Navigating ASME B31
Any specifying engineer involved in the design of pressure piping systems should, at a minimum, be familiar with ASME B31: Code for Pressure Piping.
- Become familiar with various design codes governing pressure piping, including ASME B31.
- Understand common steps involved in pressure piping system design per code requirements.
- Identify several design pitfalls that should be avoided.
When designing a pressure piping system, it is typical for the specifying engineer to indicate that system piping shall comply with a section, or sections, of ASME B31 Code for Pressure Piping. How does the engineer properly follow code requirements when designing a piping system?
To start, an engineer must determine which design code should be selected. For pressure piping systems, this is not necessarily limited to ASME B31. Other codes published by ASME, ANSI, NFPA, or other jurisdictional organizations may govern depending on the project location, application, etc. Within ASME B31 there are currently seven active individual sections.
ASME B31.1 Power Piping: This section covers piping found in electric power generating stations, industrial and institutional plants, geothermal heating systems, and central and district heating and cooling systems. This includes boiler external and non-boiler external piping for installations where an ASME Section I boiler is present. This section does not apply to equipment covered under the ASME Boiler and Pressure Vessel Code, certain low pressure heating and cooling distribution piping, and various other systems as indicated in Paragraph 100.1.3 of ASME B31.1. The origins of ASME B31.1 date back to the 1920s, with the first official edition published in 1935. It should be noted that the first edition, including addenda, was less than 30 pages long, while the current edition is now over 300 pages long.
ASME B31.3 Process Piping: This section covers piping found in petroleum refineries; chemical, pharmaceutical, textile, paper, semiconductor, and cryogenic plants; and related processing plants and terminals. This section is very similar to ASME B31.1, especially when calculating minimum wall thickness in straight pipe. Originally a part of B31.1, this section was first published separately in 1959.
ASME B31.4 Pipeline Transportation Systems for Liquids and Slurries: This section covers piping transporting products that are predominantly liquid between plants and terminals and within terminals, pumping, regulating, and metering stations. Originally a part of B31.1, this section was first published separately in 1959.
ASME B31.5 Refrigeration Piping and Heat Transfer Components: This section covers piping for refrigerants and secondary coolants. Originally a part of B31.1, this section was first published separately in 1962.
ASME B31.8 Gas Transmission and Distribution Piping Systems: This covers piping transporting products that are predominantly gas between sources and terminals, including compressor, regulating, and metering stations; and gas gathering pipelines. Originally a part of B31.1, this section was first published separately in 1955.
ASME B31.9 Building Services Piping: This section covers piping typically found in industrial, institutional, commercial, and public buildings; and multi-unit residences, which does not require the range of sizes, pressures, and temperatures covered in ASME B31.1. This section is similar to ASME B31.1 and B31.3, but is less conservative (specifically when calculating minimum wall thickness) and contains less detail. It is limited to low pressure, low temperature applications indicated in Paragraph 900.1.2 of ASME B31.9. This was first published in 1982.
ASME B31.12 Hydrogen Piping and Pipelines: This section covers piping in gaseous and liquid hydrogen service, and pipelines in gaseous hydrogen service. This section was first published in 2008.
The decision of which design code should be used ultimately lies with the owner. The introduction to ASME B31 states that “It is the owner’s responsibility to select the code section that most nearly applies to a proposed piping installation.” In some instances “more than one code section may apply to different parts of the installation.”
The 2012 edition of ASME B31.1 will be used as the primary reference for the ensuing discussion. The intent of this article is to walk the specifying engineer through some of the major steps in designing a pressure piping system compliant with ASME B31. Following the guidelines of ASME B31.1 provides a good representation of general system design. Similar design approaches are used if following ASME B31.3 or B31.9. The remaining sections of ASME B31 are used for more narrow applications that mainly apply to a specific system or application and will not be discussed further. While key steps in the design process will be highlighted here, this discussion is not meant to be exhaustive and the complete code should always be referenced during system design. All references to the text refer to ASME B31.1 unless noted otherwise.
System and design considerations
Having selected the proper code(s), the system designer must also review any system-specific design requirements. Paragraph 122 (Part 6) provides design requirements pertaining to common systems found in power piping applications such as steam, feedwater, blowoff and blowdown, instrument piping, and pressure relieving systems, among others. ASME B31.3 contains a paragraph similar to that found in ASME B31.1 but with less detail. Items of note within Paragraph 122 include system-specific pressure and temperature requirements, and the definition of the various jurisdictional limits that delineate between boiler proper, boiler external piping, and non-boiler external piping for piping attached to an ASME Section I boiler. Figure 2 shows these limits for drum-type boilers.
The system designer must identify the pressure and temperature at which the system will operate and which conditions the system should be designed to satisfy.
Per Paragraph 101.2, the internal design pressure shall not be less than the maximum sustained operating pressure (MSOP) within the piping system including the effects of static head. Piping subject to external pressure shall be designed for the maximum differential pressure anticipated during operating, shutdown, or test conditions. Additionally, ambient influences need to be accounted for. Per Paragraph 101.4, where the cooling of a fluid may reduce the pressure in the piping to below atmospheric, the piping shall be designed to withstand the external pressure or provisions shall be made to break the vacuum. Where the expansion of a fluid may increase the pressure, the piping system shall be designed to withstand the increased pressure or provision shall be made to relieve the excess pressure.
From Paragraph 101.3.2, the piping shall be designed for a metal temperature representing the maximum sustained condition expected. For simplicity, the metal temperature is typically assumed equal to the fluid temperature. The average metal temperature may be used if desired, so long as the outside wall temperature is known. Special care shall also be taken for fluid passing through a heat exchanger or leading from fired equipment, to ensure that the most severe temperature conditions are being considered.
Typically, a margin of safety is added to both maximum operating pressure and/or temperature by the designer. The magnitude of the margin depends on the application. When determining design temperature, it is also important to consider material limitations. Specifying a high design temperature (greater than 750 F) may require the use of alloy materials instead of the more standard carbon steel. The stress values in Mandatory Appendix A are only provided for those temperatures at which each material is permitted for use. For instance, stress values for carbon steel are only provided up to 800 F. Prolonged exposure of carbon steel to temperatures above 800 F may cause carbonization of the pipe, making it more brittle and susceptible to failure. If operating above 800 F, accelerated creep damage associated with carbon steel should also be considered. See Paragraph 124 for a complete discussion of material temperature limitations.
Sometimes it is within the engineer’s scope to also specify a test pressure for each system. Paragraph 137 provides guidance on pressure testing. Typically, a hydrostatic test with water at 1.5 times the design pressure will be specified; however, hoop and longitudinal stresses in the pipe should not exceed 90% of the material yield strength during the pressure test per paragraph 102.3.3 (B). For some non-boiler external piping systems, an in-service leak test may be a more practical method to check for leaks due to difficulty in isolating certain sections of the system, or simply because the system configuration allows for easy leak testing during initial service. This is acceptable with owner and engineer concurrence.
Once design conditions are established, piping can be specified. The first item to determine is what material to use. As stated previously, different materials have different temperature limitations. Paragraph 105 provides additional limitations for various piping materials. Material selection is also dependent on the system fluid, such as the use of nickel alloy for a corrosive chemical piping application, the use of stainless steel to convey clean instrument air, or the use of carbon steel with high chromium content (greater than 0.1%) to prevent flow accelerated corrosion. Flow accelerated corrosion (FAC) is an erosion/corrosion phenomenon that has been demonstrated to cause severe wall thinning and pipe failure in some of the most critical piping systems. Failure to properly account for thinning of piping components can, and has, led to drastic consequences, such as in 2007 when a desuperheating water line ruptured at KCP&L’s IATAN power station, killing two workers and seriously injuring a third.
Equations 7 and 9 in Paragraph 104.1.1 define minimum required wall thickness and maximum internal design pressure, respectively, in a straight pipe under internal pressure. Variables in these equations include maximum allowable stress (from Mandatory Appendix A), pipe outside diameter, material coefficient (as given in Table 104.1.2 (A)), and any additional thickness allowance (as described below). With so many variables involved, specifying a proper piping material, nominal diameter, and wall thickness can be an iterative process that may also incorporate fluid velocity, pressure drop, and pipe and pumping cost. Whatever the application, it is necessary that the minimum required wall thickness be verified.
Additional thickness allowances may be added to compensate for several reasons, including FAC. An allowance may be required to account for material removal due to threading, grooving, etc., required to make a mechanical joint. Per Paragraph 102.4.2, the minimum allowance should be equal to the thread depth plus the machining tolerance. An allowance may also be required to provide for additional strength to prevent damage, collapse, excessive sag, or buckling of pipe due to superimposed loads or other causes as discussed in Paragraph 102.4.4. Allowances may also be added to account for welded joints (Paragraph 102.4.3) and pipe bends (Paragraph 102.4.5). Finally, an allowance may be added to compensate for corrosion and/or erosion. Per Paragraph 102.4.1, the thickness of this allowance is in the judgment of the designer and should be consistent with the expected life of the piping.
Nonmandatory Appendix IV provides guidance for controlling corrosion. Protective coatings, cathodic protection, and electric isolation (e.g., dielectric flange) are all methods to prevent external corrosion in buried or submerged pipe. Corrosion inhibitors or internal linings may be used to prevent internal corrosion. Care should also be taken to use a hydrostatic testing water of appropriate purity, and to completely drain piping after hydrostatic testing, if necessary.
The minimum pipe wall thickness, or schedule, required per the previous calculations may not be constant over a range of pipe diameters and may require the specification of different schedules for different diameters. Corresponding values for schedule and wall thickness are defined in ASME B36.10 Welded and Seamless Wrought Steel Pipe.
When specifying piping material and performing the calculations discussed previously, it is important to ensure the maximum allowable stress values used in calculations match the material being specified. For example, if A312 304L stainless steel pipe were mistakenly specified instead of A312 304 stainless steel pipe, the wall thickness provided could be insufficient due to the significant difference in maximum allowable stress values between the two materials. Similarly, the method of pipe manufacture should also be specified properly. For example, if calculations are performed using maximum allowable stress values for seamless pipe, then seamless pipe should be specified. If not, seam welded pipe may be provided by the fabricator/erector, which could result in insufficient wall thickness due to a lower maximum allowable stress value.
As an example, assume piping is sized for water with a design temperature of 300 F and a design pressure of 1,200 psig. A 2- and 3-in. carbon steel (A53 Grade B Seamless) line will be used. Determine the proper pipe schedule to specify to meet the requirements of ASME B31.1, Equation 9. First, indicate the design conditions:
P = 1200 psig
T = 300 F
Next, determine the maximum allowable stress value at the above indicated design temperature for A53 Grade B from Table A-1. Note that the value for seamless pipe is used because seamless pipe will be specified:
SE = 17,100 psi
A thickness allowance must also be added. For this application a 1/16-in. corrosion allowance is assumed. A separate milling tolerance will be added later.
A = 0.0625 in.
The value of y is determined from Table 104.1.2(A):
y = 0.4
The 3-in. pipe will be specified first. Assuming a Schedule 40 pipe and a 12.5% milling tolerance, the maximum pressure is calculated:
Do = 3.5 in.
tm = 0.216 in.* 0.875 = 0.189 in.
Schedule 40 pipe is satisfactory for 3-in. pipe at the design conditions specified above. Next, check 2-in. pipe using the same assumptions:
Do = 2.375 in.
tm = 0.154 in.* 0.875 = 0.135 in.
The 2-in. pipe will require a heavier wall thickness than Schedule 40 at the design conditions specified above. Try a 2-in. Schedule 80 pipe:
Do = 2.375 in.
tm = 0.218 in.* 0.875 = 0.191 in.
Schedule 80 pipe is satisfactory for 2-in. pipe at the design conditions specified above.
While pipe wall thickness typically will be the limiting factor when designing for pressure, it is still important to verify that the fittings, components, and connections used are appropriate for the design conditions specified.
As a general rule, per Paragraphs 104.2, 104.7.1, 106, and 107, all valves, fittings, and other pressure-containing components manufactured in accordance with the standards listed in Table 126.1 shall be considered suitable for use under normal operating conditions at or below the specified pressure–temperature ratings specified in those standards. The user is cautioned that where certain standards or manufacturers may impose more restrictive allowances for variation from normal operation than those established by ASME B31.1, the more restrictive allowances shall apply.
At piping intersections, it is advisable to use a tee, lateral, cross, branch weld-on fitting, etc., manufactured in accordance with the standards listed in Table 126.1. In some cases a piping intersection may require a unique branch connection. Paragraph 104.3.1 provides additional requirements for branch connections to ensure there is sufficient piping material present to withstand the pressure.
To simplify design, the designer may choose to set design conditions conservatively high to meet the flange ratings of a certain pressure class (e.g. ASME Class 150, 300, etc.) defined by the pressure-temperature ratings for a particular material specified in ASME B16.5 Pipe Flanges and Flanged Fittings, or a similar standard listed in Table 126.1. This is acceptable provided it does not cause an unnecessary increase in wall thickness or other component design.
Piping structural integrity
An important part of piping design is to ensure that the piping system’s structural integrity is maintained once the effects of pressure, temperature, and external forces are applied. System structural integrity is often overlooked during design and can be one of the more costly parts of design if not done properly. Structural integrity is mainly discussed in two places, Paragraph 104.8: Analysis of Piping Components, and Paragraph 119: Expansion and Flexibility.
Paragraph 104.8 lays out the basic code equations for determining if a piping system has exceeded the code stress allowable. Those code equations are commonly referred to as the sustained loads, occasional loads, and displacement loads. The sustained loads are the effects of pressure and weight on a piping system. The occasional loads are the sustained loads plus the addition of wind, seismic, relief, and other short-term loads that might occur. It is assumed that each occasional load applied will not act simultaneously to the other occasional loads, thus each occasional load will be a separate load case when analyzed. The displacement loads are the effects of thermal growth, equipment displacements during operation, or any other displacement loading.
Paragraph 119 discusses how to handle piping expansion and flexibility in a piping system and how to determine reaction loads. A piping system’s flexibility is typically most important at equipment connections, because most equipment connections can withstand only a minimal amount of forces and moments applied at the point of connection. In most cases the thermal growth of the piping system has the highest impact on the reaction loads, so it is important to control thermal growth accordingly in the system.
To meet piping system flexibility and ensure that a system is properly supported, a good practice is to support steel piping in accordance with Table 121.5. If a designer strives to meet this table for standard support spacing, it accomplishes three things: minimizes dead weight deflection, decreases the sustain loads, and increases the available stress allowable for displacement loads. If a designer places the supports based on Table 121.5, it will typically yield dead weight displacement, or sag, less than 1/8-in. between pipe supports. Minimizing dead weight deflection helps reduce the chance of condensation pooling in piping for piping carrying vapor or gas. Following the spacing recommendations of Table 121.5 also allows the designer to reduce the sustained stress in the piping to approximately 50% of code sustained allowable. Per equation 1B, the stress allowable for displacement loads is negatively correlated with the sustained loads. So, by minimizing sustained loads it allows for the displacement stress allowables to be maximized. Suggested pipe support spacing is shown in Figure 3.
To help ensure that the piping system reaction loads are accounted for properly and code stresses are met, a common method is to perform computer-aided pipe stress analysis of the system. There are several different pipe stress analysis packages that can be used, such as Bentley AutoPIPE, Intergraph Caesar II, Piping Solutions Tri-Flex, or one of the other commercially available packages. The advantage to using computer-aided pipe stress analysis is it allows the designer to create a finite element model of the piping system that allows for easy verification and the ability to make necessary changes to a configuration. An example of a section of pipe modeled and analyzed is shown in Figure 4.
When designing a new system, typically the system designer will specify that all piping and components shall be fabricated, welded, assembled, etc., per the requirements of whichever code is being used. However, in certain retrofit or other applications, it may be beneficial for the specifying engineer to provide guidance on certain fabrication techniques, as specified in Chapter V.
A common issue encountered in retrofit applications is preheating of welds (Paragraph 131) and postweld heat treatment (Paragraph 132). Among other benefits, these heat treatments are applied to relieve stress, prevent cracking, and improve strength at the weld. Items affecting pre- and postweld heat treatment requirements include, but are not limited to, the following: P-number grouping, material chemistry, and thickness of the materials at the joint to be welded. Each material listed in Mandatory Appendix A has an assigned P-number. For preheating, Paragraph 131 provides a minimum temperature to which the base metal must be heated before welding can occur. For postweld heat treatment, Table 132 provides a holding temperature range and a length of time at which to hold the weld area. Heating and cooling rates, methods of temperature measurement, heating techniques, and other procedures should closely follow the guidelines specified by code. Unintended adverse effects to the weld area may result due to failure to properly perform heat treatment.
Another potential area of concern in a pressure piping system is pipe bends. Pipe bending can cause wall thinning that can result in insufficient wall thickness. Per Paragraph 102.4.5, pipe bends are allowed by code as long as the minimum wall thickness satisfies the same equations used to calculate minimum wall thickness for straight pipe. Typically, an allowance is added to account for wall thinning. Table 102.4.5 provides recommended bend thinning allowances for different bend radii. Pre- and/or post-bending heat treatment may be required for pipe bends as well. Paragraph 129 provides guidance for the fabrication of pipe bends.
For many pressure piping systems it is necessary to incorporate a safety, or relief, valve to prevent overpressurization in the system. For these applications, Nonmandatory Appendix II: Rules for the Design of Safety Valve Installations is a very valuable and sometimes lesser known resource.
Per Paragraph II-1.2, a safety valve is characterized by full opening pop action and is used for gas or vapor service, while a relief valve opens relative to static upstream pressure and is used primarily for liquid service.
Safety valve installations are characterized by whether they are an open or a closed discharge system. In an open discharge installation, the elbow at the outlet of the safety valve will normally discharge into a vent stack open to atmosphere. Typically, this will result in less back pressure. If enough back pressure is generated in the vent pipe, a portion of the exhaust may discharge, or blowback, from the inlet end of the vent pipe. The vent pipe should be sized large enough to prevent blowback. In a closed discharge application, pressure can build up at the safety valve outlet due to air compressing in the discharge pipe, possibly causing pressure waves to propagate. In Paragraph II-2.2.2 it is recommended that design pressure of the closed discharge pipe be greater than the steady state operating pressure by a factor of at least two. Figures 5 and 6 illustrate an open and a closed safety valve installation, respectively.
A safety valve installation can be subjected to a variety of forces that are summarized in Paragraph II-2. These forces include the effects of thermal expansion, interaction from the simultaneous discharge of multiple safety valves, seismic and/or vibration effects, and the effects of pressure during a relieving event. While the design pressure up to the safety valve outlet should match that of the run pipe, the design pressure in the discharge system is dependent on the configuration of the discharge system and the properties of the safety valve. Equations are provided in Paragraph II-2.2 to determine the pressure and velocity at the discharge elbow, vent pipe inlet, and vent pipe outlet for both an open and a closed discharge system. Using this information, the reaction forces at the various points in the discharge system can be calculated and accounted for.
A sample problem is provided in Paragraph II-7 for an open discharge application. Other methods exist for the calculation of flow properties in a safety valve discharge system, and the reader is cautioned to verify that the method used is sufficiently conservative. One such method is described in “Analysis of Power Plant Safety and Relief Valve Vent Stacks” by G. S. Liao, published by ASME in the Journal of Engineering for Power in October 1975.
Safety valves should be located a minimum distance of straight pipe away from any bends. This minimum distance is dependent on the service and geometry of the system as defined in Paragraph II-5.2.1. For installations with multiple safety valves, recommended spacing of the valve branch connections is dependent on the radii of the branch and run piping as indicated in Note (10)(c) of Table D-1. Per Paragraph II-5.7.1, it may be necessary for pipe supports located on the safety valve discharge to be connected to the run pipe instead of the adjacent structure to minimize the effects of thermal expansion and seismic interaction. A summary of these and other design considerations in the design of safety valve installations can be found in Paragraph II-5.
Obviously, it is impossible to cover all the design requirements of ASME B31 within the confines of this article. But any specifying engineer involved in the design of pressure piping systems should, at a minimum, be familiar with this design code. Hopefully, armed with the information above, the reader will find ASME B31 a more valuable and approachable resource.
Monte K. Engelkemier is a project principal at Stanley Consultants. Engelkemier is a member of the Iowa Engineering Society, NSPE, and ASME, where he serves on the B31.1 Power Piping Code committee and subcommittees. He has more than 12 years of hands-on experience in pipe system layout, design, support assessment, and stress analysis. Matt Wilkey is a mechanical engineer at Stanley Consultants. He has more than 6 years of professional experience in piping system design for various utility, municipal, institutional, and industrial clients and has been a member of ASME and the Iowa Engineering Society.