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Overview of ASME Standard B31.3

Updated on April 16, 2017
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Tamara Wilhite is a technical writer, engineer, mother of 2, and published scifi and horror author.


ASME standard B31 is the American Society of Mechanical Engineers’ code for high pressure piping. ASME B31.3 is the code for process piping and covers different types of fluids, including Class M fluids and Class D fluids.

ASME was founded to prevent boiler explosions and piping ruptures that put construction and boiler workers at risk.
ASME was founded to prevent boiler explosions and piping ruptures that put construction and boiler workers at risk. | Source

ASME Standard B31.3

ASME B31.3 specifies a list of materials that can be used in process piping. These listed materials are allowed for use in high pressure pipe designs.

ASME B31.3 also allows for the use of unlisted materials if the material can handle the acceptable stresses at the expected operational pressures. Reclaimed materials and re-used pipes are permitted within limits for ASME B31.3 pipes, whereas B31.1 does not allow reclaimed or used material.

B31.3 also requires vent systems for many types of process piping. Venting systems ensure that pressure can be relieved in a controlled manner instead of risking uncontrolled pipe ruptures. However, for category M fluids, additional limits on venting of processed fluids are required to protect bystanders.

ASME B31.3 requires process pipes to have full structural connections. Full structural connections are considered to occur when bolts and threads are properly joined. ASME standard B31.3 says that bolts should extend completely through the nut. However, if the bolt cannot extend completely through the nut, it can be short up to one thread width.

ASME B31.3 requires process piping to be arranged and supported to eliminate excessive harmful effects. ASME B31.3 requires designers to consider the stresses vibrations cause within piping as well as the operational conditions. Vibrations can be caused by pressure cycling within pipe, fluid flow turbulence, resonance caused by equipment like motors and pumps, sudden impacts and wind. Short term higher pressure loads such as snow and ice build up are can also be allowed if the piping can handle the stresses.

ASME Categories of Fluids

ASME B31.3 classifies category M fluids are those that are particularly toxic or hazardous. Figure M300 in ASME B31.3 contains a logic flow chart to help readers determine if fluids are classified as category M or category D (normal fluid service).

Generally, category fluid M service involves fluids that are dangerous if anyone exposed to even a small amount, causing irreversible harm. For class M fluid service, additional safeguards and precautions are necessary. For example, exhaust vents cannot release gas into work spaces or maintenance shafts where workers may be located.

Related Standards

ASME B31.1 is the standard for steam water loop piping used in geothermal systems and power plants. ASME B31.2 was the standard for fuel gas piping but has been replaced by ANSI standard Z223.1. ASME B31.4 covers liquid transportation pipes for moving hydrocarbons like oil and other types of liquids.

B31.5 covers heat transfer pipes and refrigeration. There is no ASME B31.6. ASME B31.7 was originally the section on nuclear piping but this type of piping is now covered under the ASME Boiler and Pressure Vessel code or ASME BPVC, section 3.

ASME B31.8 is the design code for buried gas pipes such as those used to transport natural gas. ASME B31.9 covers building services piping such as that in schools and hospitals.

There is no ASME B31.10.That section could be added at a later date.

ASME B31.11 addresses slurry piping. ASME B31.12 is specific to pipes that carry hydrogen. ASME B31.12 is similar but not exactly copied from ASME B31.3. ASME B31.3 uses the definition of high pressure based on the pressure-temperature charts given in ASME B16.5 as class 2500.


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