ASME B31 · ASME PCC-1 · Piping

ASME B31 piping codes, test pressures & bolt-up, explained

Which B31 code covers your job, how a B31.3 hydrotest number is built, how weld examination scales with fluid service, and what PCC-1 says about flange bolt-up — with a worked example.

Need the code requirements? Get the B31 codes and PCC-1 from ASME →

Which B31 code applies?

Code	Scope (general)	Typical jobs
B31.1 Power Piping	Power plants, boiler external piping, steam and utility systems	Power stations, central plants, district steam
B31.3 Process Piping	Piping in refineries, chemical, pharmaceutical, and similar plants and terminals	Plant piping inside the fence, skids, utilities in process plants
B31.4 Liquid Pipelines	Pipelines transporting liquids (crude, refined products, NGLs, CO₂ and others) between facilities	Cross-country liquid lines, pump stations, terminals' pipeline side
B31.8 Gas Pipelines	Gas transmission and distribution piping	Transmission lines, compressor stations, gas distribution mains
B31.9 Building Services	Building heating, cooling, and utility piping within limits	Commercial HVAC and utility piping

Boundaries between codes (e.g. where plant piping ends and the pipeline begins) are set by the owner's spec and the codes' scope clauses — confirm before testing. Our own summary, not the ASME text. Get the actual requirements from ASME B31.3/B31.4/B31.8 and PCC-1 (asme.org), follow the owner's piping specification and approved test procedure, and verify against the edition your contract references. Not engineering advice.

Pressure testing — how each code approaches it (concepts)

Code	Test concept	What drives the number
B31.3 hydrostatic	A multiple of design pressure, adjusted up when design temperature is hot	Design pressure × a factor × (allowable stress at test temp ÷ allowable stress at design temp), with a cap on that ratio
B31.3 pneumatic	A lower multiple of design pressure, with a relief device and extra precautions	Stored energy in compressed gas — needs owner approval and a safety plan
B31.3 alternatives	Sensitive leak test or an alternative (examination + flexibility analysis) where a pressure test isn't practical	Owner approval and the code's alternative-test clause
B31.4	Hydrostatic test to a multiple of internal design pressure, held for a set duration	Design pressure; leak-test hold where segments aren't visually inspected
B31.8	Test pressure and test medium depend on the location class and the hoop stress the line operates at	Population density along the line (location class) and design factor

Test pressure is always limited by the weakest non-isolated component (flanges, valves, instruments). Our own summary, not the ASME text. Get the actual requirements from ASME B31.3/B31.4/B31.8 and PCC-1 (asme.org), follow the owner's piping specification and approved test procedure, and verify against the edition your contract references. Not engineering advice.

Worked example — B31.3 hydrostatic test pressure

Input / step	Value	Source
Design pressure (P)	200 psig	Line list
Hydrostatic test factor	1.5	B31.3 hydrostatic test clause
Case A — design temperature near ambient (stress ratio = 1.0)	1.5 × 200 × 1.0 = 300 psig	Minimum test pressure
Case B — hot line, stress ratio (test ÷ design allowable) of 1.2 from the allowable stress table	1.5 × 200 × 1.2 = 360 psig	Example ratio — compute yours from the material's allowable stresses
Check the weakest component	Flanges, valves, instruments must tolerate the test pressure	Isolate or remove what can't
Check stresses in the test condition	Weight of water in large gas lines, supports, and test-temperature brittle-fracture risk	Engineer / approved test procedure

Worked example — one code value (the 1.5 factor); the ratio in Case B is illustrative. Always test to the approved test package.

B31.3 weld examination — how it scales by fluid service (concept)

Fluid service category	What it means	Examination approach
Category D	Nonflammable, nontoxic, low pressure and moderate temperature	Visual examination only in most cases
Normal Fluid Service	The default — most process piping	Visual plus random radiography/UT of a percentage of welds, with progressive sampling when rejects are found
Category M	Very small exposure could cause serious harm (toxic)	More extensive examination and additional requirements
High Pressure	Owner-designated high-pressure service (Chapter IX)	100% volumetric examination and tighter acceptance
Severe Cyclic Conditions	High cyclic stress	100% examination and the strictest acceptance criteria

Acceptance criteria (cracks, lack of fusion, porosity, undercut, reinforcement) come from the B31.3 acceptance table, which varies by service and examination method. Our own summary, not the ASME text. Get the actual requirements from ASME B31.3/B31.4/B31.8 and PCC-1 (asme.org), follow the owner's piping specification and approved test procedure, and verify against the edition your contract references. Not engineering advice.

ASME PCC-1 bolted flange joint assembly — the concepts

Concept	What it means	Field impact
Target bolt stress	The bolt stress you are trying to achieve, chosen for the gasket, flange, and bolts — not just the bolt's strength	Get it from the joint's bolt-up procedure; torque is only a means to reach it
Nut factor (K)	Relates torque to bolt load: torque ≈ K × diameter × load	Lubricant choice changes K a lot — use the value the procedure was built with
Legacy cross pattern	The original star/cross sequence with multiple passes at increasing torque	More passes, slower, widely understood
Alternative patterns	Quadrant and multi-bolt patterns (and patterns for multiple tools at once)	Fewer passes and faster when qualified — use only as the procedure allows
Check passes	Circular passes at full torque until nuts stop turning	Required to even out bolt load (elastic interaction)
Assembler qualification and joint records	Trained bolters and a record per joint	Tag each joint; track gasket, lube, tool, and torque

This page does not provide torque values. Our own summary, not the ASME text. Get the actual requirements from ASME B31.3/B31.4/B31.8 and PCC-1 (asme.org), follow the owner's piping specification and approved test procedure, and verify against the edition your contract references. Not engineering advice.

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Piping codes and pressure testing, in plain language

Get the actual requirements from ASME. The B31 codes and PCC-1 (Guidelines for Pressure Boundary Bolted Flange Joint Assembly) are sold by ASME. This page explains them; it doesn’t reproduce their tables or torque values.

Pick the right code first. Inside a refinery or chemical plant, piping is usually B31.3. Once it leaves the plant as a liquid pipeline it’s B31.4; gas transmission and distribution is B31.8. Power plants use B31.1 and commercial building services can use B31.9. Each code has different design factors, test rules, and examination requirements — and the owner’s spec sets the boundaries.

Worked example. A B31.3 line designed for 200 psig near ambient temperature tests hydrostatically at 1.5 × 200 = 300 psig. If the line runs hot, the code raises the test pressure by the ratio of the material’s allowable stress at test temperature to that at design temperature — with an illustrative ratio of 1.2, the test becomes 360 psig. Then check the weakest component: a flange class or valve rating can limit what you’re allowed to put on the system.

Bolt-up is its own discipline. PCC-1 treats a flange joint as a system: the target bolt stress is chosen for the gasket and flange, torque is only a way to get there (through the nut factor of the lubricant), and the tightening pattern — legacy cross or an alternative pattern — plus check passes evens out the load. Use your site’s qualified bolt-up procedure for actual values.

Related: ASME B16.5 flange pressure classes, the steel pipe schedule chart, the weld log template, and the WPS & welder continuity log. Field PM keeps test packages, NDE reports, and bolt-up records with the job.

FAQ

What is the difference between B31.3, B31.4, and B31.8?+

B31.3 covers process piping in plants and terminals; B31.4 covers pipelines carrying liquids such as crude and refined products; B31.8 covers gas transmission and distribution. Where plant piping ends and a pipeline begins is set by the codes' scope clauses and the owner's specification.

How do I calculate a B31.3 hydrotest pressure?+

Multiply the design pressure by 1.5, then by the ratio of the material's allowable stress at test temperature to its allowable stress at design temperature (the code caps that ratio). For a 200 psig line at near-ambient design temperature that gives 300 psig. The result can't exceed what the weakest non-isolated component can take.

When can you pneumatic test instead of hydrotest?+

B31.3 allows a pneumatic test at a lower multiple of design pressure when a hydrotest isn't practical — for example where water would harm the process or the supports can't carry the water weight. It stores far more energy than a hydrotest, so it needs owner approval, a pressure relief device, controlled pressurization steps, and an exclusion zone.

What is the ASME PCC-1 legacy pattern?+

The traditional cross (star) tightening sequence with several passes at increasing torque, followed by circular check passes. PCC-1 also describes alternative patterns that tighten groups of bolts or quadrants in fewer passes. Which one you use, and the target bolt stress, come from the site's qualified bolt-up procedure.

How much of the piping gets radiographed under B31.3?+

It depends on the fluid service category. Category D service is usually visual only; Normal Fluid Service gets visual plus random radiography or UT of a sample of welds with progressive sampling when defects are found; High Pressure and Severe Cyclic services get 100% examination. The owner's spec can require more.

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