Relief Valve vs Safety Valve: Pressure Protection Terms Decoded
Quick Answer
A safety valve is a spring-loaded device that opens suddenly (pop action) when set pressure is reached and is used mainly for compressible fluids such as steam, air and gas under ASME Section VIII. A relief valve opens more gradually in proportion to overpressure and is used for liquid (incompressible) service. Both are pressure relief devices; code language (ASME Section VIII, API 520/521) governs how they are sized and applied.
Terminology by Code
ASME Section VIII uses "safety valve" for compressible fluid and "safety relief valve" as a combined term. API 520/521 discuss pressure relief for refining and chemical services, where "pressure relief valve" (PRV) is the common umbrella term covering both.
| Term | Typical fluid | Opening action |
|---|---|---|
| Safety valve | Compressible (steam, gas, air) | Rapid pop at set pressure |
| Relief valve | Liquid (incompressible) | Gradual, proportional to overpressure |
| PRV (umbrella) | Either | Depends on design |
Key Pressure Terms
- Set pressure: the pressure at which the device opens.
- Accumulation: the allowed pressure rise above set during relief (percentage per code).
- Blowdown: the pressure drop below set at which the valve reseats.
- Overpressure: the pressure above the protected equipment design pressure during relief.
Sizing and Selection
Sizing follows the relieving load, not the operating pressure. API 520 gives the methods; API 521 covers the disposal system. Undersizing risks equipment overpressure, while oversizing can cause chatter and seat damage.
Practical Note
In many plants the words "relief valve" and "safety valve" are used interchangeably for any spring-loaded PRV. To avoid mistakes, the datasheet should state fluid phase, code (ASME Section VIII) and set pressure. For the broader standards picture, see the valve standards overview.
Frequently Asked Questions
What is the difference between a relief valve and a safety valve?
A safety valve opens rapidly (pop action) for compressible fluids; a relief valve opens gradually for liquid service. Both are pressure relief devices under ASME and API codes.
Which code governs safety and relief valves?
ASME Section VIII covers the construction and set pressure rules; API 520 and API 521 cover sizing and disposal system design.
Is a relief valve the same as a PRV?
PRV (pressure relief valve) is the umbrella term. A relief valve is the liquid-service type; a safety valve is the compressible-fluid, pop-action type.
Why does set pressure matter?
Set pressure is the opening point. The protected equipment design pressure must exceed set pressure plus allowed accumulation to stay within code.