PRV vs PSV: Pressure Relief Terminology
PRV and PSV are abbreviations used extensively in oil and gas engineering for pressure-protective devices. The terms are often used interchangeably in everyday practice, but they have distinct definitions in ASME and API standards.
Comparison Table
| Feature | PRV (Pressure Relief Valve) | PSV (Pressure Safety Valve) |
|---|---|---|
| Full name | Pressure Relief Valve | Pressure Safety Valve |
| Opening action | Proportional (gradual opening) | Pop action (full, rapid opening) |
| Media | Liquid (incompressible) | Gas, steam, vapor (compressible) |
| ASME definition | Relief valve for liquid service | Safety valve for compressible service |
| Common misuse | Used generically for any pressure device | Used generically for any pressure device |
| API 520/526 classification | Relief valve | Safety valve |
| Set pressure | Opens in proportion to overpressure | Pops fully open at set pressure |
| Blowdown | Gradual reseat | Snap reseat (~7-10% below set) |
Key Differences
Formal definitions per ASME Section VIII:
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PRV (Pressure Relief Valve): a valve that opens in proportion to the increase in pressure above the set pressure. Designed primarily for liquid (incompressible) service. The disc lifts progressively, relieving only enough fluid to prevent further pressure rise.
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PSV (Pressure Safety Valve): a valve that opens rapidly (pops) to its full rated capacity when the set pressure is reached. Designed primarily for gas, vapor, and steam (compressible) service. The pop action results from the expanding gas pushing against the disc once the initial seal breaks.
In practice, the oil and gas industry frequently uses both abbreviations interchangeably to mean any pressure-protective valve. P&ID legends, equipment lists, and valve schedules often label all spring-loaded pressure relief devices as “PSV” regardless of the actual service media. This informal usage is widespread but technically incorrect.
Safety Relief Valve (SRV): this is the correct term for a valve that combines both functions; popping on gas and opening proportionally on liquid. Most refinery and petrochemical pressure relief devices are safety relief valves and can serve either media.
Correct Usage in Documentation
| Document | Recommended Practice |
|---|---|
| P&ID | Use “PSV” tag number (industry convention) regardless of media |
| Datasheet | Specify type: “safety valve” (gas), “relief valve” (liquid), or “safety relief valve” (dual) |
| API 520 sizing | Size per gas or liquid equations as applicable to the service |
| ASME nameplate | Stamped with UV (safety relief) or V (safety) code symbol |
| Requisition | State full description: “spring-loaded safety relief valve per API 526” |
For sizing, orifice letters, materials, and set pressure selection, see the full guide to safety and pressure relief valves.
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