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Pressure relief valve terminology

Relief valve terms are used loosely, even in project specifications. This page gives the meaning each term has on this site. The definitions follow common industry usage. Where a project specification defines a term differently, the project specification governs.

The short answer

Pressure relief valve (PRV) is the general name. A safety valve opens fully at once and suits gas, vapor, and steam. A relief valve opens in proportion to the pressure rise and suits liquid. A safety relief valve can do either.

PSV, pressure safety valve, is the instrument tag many P&IDs and data sheets use for any of them.

Valve names

Pressure relief valve (PRV)
A valve that opens at a set pressure to protect equipment from overpressure and closes again when the pressure falls. API 520 uses it as the general name.
Pressure safety valve (PSV)
The instrument tag name on P&IDs and data sheets. In practice it means the same valve as PRV.
Safety valve
A spring-loaded valve with pop action, used on gas, vapor, and steam.
Relief valve
A spring-loaded valve that opens in proportion to the pressure rise above set pressure, used mainly on liquid.
Safety relief valve
A spring-loaded valve that works as a safety valve or a relief valve, depending on the medium.
Pressure relief device
The wider term. It covers reclosing valves and also non-reclosing devices that open once and must be replaced.
Conventional valve
A spring-loaded valve whose opening pressure, closing pressure, and capacity change with back pressure. Luofu series LFO.
Balanced bellows valve
A spring-loaded valve with a bellows that limits the effect of back pressure and keeps the medium away from the spring. Luofu series LFB.
Pilot-operated valve
A main valve held closed by system pressure and opened by a small pilot valve that senses the inlet pressure. Luofu series LFXD.
Thermal relief valve
A small relief valve that protects blocked-in liquid from expansion when it warms up. Luofu series LFXY.

Pressure terms

Pressures on a data sheet are gauge pressures unless the line says absolute. This site writes them in bar(g).

Set pressure
The inlet gauge pressure at which the valve starts to open.
Cold differential test pressure (CDTP)
The pressure set on the test bench. It corrects the set pressure for constant back pressure and for service temperature.
Maximum allowable working pressure (MAWP)
The highest gauge pressure allowed at the top of a vessel at a stated temperature. The applicable code relates the set pressure to it.
Design pressure
The pressure the equipment is designed for. The project documents give it together with the MAWP.
Operating pressure
The pressure in normal service. Give it on the data sheet, because the margin to the set pressure matters for seat tightness.
Overpressure
The pressure rise above set pressure while the valve relieves, in percent of set pressure.
Accumulation
The pressure rise above MAWP while the valve relieves, in percent of MAWP. With set pressure at MAWP, it equals overpressure.
Blowdown
The difference between set pressure and the pressure at which the valve closes again, in percent of set pressure.
Simmer
Fluid escaping between seat and disc just below set pressure, before the valve opens.
Chatter
Rapid opening and closing of the valve. It damages the seat and the guide. Oversizing and high inlet loss are among the common causes.

Back pressure terms

Back pressure
The pressure at the valve outlet.
Superimposed back pressure
Back pressure that is present before the valve opens, for example from a flare header. It can be constant or variable.
Built-up back pressure
Back pressure that builds in the outlet piping after the valve opens.
Total back pressure
Superimposed plus built-up back pressure. The sizing page shows how it decides the valve type.

Capacity and area terms

Required capacity
The flow the valve must pass in the governing relief case.
Rated relieving capacity
The certified flow of the valve at its rated overpressure. The sizing checks that it covers the required capacity.
Coefficient of discharge (Kd)
The ratio of the actual flow through the valve to the theoretical flow through a nozzle of the same area.
Effective orifice area
The nominal area that API 526 gives for each orifice letter, used in preliminary sizing.
Actual orifice area
The real minimum flow area of a valve, used with its certified coefficient of discharge.
Orifice letter
The API 526 letter, D to T, that names each standard effective area. See API 526 orifice sizes.
Lift
The travel of the disc away from the seat when the valve opens.

Valve parts

Nozzle
The inlet part whose top forms the seat. On a full nozzle valve, the nozzle carries the flow from the inlet flange to the seat.
Disc
The part that closes on the nozzle seat.
Nozzle ring
A ring screwed onto the nozzle. Its position sets the blowdown of a spring-loaded valve.
Spring and spindle
The spring holds the disc on the seat through the spindle. Set pressure is adjusted by compressing the spring.
Bonnet
The housing above the body that encloses the spring. It is closed or open.
Bellows
A flexible metal seal around the disc holder and the spindle. It balances back pressure and keeps the medium away from the spring and the guide.
Lifting lever
A lever for opening the valve by hand to test it. A plain lever is not sealed. A packed lever seals the cap against back pressure.
Test gag
A device that holds the disc shut during a hydrostatic test of the system. It is removed before the system goes into service.
Pilot
The small valve on a pilot-operated valve that senses the inlet pressure and opens or closes the main valve.

Questions engineers ask

Can the set pressure be higher than the design pressure?

For a single valve, the set pressure is referred to the MAWP of the protected equipment, as the applicable code sets out. When several valves share a duty, the code allows the additional valves to be set higher within its limits. Give the design pressure and the MAWP on the data sheet, and the sizing proposes the set pressure of each valve.

How much blowdown should I expect?

LFO and LFB spring-loaded valves and LFXD pilot-operated valves close within 10% below set pressure. LFGS steam-service valves close within 4%.

Blowdown is set with the nozzle ring on LFO and LFB valves and with the back pressure adjusting system on LFGS valves. On LFXD valves it is adjusted from outside the valve.

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