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What Is a Safety Relief Valve (SRV)? | Process Engineering Glossary
What Is a Safety Relief Valve (SRV)?
In piping engineering and process engineering, a safety relief valve (SRV) is a spring-loaded pressure relief valve that can be used in either gas and vapor service or liquid service, depending on how it is applied. It opens automatically when system pressure reaches its set pressure, discharges enough fluid to prevent excessive pressure buildup, and recloses once pressure falls back to a safe level.
Safety Valve, Relief Valve, and Safety Relief Valve
The terms describe different opening behaviors. A safety valve pops fully open almost instantly and is used for gases and vapors. A relief valve opens gradually in proportion to overpressure and is used mainly for liquids. A safety relief valve combines both: it pops open in compressible service and opens more progressively in liquid service. In everyday practice, many plants call all of these a pressure safety valve, or PSV.
Most SRVs are of the conventional relief valve type, with a spring holding the disc against the nozzle. Where backpressure is high or variable, a balanced bellows relief valve or pilot-operated relief valve may be used instead.
Applications
Vessel and Equipment Protection
SRVs protect pressure vessels, heat exchangers, and piping against blocked outlets, fire exposure, and control failures.
Liquid Thermal Relief
Small SRVs protect blocked-in liquid lines from pressure rise caused by solar heating or heat tracing. These valves typically have small orifices because only a small volume needs to be relieved.
Pump and Compressor Discharge
SRVs protect positive displacement equipment from overpressure when downstream valves are closed. The discharge is often routed back to the suction side or to a closed system.
Benefits
Automatic Protection
SRVs act without power, instruments, or operator action, making them the final layer of protection against overpressure.
Versatile Service
A single valve type can cover gas, vapor, and liquid applications.
Proven and Standardized
Design, sizing, testing, and certification follow well-established codes and standards. Valves are stamped and certified for their set pressure and capacity.
Limitations
Sensitive to Installation
High inlet pressure loss or excessive backpressure can cause unstable operation and relief valve chatter.
Requires Maintenance
Valves must be bench tested and serviced at regular intervals to confirm they open at the correct pressure and reseat tightly. Inspection intervals are usually set by the service conditions and local regulations.
Leakage Near Set Pressure
Operating too close to the set pressure can cause seat leakage, known as simmering. A common guideline is to keep normal operating pressure at least about 10 percent below set pressure.
FAQ
What is the set pressure of an SRV?
It is the inlet pressure at which the valve is adjusted to open. The set pressure is usually at or below the maximum allowable working pressure of the protected equipment.
How much can pressure rise when an SRV opens?
Codes allow a limited overpressure above set pressure while the valve relieves, commonly 10 percent for a single valve, 16 percent for multiple valves, and 21 percent for fire cases.
How is an SRV sized?
Relief valve sizing starts from the worst credible overpressure scenario and the required relieving rate, then selects an orifice area that passes that flow at relieving conditions.
Can an SRV handle two-phase flow?
Yes, but two-phase relief requires special sizing methods, because a valve sized for vapor alone can be significantly undersized when liquid is also present.
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