Pressure Relief Devices and Certification
Pressure relief is the last line of defense, and the exam tests whether you can check three things at once: set pressure, capacity, and certification.
The stamps
V - safety valves for Section I power boilers; HV - heating boiler safety valves (Section IV); UV - pressure relief valves for Section VIII vessels; UD - rupture disk devices; NV - nuclear. Capacity is established by test and certified capacities are published in the National Board's listings. Repaired devices require a VR Certificate of Authorization, a calibrated test stand, and a report of the repair.
The three checks
- Set pressure: must not exceed the item's MAWP.
- Accumulation: Section VIII Div. 1 limits pressure rise to 110% of MAWP with a single device and 116% when multiple devices are installed; Section I limits accumulation to 110%, with the highest-set valve at or below MAWP.
- Capacity: certified relieving capacity at the accumulation pressure must equal or exceed the maximum generating or input rate. Multiple smaller valves can satisfy capacity where one large valve will not fit.
Worked example. A Div. 1 vessel with MAWP 200 psig has one UV valve set at 200 psig. Allowable accumulation is 110% = 220 psig, and the valve's certified capacity is evaluated at 220 psig, not merely at its set point. If a second valve set at 180 psig were added, accumulation could reach 116% = 232 psig, and the combined certified capacity at 232 psig must cover the maximum input.
Traps. Checking set pressure only; forgetting that a rupture disk upstream of a valve derates its capacity; accepting a device with a missing or altered nameplate; returning a VR-repaired device to service without test records; assuming a high set pressure is acceptable because the plant never exceeds 150 psig.