What Is ASME B31.8?

ASME B31.8, Gas Transmission and Distribution Piping Systems, is the American Society of Mechanical Engineers standard that governs the design, fabrication, installation, testing, and maintenance of natural gas transmission and distribution pipeline systems. It is the primary code referenced by the DOT’s 49 CFR Part 192 (Transportation of Natural and Other Gas by Pipeline) and is the technical foundation for pressure testing requirements on gas pipelines across the United States.

Understanding B31.8 pressure testing requirements is essential for anyone involved in gas pipeline commissioning — operators, contractors, inspectors, and nitrogen service providers who perform the pressure testing work.

Location Class and Its Effect on Test Pressure

ASME B31.8 uses a location class system to classify pipeline segments based on the population density and land use in the area through which they pass. Location class directly determines the minimum design factor, MAOP, and required test pressure for each segment:

• Class 1, Division 1: Fewer than 10 buildings intended for human occupancy within 220 yards on either side of the centerline. Design factor 0.80 (steel). This is typical open country, farmland, or desert terrain.
• Class 1, Division 2: Same population criteria but offshore or where crossings of major rivers or highway ROWs apply. Design factor 0.72 (steel).
• Class 2: 10 to 45 buildings within the 220-yard corridor. Design factor 0.60 (steel). Suburban fringe areas.
• Class 3: More than 45 buildings or an area where the pipeline is within 100 yards of a building or small, well-defined outside area (playground, recreation area, theater, etc.) occupied by 20 or more people. Design factor 0.50 (steel). Suburban and light commercial areas.
• Class 4: Areas where multistory buildings are prevalent and traffic is heavy. Design factor 0.40 (steel). Dense urban areas.

Minimum Pressure Test Requirements by Location Class

ASME B31.8 Section 841.122 specifies minimum test pressure requirements for steel gas pipelines:

Location Class	Minimum Test Pressure (hydrostatic)	Minimum Test Pressure (pneumatic/N₂)
Class 1, Division 1	1.1 × SMYS (or 1.1 × MAOP)	1.1 × MAOP
Class 1, Division 2	1.25 × SMYS (or 1.25 × MAOP)	1.25 × MAOP
Class 2	1.25 × SMYS (or 1.25 × MAOP)	1.25 × MAOP
Class 3	1.5 × SMYS (or 1.5 × MAOP)	Not permitted in most cases
Class 4	1.5 × SMYS (or 1.5 × MAOP)	Not permitted in most cases

Note: SMYS = Specified Minimum Yield Strength of the pipe material. For new construction, test pressure is typically calculated as a percentage of SMYS. For qualification of MAOP on existing lines, test pressure is expressed as a multiple of MAOP.

Test Hold Time Requirements

B31.8 does not specify a universal hold time — it requires that the test be held for a “minimum of 8 hours” for steel onshore pipelines under 49 CFR 192.505(b), which incorporates B31.8 by reference. Some operator-specific specifications require longer hold times — 24 or 72 hours — particularly for high-consequence area (HCA) segments or novel construction materials.

During the hold period, pressure must remain stable within the tolerance defined by the temperature correction method being used. A pressure drop that cannot be explained by temperature correction constitutes a test failure.

Acceptable Test Mediums Under B31.8 and 49 CFR 192

49 CFR 192.505 lists the acceptable test mediums for new steel gas transmission pipelines:

• Water (hydrostatic test): Preferred method, required for Class 3 and Class 4 pipelines. Water is incompressible, creating a safer test than gas. Any failure rapidly depressurizes without explosive release.
• Natural gas or inert gas (pneumatic test): Permitted for Class 1 and Class 2 locations only. Nitrogen is the inert gas of choice for pneumatic pipeline pressure testing. Benefits: no dewatering required, no hydrotest water disposal, faster commissioning timeline. Risk: compressed gas stores significant energy — a pneumatic test failure at high pressure can be catastrophic.
• Air: Permitted in some non-hazardous applications but not recommended for gas pipelines due to oxygen contamination risk.

Nitrogen Pressure Testing — When and How

Nitrogen pressure testing is permitted by B31.8 and 49 CFR 192 for Class 1 and Class 2 location pipeline segments. The advantages over hydrostatic testing are significant: no water handling, no dewatering, no post-test nitrogen drying, and a faster path from test completion to gas commissioning.

Key nitrogen pressure testing safety requirements:

• Exclusion zone: All non-essential personnel must be clear of the test segment during pressurization. A rupture under pneumatic test pressure releases enormous energy.
• Staged pressurization: Pressure is typically increased in increments of 10% of test pressure, with holds and visual inspection at each step before continuing to the next increment.
• Temperature monitoring: Temperature correction of pressure readings is required for nitrogen pressure tests just as for hydrostatic tests.
• Documentation: All nitrogen pressure tests must be documented per the same requirements as hydrostatic tests.

NitroTech provides nitrogen for pressure testing operations on Class 1 and Class 2 natural gas pipeline segments nationwide, with equipment capable of reaching up to 2,500 psig delivery pressure. Learn more about pipeline nitrogen services or request a quote.