When designing a pressure piping system, the designating engineer will often specify that the system piping should conform to one or more parts of the ASME B31 Pressure Piping Code.How do engineers properly follow code requirements when designing piping systems?
First, the engineer must determine which design specification should be selected.For pressure piping systems, this is not necessarily limited to ASME B31.Other codes issued by ASME, ANSI, NFPA, or other governing organizations may be governed by project location, application, etc.In ASME B31, there are currently seven separate sections in effect.
ASME B31.1 Electrical Piping: This section covers piping in power stations, industrial and institutional plants, geothermal heating systems, and central and district heating and cooling systems.This includes boiler exterior and non-boiler exterior piping used to install ASME Section I boilers.This section does not apply to equipment covered by the ASME Boiler and Pressure Vessel Code, certain low pressure heating and cooling distribution piping, and various other systems described in paragraph 100.1.3 of ASME B31.1.The origins of ASME B31.1 can be traced back to the 1920s, with the first official edition published in 1935.Note that the first edition, including the appendices, was less than 30 pages, and the current edition is over 300 pages long.
ASME B31.3 Process Piping: This section covers piping in refineries; chemical, pharmaceutical, textile, paper, semiconductor, and cryogenic plants; and associated processing plants and terminals.This section is very similar to ASME B31.1, especially when calculating the minimum wall thickness for straight pipe.This section was originally part of B31.1 and was first released separately in 1959.
ASME B31.4 Pipeline Transportation Systems for Liquids and Slurry: This section covers piping that transports primarily liquid products between plants and terminals, and within terminals, pumping, conditioning, and metering stations.This section was originally part of B31.1 and was first released separately in 1959.
ASME B31.5 Refrigeration Piping and Heat Transfer Components: This section covers piping for refrigerants and secondary coolants.This part was originally part of B31.1 and was first released separately in 1962.
ASME B31.8 Gas Transmission and Distribution Piping Systems: This includes piping to transport primarily gaseous products between sources and terminals, including compressors, conditioning and metering stations; and gas gathering piping.This section was originally part of B31.1 and was first released separately in 1955.
ASME B31.9 Building Services Piping: This section covers piping commonly found in industrial, institutional, commercial, and public buildings; and multi-unit dwellings that do not require the size, pressure, and temperature ranges covered in ASME B31.1.This section is similar to ASME B31.1 and B31.3, but is less conservative (especially when calculating minimum wall thickness) and contains less detail.It is limited to low pressure, low temperature applications as indicated in ASME B31.9 paragraph 900.1.2.This was first published in 1982.
ASME B31.12 Hydrogen Piping and Piping: This section covers piping in gaseous and liquid hydrogen service, and piping in gaseous hydrogen service.This section was first published in 2008.
Which design code should be used is ultimately up to the owner.The introduction to ASME B31 states, “It is the owner’s responsibility to select the code section that most closely approximates the proposed piping installation.” In some cases, “multiple code sections may apply to different sections of the installation.”
The 2012 edition of ASME B31.1 will serve as the primary reference for subsequent discussions.The purpose of this article is to guide the designating engineer through some of the main steps in designing an ASME B31 compliant pressure piping system.Following the guidelines of ASME B31.1 provides a good representation of general system design.Similar design methods are used if ASME B31.3 or B31.9 is followed.The remainder of ASME B31 is used in narrower applications, primarily for specific systems or applications, and will not be discussed further.While key steps in the design process will be highlighted here, this discussion is not exhaustive and the complete code should always be referenced during system design.All references to text refer to ASME B31.1 unless otherwise stated.
After selecting the correct code, the system designer must also review any system-specific design requirements.Paragraph 122 (Part 6) provides design requirements related to systems commonly found in electrical piping applications, such as steam, feedwater, blowdown and blowdown, instrumentation piping, and pressure relief systems.ASME B31.3 contains similar paragraphs to ASME B31.1, but with less detail.Considerations in paragraph 122 include system-specific pressure and temperature requirements, as well as various jurisdictional limitations delineated between the boiler body, boiler external piping, and non-boiler external piping connected to ASME Section I boiler piping. definition.Figure 2 shows these limitations of the drum boiler.
The system designer must determine the pressure and temperature at which the system will operate and the conditions the system should be designed to meet.
According to paragraph 101.2, the internal design pressure shall not be less than the maximum continuous working pressure (MSOP) within the piping system, including the effect of static head.Piping subjected to external pressure shall be designed for the maximum differential pressure expected under operating, shutdown or test conditions.In addition, environmental impacts need to be considered.According to paragraph 101.4, if cooling of the fluid is likely to reduce the pressure in the pipe to below atmospheric pressure, the pipe shall be designed to withstand external pressure or measures shall be taken to break the vacuum.In situations where fluid expansion may increase pressure, piping systems should be designed to withstand the increased pressure or measures should be taken to relieve excess pressure.
Beginning in Section 101.3.2, the metal temperature for piping design shall be representative of the expected maximum sustained conditions.For simplicity, it is generally assumed that the metal temperature is equal to the fluid temperature.If desired, the average metal temperature can be used as long as the outer wall temperature is known.Particular attention should also be paid to fluids drawn through heat exchangers or from combustion equipment to ensure the worst temperature conditions are taken into account.
Often, designers add a safety margin to the maximum working pressure and/or temperature.The size of the margin depends on the application.It is also important to consider material constraints when determining the design temperature.Specifying high design temperatures (greater than 750 F) may require the use of alloy materials rather than the more standard carbon steel.The stress values in Mandatory Appendix A are provided only for the permissible temperatures for each material.For example, carbon steel can only provide stress values up to 800 F.Prolonged exposure of carbon steel to temperatures above 800 F may cause the pipe to carbonize, making it more brittle and prone to failure.If operating above 800 F, the accelerated creep damage associated with carbon steel should also be considered.See paragraph 124 for a full discussion of material temperature limits.
Sometimes engineers can also specify test pressures for each system.Paragraph 137 provides guidance on stress testing.Typically, hydrostatic testing will be specified at 1.5 times the design pressure; however, the hoop and longitudinal stresses in the piping shall not exceed 90% of the yield strength of the material in paragraph 102.3.3 (B) during the pressure test.For some non-boiler external piping systems, in-service leak testing may be a more practical method of checking for leaks due to difficulties in isolating parts of the system, or simply because the system configuration allows for simple leak testing during initial service. Agreed, this is acceptable.
Once the design conditions are established, the piping can be specified.The first thing to decide is what material to use.As mentioned earlier, different materials have different temperature limits.Paragraph 105 provides additional restrictions on various piping materials.Material selection also depends on the system fluid, such as nickel alloys in corrosive chemical piping applications, stainless steel to deliver clean instrument air, or carbon steel with a high chromium content (greater than 0.1%) to prevent flow accelerated corrosion.Flow Accelerated Corrosion (FAC) is an erosion/corrosion phenomenon that has been shown to cause severe wall thinning and pipe failure in some of the most critical piping systems.Failure to properly consider thinning of plumbing components can and has had serious consequences, such as in 2007 when a desuperheating pipe at KCP&L’s IATAN power station burst, killing two workers and seriously injuring a third.
Equation 7 and Equation 9 in paragraph 104.1.1 define the minimum required wall thickness and maximum internal design pressure, respectively, for straight pipe subject to internal pressure.The variables in these equations include the maximum allowable stress (from Mandatory Appendix A), the outside diameter of the pipe, the material factor (as shown in Table 104.1.2 (A)), and any additional thickness allowances (as described below).With so many variables involved, specifying the appropriate piping material, nominal diameter, and wall thickness can be an iterative process that may also include fluid velocity, pressure drop, and piping and pumping costs.Regardless of the application, the minimum wall thickness required must be verified.
Additional thickness allowance may be added to compensate for various reasons including FAC.Allowances may be required due to the removal of threads, slots, etc. material required to make mechanical joints.According to paragraph 102.4.2, the minimum allowance shall be equal to the thread depth plus the machining tolerance.Allowance may also be required to provide additional strength to prevent pipe damage, collapse, excessive sag, or buckling due to superimposed loads or other causes discussed in paragraph 102.4.4.Allowances can also be added to account for welded joints (paragraph 102.4.3) and elbows (paragraph 102.4.5).Finally, tolerances can be added to compensate for corrosion and/or erosion.The thickness of this allowance is at the designer’s discretion and shall be consistent with the expected life of the piping in accordance with paragraph 102.4.1.
Optional Annex IV provides guidance on corrosion control.Protective coatings, cathodic protection, and electrical isolation (such as insulating flanges) are all methods of preventing external corrosion of buried or submerged pipelines.Corrosion inhibitors or liners can be used to prevent internal corrosion.Care should also be taken to use hydrostatic test water of the appropriate purity and, if necessary, to completely drain the piping after hydrostatic testing.
The minimum pipe wall thickness or schedule required for previous calculations may not be constant across the pipe diameter and may require specifications for different schedules for different diameters.Appropriate schedule and wall thickness values are defined in ASME B36.10 Welded and Seamless Forged Steel Pipe.
When specifying the pipe material and performing the calculations discussed earlier, it is important to ensure that the maximum allowable stress values used in the calculations match the specified material.For example, if A312 304L stainless steel pipe is incorrectly designated as A312 304 stainless steel pipe, the wall thickness provided may be insufficient due to the significant difference in maximum allowable stress values between the two materials.Likewise, the method of manufacture of the pipe shall be appropriately specified.For example, if the maximum allowable stress value for seamless pipe is used for the calculation, seamless pipe should be specified.Otherwise, the manufacturer/installer may offer seam welded pipe, which may result in insufficient wall thickness due to lower maximum allowable stress values.
For example, suppose the design temperature of the pipeline is 300 F and the design pressure is 1,200 psig.2″ and 3″.Carbon steel (A53 Grade B seamless) wire will be used.Determine the appropriate piping plan to specify to meet the requirements of ASME B31.1 Equation 9.First, the design conditions are explained:
Next, determine the maximum allowable stress values for A53 Grade B at the above design temperatures from Table A-1.Note that the value for seamless pipe is used because seamless pipe is specified:
Thickness allowance must also be added.For this application, a 1/16 inch.Corrosion allowance is assumed.A separate milling tolerance will be added later.
3 inches.The pipe will be specified first.Assuming a Schedule 40 pipe and a 12.5% milling tolerance, calculate the maximum pressure:
Schedule 40 pipe is satisfactory for 3 inches.tube in the design conditions specified above.Next, check 2 inches.The pipeline uses the same assumptions:
2 inches.Under the design conditions specified above, the piping will require a thicker wall thickness than Schedule 40.Try 2 inches.Schedule 80 Pipes:
While pipe wall thickness is often the limiting factor in pressure design, it is still important to verify that the fittings, components and connections used are suitable for the specified design conditions.
As a general rule, in accordance with paragraphs 104.2, 104.7.1, 106 and 107, all valves, fittings and other pressure-containing components manufactured to the standards listed in Table 126.1 shall be deemed suitable for use under normal operating conditions or below those standards pressure-temperature ratings specified in .Users should be aware that if certain standards or manufacturers may impose stricter limits on deviations from normal operation than those specified in ASME B31.1, the stricter limits shall apply.
At pipe intersections, tees, transverses, crosses, branch welded joints, etc., manufactured to the standards listed in Table 126.1 are recommended.In some cases, pipeline intersections may require unique branch connections.Paragraph 104.3.1 provides additional requirements for branch connections to ensure that there is sufficient piping material to withstand the pressure.
To simplify the design, the designer may choose to set the design conditions higher to meet the flange rating of a certain pressure class (eg ASME class 150, 300, etc.) as defined by the pressure-temperature class for specific materials specified in ASME B16 .5 Pipe flanges and flange joints, or similar standards listed in Table 126.1.This is acceptable as long as it does not result in an unnecessary increase in wall thickness or other component designs.
An important part of piping design is ensuring that the structural integrity of the piping system is maintained once the effects of pressure, temperature and external forces are applied.System structural integrity is often overlooked in the design process and, if not done well, can be one of the more expensive parts of the design.Structural integrity is discussed primarily in two places, Paragraph 104.8: Pipeline Component Analysis and Paragraph 119: Expansion and Flexibility.
Paragraph 104.8 lists the basic code formulas used to determine whether a piping system exceeds code allowable stresses.These code equations are commonly referred to as continuous loads, occasional loads, and displacement loads.Sustained load is the effect of pressure and weight on a piping system.Incidental loads are continuous loads plus possible wind loads, seismic loads, terrain loads and other short-term loads.It is assumed that each incidental load applied will not act on other incidental loads at the same time, so each incidental load will be a separate load case at the time of analysis.Displacement loads are the effects of thermal growth, equipment displacement during operation, or any other displacement load.
Paragraph 119 discusses how to handle pipe expansion and flexibility in piping systems and how to determine reaction loads.Flexibility of piping systems is often most important in equipment connections, as most equipment connections can only withstand the minimum amount of force and moment applied at the connection point.In most cases, the thermal growth of the piping system has the greatest effect on the reaction load, so it is important to control the thermal growth in the system accordingly.
To accommodate the flexibility of the piping system and to ensure that the system is properly supported, it is good practice to support steel pipes in accordance with Table 121.5.If a designer strives to meet the standard support spacing for this table, it accomplishes three things: minimizes self-weight deflection, reduces sustained loads, and increases available stress for displacement loads.If the designer places the support in accordance with Table 121.5, it will typically result in less than 1/8 inch of self-weight displacement or sag.between the tube supports.Minimizing self-weight deflection helps reduce the chance of condensation in pipes carrying steam or gas.Following the spacing recommendations in Table 121.5 also allows the designer to reduce the sustained stress in the piping to approximately 50% of the code’s continuous allowable value.According to Equation 1B, the allowable stress for displacement loads is inversely related to sustained loads.Therefore, by minimizing the sustained load, the displacement stress tolerance can be maximized.The recommended spacing for pipe supports is shown in Figure 3.
To help ensure that piping system reaction loads are properly considered and that code stresses are met, a common method is to perform a computer-aided piping stress analysis of the system.There are several different pipeline stress analysis software packages available, such as Bentley AutoPIPE, Intergraph Caesar II, Piping Solutions Tri-Flex, or one of the other commercially available packages.The advantage of using computer-aided piping stress analysis is that it allows the designer to create a finite element model of the piping system for easy verification and the ability to make necessary changes to the configuration.Figure 4 shows an example of modeling and analyzing a section of pipeline.
When designing a new system, system designers typically specify that all piping and components should be fabricated, welded, assembled, etc. as required by whatever code is used.However, in some retrofits or other applications, it may be beneficial for a designated engineer to provide guidance on certain manufacturing techniques, as described in Chapter V.
A common problem encountered in retrofit applications is weld preheat (paragraph 131) and post-weld heat treatment (paragraph 132).Among other benefits, these heat treatments are used to relieve stress, prevent cracking, and increase weld strength.Items that affect pre-weld and post-weld heat treatment requirements include, but are not limited to, the following: P number grouping, material chemistry, and thickness of material at the joint to be welded.Each material listed in Mandatory Appendix A has an assigned P number.For preheating, paragraph 131 provides the minimum temperature to which the base metal must be heated before welding can occur.For PWHT, Table 132 provides the hold temperature range and length of time to hold the weld zone.Heating and cooling rates, temperature measurement methods, heating techniques, and other procedures should strictly follow the guidelines set forth in the code.Unexpected adverse effects on the welded area can occur due to failure to properly heat treat.
Another potential area of concern in pressurized piping systems is pipe bends.Bending pipes can cause wall thinning, resulting in insufficient wall thickness.According to paragraph 102.4.5, the code allows bends as long as the minimum wall thickness satisfies the same formula used to calculate the minimum wall thickness for straight pipe.Typically, an allowance is added to account for wall thickness.Table 102.4.5 provides recommended bend reduction allowances for different bend radii.Bends may also require pre-bending and/or post-bending heat treatment.Paragraph 129 provides guidance on the manufacture of elbows.
For many pressure piping systems, it is necessary to install a safety valve or relief valve to prevent overpressure in the system.For these applications, the optional Appendix II: Safety Valve Installation Design Rules is a very valuable but sometimes little-known resource.
In accordance with paragraph II-1.2, safety valves are characterized by a fully open pop-up action for gas or steam service, while safety valves open relative to upstream static pressure and are used primarily for liquid service.
Safety valve units are characterized by whether they are open or closed discharge systems.In an open exhaust, the elbow at the outlet of the safety valve will usually exhaust into the exhaust pipe to atmosphere.Typically, this will result in less back pressure.If sufficient back pressure is created in the exhaust pipe, a portion of the exhaust gas may be expelled or backflushed from the inlet end of the exhaust pipe.The size of the exhaust pipe should be large enough to prevent blowback.In closed vent applications, pressure builds up at the relief valve outlet due to air compression in the vent line, potentially causing pressure waves to propagate.In paragraph II-2.2.2, it is recommended that the design pressure of the closed discharge line be at least two times greater than the steady state working pressure.Figures 5 and 6 show the safety valve installation open and closed, respectively.
Safety valve installations may be subject to various forces as summarized in paragraph II-2.These forces include thermal expansion effects, the interaction of multiple relief valves venting simultaneously, seismic and/or vibration effects, and pressure effects during pressure relief events.Although the design pressure up to the outlet of the safety valve should match the design pressure of the down pipe, the design pressure in the discharge system depends on the configuration of the discharge system and the characteristics of the safety valve.Equations are provided in paragraph II-2.2 for determining pressure and velocity at the discharge elbow, discharge pipe inlet, and discharge pipe outlet for open and closed discharge systems.Using this information, the reaction forces at various points in the exhaust system can be calculated and accounted for.
An example problem for an open discharge application is provided in paragraph II-7.Other methods exist for calculating flow characteristics in relief valve discharge systems, and the reader is cautioned to verify that the method used is sufficiently conservative.One such method is described by GS Liao in “Power Plant Safety and Pressure Relief Valve Exhaust Group Analysis” published by ASME in the Journal of Electrical Engineering, October 1975.
The relief valve should be located at a minimum distance of straight pipe away from any bends.This minimum distance depends on the service and geometry of the system as defined in paragraph II-5.2.1.For installations with multiple relief valves, the recommended spacing for valve branch connections depends on the radii of the branch and service piping, as shown in Note (10)(c) of Table D-1.In accordance with paragraph II-5.7.1, it may be necessary to connect piping supports located at relief valve discharges to operating piping rather than adjacent structures to minimize the effects of thermal expansion and seismic interactions.A summary of these and other design considerations in the design of safety valve assemblies can be found in paragraph II-5.
Obviously, it is not possible to cover all design requirements of ASME B31 within the scope of this article.But any designated engineer involved in the design of a pressure piping system should at least be familiar with this design code.Hopefully, with the above information, readers will find ASME B31 a more valuable and accessible resource.
Monte K. Engelkemier is the project leader at Stanley Consultants.Engelkemier is a member of the Iowa Engineering Society, NSPE, and ASME, and serves on the B31.1 Electrical Piping Code Committee and Subcommittee.He has over 12 years of hands-on experience in piping system layout, design, bracing evaluation and stress analysis.Matt Wilkey is a Mechanical Engineer at Stanley Consultants.He has over 6 years of professional experience designing piping systems for a variety of utility, municipal, institutional and industrial clients and is a member of ASME and the Iowa Engineering Society.
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