Piping Support and Flexibility

62 

Full text

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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.

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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

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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

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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

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IDENTIFYING OPERATIONAL, MAINTENANCE, AND SAFETY FACTORS THAT INFLUENCE PIPE LAYOUT

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

Operations Requirements

Operating and control points such as valves, flanges, instruments, sample points, drains, and vents 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 be reached.

Clearances Above and Below Piping

There must be enough clearance above and below the pipe to perform some basic operations on 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, specifies clearance requirements as follows:

• Above-grade piping shall be supported to provide a clearance between the bottom of pipe 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 (12 in.).

- 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, or valve and any structure not used to support the piping, shall be 0.3 m (1 ft) over a length up to three pipe diameters, or a minimum of 0.6 m (2 ft) if the length is longer. Clearances between aboveground piping and structures shall provide reasonable access for inspection and freedom of 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 other equipment that may require manual operation.

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

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

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); and

f) 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 block valves, the maximum section length shall be the weighted average of the distances that are specified for the classes involved, provided that the pipe in the entire section meets the requirements 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 will prevent backflow in case of an upstream line rupture or an emergency in the plant that feeds the pipeline. If the pipeline is designed for bidirectional flow service, a check valve and a parallel block valve shall be installed. If a check valve was installed on an existing outgoing pipeline 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 the bypass line.

Vents and Drains – Permanent vents and drains with plugged or blinded valves, and provision for blow-down of pipeline sections at selected locations, shall be installed only as required by the operating department for emergency conditions. Temporary vent valves, if required during 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 of a piping system. Operators need enough space around the scraper trap to remove and reinsert scrapers. Saudi Aramco Standard Drawing AC-036541 shows the layout for a typical scraper trap. SAES-L-045, Paragraph 6.4, also specifies the following with regard to scraper traps for pipelines:

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

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

• The pipe size, layout and supports shall be designed such that all hydraulic requirements are 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 avoided as 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 , specifies

requirements for corrosion considerations.

• Carbon steel piping that is in potentially corrosive fluid service shall be provided with corrosion monitoring fittings and/or drop-out spools subject to the approval of the assigned specialist in the Engineering Services Organization for Project Proposals and/or final design. The piping layout shall contain no sections or branches longer than three pipe diameters which are normally or frequently stagnant (dead ends). Corrosive water or other deposits can collect at the bottom in such nonself-draining sections and cause accelerated 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 for pipeways.

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

Pump and Compressor Piping

There must be enough space to operate pumps and compressors. SAES-L-014, Design of

Pump and Compressor Station Piping, Paragraphs 3 and 4, specify requirements for suction

and 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 suction pipe shall be laid out to provide a balanced flow at the entry of the pump, in particular to horizontal, double suction and double volute-type pumps. Piping to pumps that operate in parallel shall be laid out in a symmetrical manner to ensure equal distribution of the flow to each pump.

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

Long taper reducers shall be used. The top of pump suction piping shall be such that gas cannot 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 pipe length immediately upstream of the suction nozzle of the pump.

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

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Discharge Piping – Discharge piping immediately downstream of the discharge nozzle shall

have 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 be accomplished by proper sizing, suitable flow pattern, use of long taper reducers, dampening anchors, etc.

Thermal Expansion Clearances

When the pipe heats up or cools down, thermal expansion or contraction of the pipe will occur. Sufficient clearance should be provided between adjacent lines and between lines and structures 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. The following 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 or vibration.

• 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 structural members.

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Maintenance Requirements

The piping system must be laid out so that its components can be inspected, repaired, or replaced 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 require maintenance.

• Access near rotating equipment is important because cranes must reach the equipment when 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 the removal of tube bundles for cleaning and alignment. These tube bundles are over 6.1 m (20 ft.) 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 includes access for firefighting equipment and fire prevention.

Access Clearance for Firefighting Equipment

Firefighting equipment needs clearance to access major pieces of equipment, such as heat exchangers, vessels, and tankage. Pipeways must be routed and designed to provide the necessary 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 containing hazardous fluids that could rupture.

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

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THE FUNCTION OF THE DIFFERENT TYPES OF SUPPORTS AND RESTRAINTS USED IN PLANT AND TRANSPORTATION PIPING

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

A piping system needs supports and restraints because of the various loads that are imposed upon it. Supports and restraints are often needed to permit the piping system to function under 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 sustained longitudinal stress in the pipe within allowable limits, or to limit pipe sag to avoid process flow problems. Restraints are used to control or direct the thermal movement of a piping system. The control of thermal movement may be necessary either to keep pipe thermal expansion stresses within allowable limits, or to limit the loads that are imposed on connected equipment. Restraints also may be necessary to absorb other loads imposed on a piping system and thus to limit pipe deflection and the resultant stresses. Examples of such loads to consider are wind, earthquake, slug flow, water hammer, and other dynamic loads.

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

Selection of a particular type of support or restraint to use in a particular situation depends on such 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 of detailed engineering which is beyond the scope of this course. The following discusses the major 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. The weight comes from the pipe itself, its contents, insulation or lining (if any), and other piping components such as valves, flanges, etc. Restraints are devices which prevent, resist, or limit the free thermal movement of piping, or absorb other applied loads so that they do not have a detrimental impact on the pipe or connected equipment. Depending on the particular situation, 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 to permit vertical thermal expansion is required. A rigid support always will prevent vertical movement downward, also will prevent vertical thermal movement upward sometimes, and will permit lateral movement and rotation. A rigid support is the more common of the two support classes.

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

• Two different shoe support concepts are shown. The choice between the two depends on the pipe diameter and the load to be carried. The design with the single vertical member would be used with small diameter, lightly loaded pipe. The design with two vertical members spreads the applied load over a larger portion of the pipe wall, reduces the local stress in the pipe wall, and would be used for larger pipe diameters and greater loads.

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

• Pipe supports are often made using sections of pipe to provide support, rather than structural members. This type is called a "dummy" support, indicating that there is no flow in the pipe section that is providing the support.

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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 Publishing

Company, 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 pipe and carry piping weight loads in tension. Pipe hangers, as shown in Figure 2, are typically one or more structural steel rods bolted to a pipe attachment and to the overhead member. A hanger rod is designed to move freely both parallel and perpendicular to the pipe axis, and not to restrict thermal expansion in these directions. A hanger will prevent movement both down

and up, and therefore cannot be used to provide support at locations where any vertical

thermal movement will occur. Many types and sizes of pipe supports can be found in a typical vendor's pipe hanger catalog.

HANGERS

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Flexible or Resilient Supports

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

• In the variable-load-type flexible support, the load exerted by the spring on the pipe changes as a result of the pipe thermal movement imparted to the spring. The amount of this load change equals the amount of thermal moment multiplied by the spring constant for the spring. The spring is selected such that it provides the correct amount of supporting 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 pipe remains constant throughout the entire moment range of the support. This is accomplished by using a pivoting, lever arm mechanism. This type of support is used in situations where the load variation in a variable-load-type spring is too large to be accommodated by the piping system, or where the thermal movement is over 75 mm (3 in.).

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

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FLEXIBLE SUPPORTS

Source: Piping Stress Handbook, Second Edition by Victor Helguero M. Copyright © 1986 by Gulf

Publishing 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 by directing or limiting it.

A piping system is generally considered to be totally restrained at its end connection points to equipment. Restraints control, limit, or redirect the thermal movement to either reduce the thermal stress in the pipe or the loads exerted on equipment connections due to thermal movement.

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

This includes wind, earthquake, slug flow, water hammer, or flow-induced vibration which 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 type to use and its specific design details depends primarily on the direction of pipe movement that must be restrained, the location of the restraint point, and the magnitude of the load that must be absorbed. It is also possible to restrain more than one direction at one location in a piping system, or to combine a restraint with a support. Figure 4 provides several examples of restraints.

Guides

A guide is a particular type of restraint. It is used in situations where movement along the pipe axis must be permitted while movement perpendicular to the pipe axis in one or both directions must be prevented. Depending on the particular guide details employed, pipe rotation may or may not be restricted. Common situations where guides are used are in long pipe runs on a pipe rack to control thermal movement and prevent buckling, and in straight pipe runs down the side of a tower to prevent wind-induced movement and control thermal expansion. 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

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Anchors

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

RESTRAINTS/ANCHORS

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Sample Problem 1

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

The following additional information is also available:

• It is unlikely that the pump nozzles can tolerate the load resulting from thermal expansion 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 vertical run 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 (150 ft.) long run from imposing high loads on the pump nozzles, and where might be a good place to try placing it?

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SCHEMATIC FOR SAMPLE PROBLEMS

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Solution

a. Note that, since Locations 1 and 2 are at the top of vertical runs directly over the pump nozzles 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 would not carry any weight. A hanger-rod-type support would be even less effective, since it would restrain the upward thermal movement and result in a large compressive load being imposed on the pump nozzles.

b. A spring support should be used at Location 6 also, for similar reasons as for Locations 1 and 2. Note that if a rigid support was used at Location 6, it would stop the vertical movement downward and cause a very high force and bending movement on the T-101 nozzle.

c. A North/South and East/West lateral guide should be used in the vertical run at Location 7. The guide will absorb the wind load while still permitting free vertical thermal expansion. The exact location of this guide in the vertical run, or whether there should be more than one, would still need to be determined. Since the guide will stop North/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 tower nozzle.

d. A North/South restraint (i.e., a stop) must be used to prevent thermal expansion in the 46 m (150 ft.) long run from imposing large thermal loads on the pump nozzles. The objective is to direct this thermal expansion away from the pumps and into the rest of the system. 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 both pumps from the effects of the large, North/South thermal movement.

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

This section discusses criteria to determine how many supports are needed in a pipe section to ensure that it does not get overstressed or sag too much. This is based on the weight of the pipe 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 can determine the number of supports that are needed and where they are needed. This will allow him to calculate the load on each support, and therefore permit doing the detailed design of each support. For example, if the load is high, the support must be designed to spread it out over a larger area on the pipe to reduce localized pipe stresses. Depending on the complexity of the piping layout, additional weight loads to be applied, the nature of the fluid service and operations, the applied loads at support points and the required support spacing may be determined by hand calculations, available tables, or by a detailed analysis using a piping flexibility 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 evaluated by using other equations to account for differences in pipe geometry or loading, or a piping flexibility analysis computer program. Discussion of other hand-calculation techniques is beyond the scope of this course. The general requirements for a computer analysis are discussed later in this module. With the general availability and ease of use of computer programs, use of hand calculations and table solutions are generally confined to relatively simple 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 the maximum permitted span.

• If the situation is beyond the limitations of the tables, calculating the maximum permitted span given stress and deflection criteria, using either hand calculations or a computer 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 conveniently located structural steel. Spacing of the supports in a pipe rack is usually based on supporting the weakest pipe, although larger spans are acceptable if sagging and pockets in smaller lines is not objectionable. Small lines can be supported off larger lines, bundled with other small lines, or increased in size to be self-supporting.

Allowable spans for horizontal lines are influenced by limits on longitudinal stress or deflection to avoid interference with the nearby pipe or structure, or to avoid excessive sagging that could be detrimental to fluid flow. The span may also be chosen to change the pipe'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 applicable ASME/ANSI B31 Code equation and table. The sum of the longitudinal stresses due to weight and pressure must be limited to the pipe material allowable stress.

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

Deflection Criteria

Deflection under weight effects is generally of secondary importance in piping just as it is in structures. In most process units, however, the deflection should be kept within reasonable limits to minimize pocketing of liquids at low points. Appearance may also be a factor. The maximum deflection is typically limited to the smaller of 25 mm (1 in.) or half the normal pipe 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 deflection criteria. 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/ANSI B31 Code. Note that this assumes that the pipewall thickness exactly matches that required for internal pressure. The longitudinal pressure stress thus also equals half the ASME/ANSI B31 Code allowable stress. This is a simplified 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 the Pipe Properties Table discussed in MEX 101.03. The weight of the pipe must include consideration of the pipe material, contained fluid, external insulation, and internal lining, in lb/ft. Determining the weight of insulation and lining is beyond the scope of this course. The equations used are based on a mean between a uniformly loaded beam simply supported at both ends and one with both ends fixed.

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Maximum Span Tables

Saudi Aramco Standard Drawing AC-036697 also provides maximum allowable spans for unrestrained pipelines based on pipe sizes from 350 mm to 1,500 mm (14 in. to 60 in.) of specified thicknesses, maximum allowable internal pressure, and specified wear pad or saddle details to distribute the load or saddles. This is included in Work Aid 1, and may be used as a convenience 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 of supports required in the 45.7 m (150 ft.) North/South run. For this work, assume the following:

• 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 is 130.3 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 be assumed 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 1 13.5 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

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Loads on Supports

The loads imposed on supports must be considered in the detailed support design to ensure that they are not overstressed, and that they do not overstress the pipe, locally. The loads on the supports will, in turn, be transmitted to other structural members and foundations which also must be designed considering the applied loads. The design of these elements must ensure 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 the applied 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 in the pipe wall resulting from the applied load. In the extreme, high-weight loads at support points could cause the pipe wall to locally deform. Therefore, the support attachment detail must spread the load enough along and around the pipe wall to keep the local stresses in the pipe wall within reasonable limits.

These detailed support design considerations may, in some cases, require the support span to be reduced even if the overall pipe stress and deflection criteria are met. This would occur if the support load is so high that the detailed design becomes impractical, or is more expensive than 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 piping components 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. If the line is not hydrostatically tested, the weight of the line contents is sufficient. Spring hangers are normally designed for the weight of the line contents, so additional support may 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 load

must be considered to the extent that it influences the structure to which the support is attached. 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 be applied to the support that acts opposite to the direction of pipe motion. The support and associated structure must be designed for this frictional load.

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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. The magnitude of these stresses determines whether or not the load needs to be distributed over a wider area. If the load needs to be distributed, then reinforcement pads, saddles or wider pipe shoes are typically used.

Saudi Aramco Standard Drawings AD-036253, AD-036252, and AD-036999 provide standard details for pipe shoes, pads and saddles. Note that these details are based on pipe diameter. More load spreading is required as the diameter increases since the pipe wall becomes 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 a profound impact, as indicated on the next page by the requirements of SAES-L-002 and SAES-L-011. Vortex shedding induced vibration caused by wind may become a problem with piping that is more than about 10 m (30 ft.) long. This generally occurs with piping that runs up along the length of a vertical tower, or for long horizontal runs in exposed locations 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 alternating force to act perpendicular to the wind direction and can make the pipe vibrate. If the frequency of vortex shedding corresponds to a mechanical natural frequency of the piping system, resonant vibration could cause pipe fatigue failure. Analyzing and solving vortex shedding vibration problems is best handled by applying certain principles that include dimensionless parameters and experimental data, which often requires using computer programs. Further discussion regarding vortex shedding is beyond the scope of this course.

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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 vibration where applicable.

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

• The location of supports influences the mechanical natural frequency of the piping system. Thus, they will affect any evaluation of wind-induced vibration since the vibration forcing frequency must be compared to the piping system mechanical natural frequency 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% to mitigate wind-induced resonant vibration of the pipelines. The basic support spacing shall be selected so that the natural frequency of the pipeline in the operating condition is outside the range of wind-induced frequencies, plus or minus 10%, for any wind speed above 9 m/s (20 mph) which will cause vortex shedding.

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DETERMINING THE NEED FOR A PIPING THERMAL FLEXIBILITY / WEIGHT ANALYSIS

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 thermal stresses or produce excessive forces and moments at connected equipment. If a system does not 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 in stresses 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 these thermal movements are allowed to occur without any restraint whatsoever, no pipe stresses or reaction loads result. However, in real systems, stresses are developed in the pipe and moments and forces are imposed on the connected equipment and at supports and restraints installed in the system. The basic problem is to determine the internal pipe stresses and the external loads, and then decide if they are acceptable. A thermal flexibility analysis is done to ensure that the piping system is laid out, supported, and restrained such that the thermal stresses 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 design problem. Determining when a detailed analysis is needed depends on the complexity of the system and the design conditions of each individual situation. There is no single definition of whether to perform an analysis or not that applies to every situation. The following are some guidelines to help the engineer determine when a thermal flexibility analysis is required. Generally speaking, if a detailed analysis is required based on temperature considerations, a weight 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 to consider and usually more than one way to design the system. It requires knowledge of how the operating and design conditions of a piping system influence its overall design, and the supports and restraints required for the system.

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

• To control movement of the pipe to reduce stress that may cause fatigue failure and loads 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 its relatively slender proportions when compared to structural steel systems. Elevated design temperatures or various operating scenarios may cause sufficient pipe thermal stress or reduce material strength such that supplementary structural assistance to support the piping system is required. It is also often necessary to limit the pipe movement at specific locations in order to protect 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 that localized stresses in the pipe wall are kept within allowable limits. This is especially relevant in large-diameter piping systems with relatively thin walls (i.e., outside diameter/thickness ratio over about 95) or in very high-temperature systems. In such cases, support/restraint design details that spread the local loads over larger areas of the pipe wall are typically used to reduce local stresses.

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

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TYPICAL SCHEMATIC OF PUMP SUCTION MANIFOLD

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Approaches

Because of the complexity of the piping flexibility and support design process, there is no single procedure or design method applicable for all situations. Considering this, the engineer can approach support and flexibility design in many ways. The following is a basic way of approaching 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 of elbows (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, orientation of 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 an appropriate computer program to perform 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 the complexity 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, design temperature, the type of equipment connected to the system, and the complexity of the process operations. There are no standard guidelines that will provide the engineer with specific 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 the applicable ASME/ANSI B31 Code for allowable stresses provide guidelines that help determine 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 rotating equipment.

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

• The piping duplicates a successfully operating system.

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• The piping is of uniform size, has no more than two points of fixation, has no intermediate 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 thermal

movements to be absorbed by the piping system in the three coordinate directions, 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-bends with L/U over 2.5 or saw-tooth patterns, for large-diameter thin-walled pipe, nor for systems having large end movements not along the direction that connects anchor points.

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

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

• Accurate calculations are also advisable when the fluid service is hazardous and the pipe material 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 following guidelines may be used to help determine when a detailed analysis is needed.

TYPE OF PIPING PIPE SIZE DIFFERENTIALMAXIMUM

FLEXIBILITY 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 any

For air-fin heat exchangers ≥ 100 ≥ 4 any

For tankage ≥ 300 ≥ 12 any

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

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Saudi Aramco Flexibility Requirements

Saudi Aramco Engineering Standard SAES-L-011, Flexibility, Support, and Anchoring of

Piping, 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 thermal expansion and movements in all piping to show compliance with the design criteria of the Code (i.e., allowable stresses). The exception is for aboveground plant piping without substantial axial restraint that can be readily judged to have adequate flexibility by comparison with successfully operating existing systems.

• This requirement establishes the need for a detailed (formal) analysis, unless it can be readily established that the piping system has adequate flexibility. The guidelines discussed 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 part of 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 the system or if changes to it are needed in the future.

• The formal analysis shall include computer calculations using a generally accepted computer program for piping flexibility analysis, except when the system is considered fully restrained. A sufficient number of calculations shall be made to establish the most severe combinations of load conditions which result in the highest combined piping stresses 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 done using a computer program. Thus, hand calculations and/or chart form solutions are not acceptable for such systems. It also indicates that multiple calculations may be needed to determine the operating scenario that will govern the system design. For example, take the case of a piping system with two pumps, one of which is a spare. Either pump may be operating while the other is down, and both will be running for a short period while they are being switched. Except for a perfectly symmetric piping system layout, it is usually necessary to perform calculations for all three operating scenarios to establish the one case that governs the design. This is because different portions of the system will be hot while others are cold

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DETERMINING THE REQUIRED DESIGN CONDITIONS FOR A THERMAL FLEXIBILITY/WEIGHT ANALYSIS

If a detailed piping flexibility analysis is required, it will normally be done using a recognized computer program such as Caesar II, Simflex, or Triflex. Saudi Aramco engineers will typically use the Simflex program. Such a program has the capability to consider any combination 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 listed below. Anything that influences the thermal flexibility of the piping system can be an applicable 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 the

longitudinal 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 is analyzed. The applicability and impact of the other considerations depend on the particular situation.

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 any orientation. Loads due to thermal expansion, wind, pipe weight, etc., may be considered. Forces, moments, and deflections may be applied and/or evaluated. Further detailed discussion of piping flexibility analysis is beyond the scope of this course.

Saudi Aramco Requirements

Saudi Aramco Engineering Standard SAES-L-011 contains additional detailed requirements that 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 the pipe wall due to any loads from supports, anchors, and/or soil pressure or other external force, except when transmitted through a full encirclement sleeve welded circumferentially to the pipe.

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

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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 in the direction opposite to the expected displacements, where such friction would tend to reduce the piping flexibility significantly.

Displacement calculated by the computer program must agree with the assumed direction 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:

MATERIALS FRICTION FACTOR Steel to steel 0.40 Teflon to steel 0.20 Teflon to Teflon 0.10 Sand to pipewrap 0.25

Sand to plastic coating 0.20

Sand 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 reduce the flexibility of the pipe and increase reaction loads. Friction forces must be included in the design of any associated structural members as an additional applied load.

• The extent that friction loads must be considered in any formal flexibility analysis depends on the particular circumstances. In most cases, the magnitudes of the friction forces are much smaller than the thermal loads developed in the system and can be safely ignored. However, in situations where the friction loads are relatively high, they should be directly included in the flexibility analysis to accurately evaluate their impact. Computer programs that are available today easily permit this by allowing a friction factor 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, where support-point loads are relatively high. In these cases, the friction forces can become high enough to restrict the thermal expansion of the pipe.

- Systems that are connected to load-sensitive equipment where there are large reaction loads at supports or restraints located near the equipment nozzle. In these cases, 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 use sliding 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 specified minimum yield stress (SMYS) of the pipe material, all structural attachments which transfer loads to the pipe through welds shall be welded to full-encirclement sleeves or saddle pads of at least 90° with rounded corners, unless a comprehensive stress analysis is made to prove the adequacy of the design. ASME/ANSI B31.4 Paragraph 421.1(d) and ASME/ANSI B21.8 Paragraph B34.5(B), if applicable, may not be waived. All welds to the pipe shall be continuous.

This requirement ensures that the applied external load will be distributed around the complete circumference of the pipe, unless a stress analysis indicates that this is not necessary. This reduces the local stresses in the pipe wall in situations where it is already 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 the run pipe. This can be accomplished by minimizing the length of the supports and/or increasing the size of the supports.

This requirement recognizes that a dummy support will result in an additional bending moment being applied to the pipe at the attachment point, and this bending moment must be considered in its design. Minimizing the support length reduces the magnitude of the moment to be considered. Increasing the support size, i.e. its diameter, increases the strength of the support itself and spreads the applied moment over a larger area of the pipe. Spreading the applied moment over a larger area on the pipe wall reduces the local 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 be imposed on the pipe by normal and abnormal operating conditions. This results in the largest pipe stress to be considered in fatigue failure evaluation, and the largest reaction loads imposed on equipment end connections, supports, and restraints. The following table provides guidelines to determine the temperatures to consider in a flexibility analysis. Note that more than one of these items might require consideration in a particular system and lead to the need for multiple computer calculations to identify the case that governs the system design.

NORMAL TEMPERATURE CONDITIONS TO CONSIDER

Stable Operation Gives the temperature range expected for most of the time a plant is in operation. Some margin above

equipment operating temperature, i.e., use of the design temperature, allows for process flexibility.

Startup and Shutdown Must be examined to determine if the heating or cooling cycles pose flexibility problems. For example, if a tower is heated while some attached piping remains cold, the piping flexibility should be checked. Regeneration and

Decoking 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 to determine if the piping is adequate for the expected variations of temperature, for no flow in some of the piping, and for switching from one piece of equipment to another. A common example is the piping for two or more pumps with one or more spares.

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ABNORMAL CONDITIONS

Loss of Cooling Medium Flow

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

Steamout for Air or Gas Freeing

Most on-site equipment and lines and many off-site lines are freed of gas or air by the use of steam. For 862 kPa (125 psi.) steam, 149°C (300°F) is used for the metal temperature. Piping connected to equipment which will be steamed out, especially piping connected to upper parts of towers, should be checked for the tower at 149°C (300°F) and the piping at ambient plus 28°C (50°F). This situation may govern the flexibility of lines connected to towers that operate at less than 149°C (300°F) or have a smaller temperature variation from top to bottom.

No Process Flow While

Heating Continues If process flow can be stopped while heat is still beingapplied, the piping flexibility should be checked for the maximum metal temperature. Such situations can occur with steam tracing and steam jacketing.

Metal temperatures that govern the flexibility design of a piping system are not necessarily the ones associated with the most severe coincident pressure and temperature which govern the 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 design while 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 total longitudinal 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 all buried and aboveground fully-restrained pipelines. The design shall be based on the expected temperature rise during operation as well as the maximum anticipated decrease in temperature after tie-in.

• This requirement results in the use of the maximum expected temperature range in the design of a fully-restrained pipeline. This then yields the maximum possible thermal

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Number of Cycles to be Considered

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

displacement cycle under analysis, psi.

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

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

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

Figure

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References

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