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Key takeaways
- ASME B31.3 covers process piping inside plants, terminals and platforms; B31.4 covers liquid pipelines; B31.8 covers gas transmission and distribution pipelines.
- The code boundary is usually set at a defined point near the plant fence, such as the first isolation valve or the pig launcher, and must be written down for every project.
- The codes use different design philosophies, so the same pipe can need a different wall thickness on each side of the boundary.
Three codes, three jobs
The ASME B31 family is split by what the piping does rather than by what it carries. B31.3 Process Piping applies to piping in petroleum refineries, chemical and pharmaceutical plants, gas processing plants, terminals and offshore production facilities. B31.4 covers pipeline systems that transport liquids such as crude oil, condensate and refined products between facilities. B31.8 covers gas transmission and distribution pipelines and the compressor and metering stations along them.
On a typical project you will meet all three: B31.3 inside the processing plant, B31.4 for the crude export line, and B31.8 for the gas export or fuel gas supply line.
Where does one code stop?
Each code defines its own scope, and the scopes overlap at the plant edge. That is why every project needs a code break written into the design basis and shown on the P&IDs. Common choices are:
- the first isolation valve outside the plant battery limit,
- the pig launcher or receiver,
- an insulating joint at the fence line,
- the riser top flange on an offshore platform.
On the plant side, everything is designed, examined and tested to B31.3. On the pipeline side, it follows B31.4 or B31.8. A spec break on the P&ID often coincides with the code break, because material, wall thickness and testing requirements all change there.
Different design philosophies
B31.3 sets allowable stress from material strength at temperature, typically the lower of one third of tensile strength and two thirds of yield strength for most metals below the creep range. Pipelines work differently: B31.4 and B31.8 calculate hoop stress as a fraction of the pipe’s specified minimum yield strength (SMYS) using a design factor.
For gas pipelines, B31.8 ties the design factor to the location class, which reflects how many buildings are near the line. The more people nearby, the lower the factor and the thicker the wall. Typical values range from 0.72 in remote areas down to 0.40 in dense urban areas.
| Item | B31.3 | B31.4 | B31.8 |
|---|---|---|---|
| Typical scope | Plants, terminals, platforms | Liquid pipelines | Gas pipelines and stations |
| Stress basis | Allowable stress at temperature | Fraction of SMYS | Fraction of SMYS by location class |
| Typical pipe | ASTM A106, A333, stainless, duplex | API 5L | API 5L |
Offshore
Offshore, the platform topsides are B31.3, but the pipelines leaving the platform often follow B31.4 or B31.8 (both have offshore chapters) or DNV-ST-F101 for submarine pipeline systems. The riser is where the debate happens, so agree the boundary early with the pipeline team and the certifying authority.
Code reference
ASME B31.3 Process Piping: plants, terminals and offshore facilities.
ASME B31.4 Pipeline Transportation Systems for Liquids and Slurries.
ASME B31.8 Gas Transmission and Distribution Piping Systems.
DNV-ST-F101 Submarine pipeline systems.
What to take back to your desk
Before sizing a single line near the fence, find the code break in the design basis. If it isn’t written down, raise it. A missing boundary causes wrong wall thicknesses, wrong test pressures and arguments during commissioning.
Educational content. Always design to your project specification.
