Pressure-Relieving Devices
What is covered
API 510 covers the devices that protect vessels: direct-spring and pilot-operated pressure relief valves, rupture disks, and rupture pins. Each device must be sized for the governing relief scenario (API 520/521), set no higher than the vessel's MAWP, and inspected, tested, and maintained per API 576.
Inspection and testing
A PRD inspection includes verifying the nameplate and set pressure against the vessel's current rating, external visual examination of the valve, checking inlet and outlet piping, supports, and that any block valves are locked or sealed open, and bench testing to record as-found and as-left set pressures. Rupture disks are checked for correct burst rating, correct orientation, physical damage, and - where a disk protects a relief valve - a telltale or gauge that reveals disk failure. Intervals are set by the owner/user based on service experience and can run as long as five years for relief valves in benign service; corrosive or fouling service justifies much shorter cycles.
Repairs and setting
Set-pressure adjustment and repair of a relief valve is not routine maintenance: it must be performed by a qualified device repair organization (holder of the National Board VR symbol), and the as-left set pressure documented.
Traps
- Setting a valve above vessel MAWP, or confusing set pressure with relieving pressure (set plus the accumulation allowance - typically 10% for a single valve, 16% for multiple valves, 21% in a fire case).
- Forgetting that a rupture disk's marked burst pressure plus manufacturing tolerance must not exceed MAWP.
- Ignoring inlet-piping loss (rule of thumb: keep it at or below 3% of set pressure for a conventional valve) and outlet back pressure.
Rule of thumb
No device protects a vessel whose MAWP is lower than the device's set pressure - reconcile the vessel rating to the set point before anything else.