzle diameter. It involves considering nozzle and tank geometry, nozzle location on thetank shell, multiple equations and graphs, and a graphical solution. Discussion of thisprocedure is beyond the scope of this course.

Extent of Analysis

The extent of a piping system flexibility and weight analysis depends on the situation.Because the overall purpose of the analysis is to provide enough flexibility for the system, theengineer must analyze the right combination of operating conditions to determine where, andif, additional flexibility is needed to reduce pipe stresses or loads at end points. The engineermust also decide if it is desirable and acceptable to not include portions of a large, complexsystem in the analysis in order to simplify the modeling. For example, including a 100 mm(4 in.) diameter branch run in the model of an extensive, 600 mm (24 in.) diameter mainsystem may not be necessary. Judicious installation of anchors or other restraints in a largesystem could also help simplify the modeling.

The following steps are involved to confirm the acceptability of the planned piping design:

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

• Determine conditions of end restraint and anchor movements, including deflections ofvessel shells and equipment casings.

• Locate intermediate points of restraint and define any limitations that they impose onpiping movement. This includes spring hangers and counterweights that are installed forsustained weight loads.

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

• Compare the results with the allowable stress range for thermal expansion stresses, theallowable stress at design temperature for weight-plus-pressure stresses, and theapplicable load criteria for connected equipment.

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Providing Additional Thermal Flexibility

The initial piping system layout may not be satisfactory for thermal flexibility stresses orloads on connected equipment. The following guidelines may help the situation.

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

• Install expansion joints. However, SAES-L-011, Paragraph 2.8, makes this theexception by imposing the following restrictions:

- The use of corrugated metal pipe sections or creased bends to reduce the stiffnessis prohibited for all pressure services. Swivel joints, expansion joints, flexible pipeor hose, or similar devices shall be used only when approved by the ChiefEngineer through the waiver process.

- Expansion joints represent a "weak link" in a piping system. They may affect thelife of the system since they are more susceptible to damage, and can createmaintenance and operational problems. Thus, the use of expansion joints shouldonly be considered as a last resort, and only through the waiver process.

• Make use of cold spring to prestress a piping system so that loads and moments areminimized when the piping is hot. However, cold spring does not affect pipingflexibility stresses because the stress range from cold to hot must be considered. Coldspring should be avoided for piping that is connected to rotating equipment since it isdifficult to control accurately.

• Strategically locate restraints such as guides, directional anchors, and limit stops, tominimize thermal and friction loads at equipment. Restraints could also be used todirect pipe thermal expansion into a section of the system that has more inherentflexibility to absorb it.

• Use spring supports if large vertical thermal movements are expected, or if thermalexpansion causes pipe to lift off fixed supports. Avoid fixed supports that result in largethermal stresses.

• Use Teflon bearing pads at supports for large-diameter pipe or other large weight loadsif friction loads are excessive. Friction loads can accumulate along the line and createunacceptable loads at equipment connections, or create the need for stronger structuralmembers.

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Sample Problem 3

Refer again to Figure 6 and the information that was provided in Sample Problems 1 and 2.For the system illustrated, either P-101A or P-101B is required to be in operation for theprocess application. Assume that all the supports and restraints discussed in Sample Problems1 and 2 will be installed. There are no similar systems to this in operation. It is now necessaryto answer the following questions:

a. Is a formal piping flexibility analysis required for this system?

b. What design conditions and operating variations should be considered?

c. Specify whether equipment nozzle loads must be considered and if so, the basis to beused for this evaluation and the loads that must be considered.

d. Assuming the thermal loads imposed on the pump nozzles are found to be too high,what design concepts might be worth considering to try to reduce them to acceptablelimits?

Solution

a. The simple criteria from ASME/ANSI B31.3 to exempt systems from formal analysiscannot be used since this system has more than two fixed points and is connected toload-sensitive equipment. Referring to the table in Work Aid 1, this system far exceedsthe diameter guideline for when a formal analysis should be done for pump piping.Therefore, formal flexibility analysis is required.

b. Note that the following additional information must be obtained before a formal analysiscan be done.

• Some layout dimensions are missing.

• The type of elbows must be determined.

• The type of branch connection South of Location 4 must be specified.

• The thermal movements at the pump nozzles must be determined. These willdepend on the location of the nozzle with respect to the fixed point on the pumpcasing.

• The thermal movements at the T-101 nozzle must be determined. This will dependon the elevation of the nozzle with respect to the tower support and the radius ofthe tower.

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A thermal analysis, a weight analysis, and a combined thermal-plus-weight analysiswould definitely be required to design the system. A wind analysis could be consideredfor completeness. However, windloads on the pipe are not likely to govern the design,especially with the guiding assumed in the vertical run at Location 6.

Three different scenarios must be analyzed to account for the pump operating cases: P-101A operating with P-101B spared; the opposite case; and both pumps operating,corresponding to the period of time when the pumps are being switched. The portion ofpipe between the branch connection and the nonoperating pump is considered to be atambient temperature for the first two cases, while the rest of the system is at designtemperature.

c. Nozzle loads at P-101 A/B must be evaluated using API 610 based on the 300 mm (12in.) nozzle diameter. To be precise, the nozzle loads at T-101 could be evaluated aswell, but in this situation should not be necessary. The pipe diameter is not that large,and the pump nozzles are much more sensitive than the vessel nozzle. Thus, any pipingsystem design that achieves acceptable pump-nozzle loads will result in very low pipingstresses, and it is unlikely that the vessel-nozzle loads will be high. Should a vessel-nozzle load evaluation be considered necessary, additional information regarding vesseland nozzle details would be required.

d. If thermal loads on the pump are still too high, the two most likely possible actions toconsider are to add more restraint(s), or provide additional flexibility by adding moreoffsets or an expansion loop. The first approach is the more attractive if it works since itwould be much less expensive and not require additional plot space.

In this situation, it is necessary to identify the direction in which the high force orbending moment is acting. This is easily found from the results of the flexibilityanalysis. Then try to determine what might be causing it, and then make a designchange to counteract it. For example, if East/West deflections at Locations 1 and 2cause high bending moments at the nozzles, adding East/West stops at these locationsmight solve the problem.

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USE OF ANCHORS FOR BURIED PIPING SYSTEMS AND DESIGN TOOLS

Pipeline Forces to Be Resisted

In buried piping applications, such as pipelines, the piping system is continuously supportedby the soil, and anchors are used to absorb the thrust loads that result from restrained thermalexpansion and limit pipe end movements. There are special considerations for anchoringunderground piping that the engineer must consider in pipeline applications. Specifically, theresistance of the anchor to movement (discussed below), and external loads at roads andrailroad crossings, must be considered.

Thermal expansion calculations are needed for buried lines if large temperature changes areexpected. Anchors are needed to limit movement at the ends of the pipeline, at changes indirection, and at changes in pipe size. Excessive movement of a buried pipeline can causeshifting of the soil that supports the pipe, subsidence of the cover, or damage the external pipecoating (if one is installed). In extreme cases, excessive movement can eventually lead tooverstress in the pipe and/or inadequate cover depth, or external pipe corrosion.

For buried, restrained pipelines, the net longitudinal compressive stress in the pipe due totemperature rise and pressure is calculated as follows, in accordance with ASME/ANSIB31.4:

S L = = E α T 2 − − T 1 ( ( ) ) − − ν S h

where: SL = Longitudinal compressive stress, psi.

Sh = Hoop stress due to pressure, psi.

T1 = Temperature at time of installation, °F.

T2 = Maximum or minimum operating temperature, °F.

E = Modulus of elasticity, psi.

α = Coefficient of thermal expansion, in./in./°F.

ν = Poisson's ratio.

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ASME/ANSI B31.4 limits SL to 90% of the pipe SMYS. The equation shows that it ispreferable to have the tie-in temperature as close as possible to the operating temperature tominimize SL. Multiplying this calculated stress by the pipe cross-sectional area yields theforce that must be resisted by the anchor to limit pipeline movement during operation.However, the maximum force for which the anchor must be designed occurs when there is nopressure in the line (i.e., Sh is zero). This situation occurs when the line is empty, or is filledwith liquid but is not in operation.

ASME/ANSI B31.4 does not require that the bending stress in buried pipelines, Sb, beincluded in the calculation of SL. However, SAES-L-003 requires that all sustained loads andconstraining forces also be included in the calculation of SL. These loads result in anadditional bending stress, and are typically caused by pipe misalignment during installationand elastic bends in the pipe that are required to conform to the ground profile. SaudiAramco limits the maximum bending stress, Sb, as follows:

Sb ≤ 0.9 (SMYS) - 0.7 Sh - Eα (T2 - T1)

The maximum permitted bending stress can then be calculated based on the known values ofthe other parameters. In some cases, it may be necessary to increase the pipewall thicknessbeyond what is required for internal pressure alone. This thickness increase might be neededto provide a high enough value of Sb to permit pipeline installation using practical limits onmisalignment tolerances and elastic bend requirements.

Saudi Aramco often uses computer programs to calculate SL, Sb and the resulting forces forwhich the pipeline anchors must be designed because multiple parameters are involved. TheConsulting Services Department should be contacted when these calculations are required.

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Types of Anchors

Before discussing the forces available to resist anchor movement which may result from Sb,the following describes Saudi Aramco requirements for the primary types of anchors forrestrained end and intermediate anchors, as stated in SAES-L-011.

• Aboveground restrained pipelines shall be provided with end anchors that are designedto withstand the full thrust and pull forces due to thermal expansion and contraction, anddue to internal fluid pressure considering Poissons ratio, with a maximum anchordeflection of6 mm (0.25 in.).

• The potential end movements of buried pipelines shall be conservatively estimated. Ifthese movements exceed 50 mm (2 in.), a full-thrust or drag anchor shall be providedand be designed per SAES-L-044 to limit the end movement to less than 25 mm (1 in.).

• Differential-thrust anchors shall be provided on aboveground restrained pipelines wherethere is a change in thrust, such as due to a change in pipe diameter or wall thickness,except when the associated local axial movement of the line can be shown to be lessthan 6 mm (0.25 in.).

• Any change in direction in the horizontal and/or vertical plane of abovegroundrestrained pipelines requires one or more deflection anchors designed to resist theresultant forces on each side of the deflection. Deflection anchors may be required onburied pipelines if the passive soil restraint against the pipe alone is not sufficient tofully restrain the line.

• Saudi Aramco Standard Drawing AB-036415 illustrates typical details for concrete-thrust block anchors.

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Forces Available to Resist Anchor Movement

The previous discussion described the pipeline forces that must be resisted, the anchor typesthat may be used, and limitations on anchor movement. This section will conclude withdiscussion of the forces that are available to resist movement of an anchor. A combination ofthe soil cover depth, soil friction, and anchors are used to restrain thermal expansion.

The full-thrust anchor force, or differential anchor force, without any reduction on account ofsoil friction due to assumed movement, shall be used for the design and stress calculations ofthe anchor. This includes structural steel design, welding details, attachment to the pipe,stability of the anchor against overturning, concrete stresses, and reinforcing bar selection.Solely for sizing a concrete-block drag anchor, credit may be taken for the soil friction on thelength of pipe assumed to be a maximum of 25 mm (1 in.) at the drag anchor.

The forces that are available to resist anchor movement come from the direct bearing loadsbetween the anchor and the surrounding material, friction loads between the anchor and thesurrounding material, and friction between the soil and pipe. SAES-L-044, Anchors forCross-Country Pipelines, provides design criteria for sizing concrete anchors. These designcriteria consider the following factors:

• Whether the anchor is in a rock area or in granular, well-compacted soil that is above thewater table. This determines the bearing load and friction factors that are applicable atthe anchor.

• Anchor depth beneath the surface.

• Anchor size (height and width). This determines the effective anchor bearing load basedon specified values of bearing pressure.

• Friction on both sides of the anchor assuming a specified active soil pressure andfriction coefficient.

• Friction on the bottom of the anchor assuming a specified friction coefficient.

• The axial friction force that accumulates along the moving length of pipeline that isadjacent to a drag anchor. This pipe friction reduces the load that must actually beabsorbed by the anchor.

• In order to maximize the stability of anchors, the resultant of all anchor forces shall havea line of action that is close to the centerline of the pipeline. This will minimize thetendency for the applied loads to rotate the anchor. Any resulting overturning momenton buried anchors shall be resisted by the weight of the anchor times the distance fromthe center of gravity to the leading face.

Participants should refer to SAES-L-044 and ADP-L-044 for additional details.

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WORK AID 1: CRITERIA FOR DETERMINING MAXIMUM SUPPORT SPACING

The maximum spacing between two supports is based on the allowable stress and deflectioncriteria. This is determined through two calculations.:

• A calculation based on stress limits:

L ≤ 0 . 8 Zf s

W

• A calculation based on deflection limits:

L ≤ EI ∆

13 . 5 W 4

where: L = Length of span, ft.fs = (1/2) x allowable stress at maximum temperature per applicable

ASME/ANSI B31 Code, psi. Note that this assumes that the pipewallthickness exactly matches that required for internal pressure. Thelongitudinal pressure stress thus also equals half the ASME/ANSIB31 Code allowable stress. This is a simplified assumption, but isconservative for situations where excess wall thickness is actuallyprovided.

∆ = Maximum deflection, in., equal to the smaller of 25 mm (1 in.) or halfthe nominal pipe diameter

W = Weight of pipe, including commodity and insulation if any, lb/ft.E = Modulus of elasticity of pipe, psi.I = Moment of inertia of the pipe, in.4Z = Section modulus of the pipe, in.3

Check which loads should be considered in the design of supports:

[ ] Weight of pipe, insulation, and lining.

[ ] Weight of line contents based on water or the line contents, whichever is larger.

[ ] Lateral loads due to wind.

[ ] Lateral loads due to movement of lines and support.

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MAXIMUM SPAN TABLES

FIGURE 9

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Engineering Encyclopedia Piping & Valves

Piping Layout Support & Flexibility

Saudi Aramco DeskTop Standards 57

GLOSSARY

anchor A rigid restraint providing substantially full fixation, ideallynot allowing translational or rotational displacement of thepipe along any of the three reference axes. It is employed forpurposes of restraint but usually serves equally well as arestraint, support, or brace.

brace A device primarily intended to resist piping displacement dueto the action of any forces other than those due to thermalexpansion or gravity.

cold spring The intentional stressing and elastic deformation of the pipingsystem during the erection cycle to permit the system to attainmore favorable reactions and stresses in the operatingcondition.

constant-effortsupport

A support which is capable of applying a relatively constantforce at any displacement within its useful operating range(i.e., a counterweight or compensating spring device).

damping device A dashpot or other frictional device which increases theresistance of a system to vibration. It offers high resistanceagainst rapid displacements caused by dynamic loads, whilepermitting essentially free movement under gradually applieddisplacement such as from thermal expansion.

expansion joint A flexible pressure-containing component of a piping systemwhich is designed to absorb thermal movement.

guide A device preventing rotation about one or more axes due tobending moment or torsion. In common usage, a guidenormally permits translation along the pipe axis but preventstranslation perpendicular to the pipe axis.

hanger A support by which piping is suspended from a structure orother fixed point located above it, and which functions bycarrying the piping load in tension.

Page 62: Piping Layout, Support, And Flexibility

Engineering Encyclopedia Piping & Valves

Piping Layout Support & Flexibility

Saudi Aramco DeskTop Standards 58

limit stop A device which restricts translational movement to a limitedamount in one direction along any single axis. Paralleling thevarious stops, there may also be: double-acting limit stops,two-axis limit stops, etc.

resilient orflexible support

A support which includes one or more largely elastic members(i.e., a spring).

resting orsliding support

A device for providing support from beneath the piping, butoffering no resistance other than frictional to horizontalmotion.

restraint Any device which prevents, resists, or limits the free thermalmovement of piping.

rigid (solid)support

A support providing stiffness in at least one direction that iscomparable to that of the pipe.

stop A device which permits rotation but prevents translationalmovement in at least one direction along any desired axis. Iftranslation is prevented in both directions along the same axis,the term "doubling-acting stop" is preferably applied. Incommon usage, a stop normally acts along the direction of thepipe axis.

support A device used to sustain a portion of piping system weightplus any superimposed vertical loadings.

two-axis stop A device which prevents translational movement in onedirection along each of two axes. A "two-axis double-actingstop" prevents translational movement in the plane of the axeswhile allowing such movement normal to the plane.