Pressure Relief Valve Capacity Certification and Code Compliance

Pressure relief valves protecting ASME Code vessels must be capacity-certified by the National Board of Boiler and Pressure Vessel Inspectors. Understanding the certification process, code stampings, and documentation requirements is essential for process safety engineers and inspection teams.

ASME Code Sections Governing PRV Selection

• Section I: power boilers, requires pressure-relief valves with National Board certificate of authorization

• Section VIII Division 1: unfired pressure vessels, UG-125 through UG-136 govern overpressure protection

• Section III: nuclear components, extremely stringent design and testing requirements

• B31.1 and B31.3: piping systems, reference Section I and VIII for valve selection

National Board Capacity Certification

All PRVs protecting ASME Code vessels must bear the NB symbol stamp. Manufacturers must hold a National Board Certificate of Authorization (VR stamp for repair organizations, V stamp for new construction). Capacity is certified based on flow testing at an ASME-accredited test laboratory. The rated capacity is 90 percent of the measured capacity to provide a conservative margin.

Orifice Designation and Sizing

ASME Section VIII provides standard orifice designations (D through T) for spring-loaded PRVs with corresponding effective flow areas. Sizing uses the formula: A = W divided by (C times K times P1 times sqrt(M divided by T times Z)) for gas service, where C is a gas constant, K is the coefficient of discharge (typically 0.975 times 0.90 equals 0.878 for ASME), P1 is relieving pressure, M is molecular weight, T is relieving temperature, and Z is compressibility.

Set Pressure, Accumulation, and Overpressure

• Set pressure: pressure at which the PRV begins to open; must not exceed the maximum allowable working pressure (MAWP)

• Accumulation: pressure increase above MAWP during PRV discharge; ASME allows 10 percent for single PRV, 16 percent for fire case

• Overpressure: difference between actual relieving pressure and set pressure, used in sizing calculations (typically 10 percent)

• Blowdown: pressure reduction below set pressure before PRV reseats, typically 7 to 10 percent

Documentation and Traceability

Each PRV installation requires a data sheet listing tag number, set pressure, relieving capacity, orifice size, inlet and outlet connection, back pressure correction, and code compliance. The valve nameplate must show the manufacturer's name, model number, set pressure, capacity in SCFM air equivalent (for gas) or GPM water equivalent (for liquid), orifice designation, and NB number. Maintain records throughout the valve's service life for each test and recertification.