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.