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Glossary of Relief Valve Terms

Why a Glossary of Relief Valve Nomenclature?

Relief Valves and Bursting Disks both share a range of terminology that isn't usually

encountered in other areas of instrumentation engineering. In fact in some organisations it ismechanical or process engineers who specify these devices, not C&I engineers.

Accumulation

The pressure increase above the Maximum Allowable Working Pressure of a vessel during

discharge through a pressure relief device. It is either expressed in pressure units or percentage.

Actual Discharge Area

The measured minimum net area that determines the flow through a valve.

Back Pressure

The pressure that exists at the outlet of a pressure relief device as a result of the pressure in the

discharge system e.g. flare system, blowdown system etc. It is the sum of the superimposed and

built up back pressure.

Balanced Pressure Relief Valve

A spring loaded pressure relief valve that incorporates a means for minimising the effect of back

pressure on the valve's performance characteristics.

BlowdownThe difference between the set pressure and the closing pressure of a pressure relief valve,

expressed either as a percentage of the set pressure, or in pressure units.

Built Up Back Pressure

The increase in pressure in the discharge header that develops as a result of flow after the relief

device opens.

Closing Pressure

The value of decreasing inlet static pressure at which the valve disk re-establishes contact with

the seat or at which lift becomes zero.

Cold Differential Test Pressure

The pressure at which the pressure relief valve is adjusted to open on the bench. It includes

corrections for back pressure and/or temperature.

Conventional Pressure Relief Valve

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A spring loaded pressure relief valve whose performance characteristics are directly affected by

changes in the back pressure on the valve.

Curtain Area

The area of the cylindrical or conical discharge openening between the seating surfaces above

the nozzle seat created by the lift of the disk.

Design Pressure

The most severe conditions of coincident temperature and pressure expected during operation. If

Maximum Allowable Working Pressure has not been established then design pressure is often

used instead. Discharge Capacity

The actual flow rate of discharge, usually expressed in mass flow units, though can be expressed

in volumetric terms.

Equivalent CapacityA flow rate calculated from the capacity of the valve when relieving a test fluid e.g. water, air or

steam.

Huddling Chamber

An annular chamber in a pressure relief valve located beyond the seat for the purpose of

generating a rapid opening.

Leak Test Pressure

The inlet static pressure at which a seat leak test is performed. API 527 (Seat Tightness of

Pressure Relief Valves) specifies 90% of set pressure.

Lift

The actual travel of the disk away from the seat when a valve is relieving.

Maximum Operating Pressure

The maximum pressure expected during operation.

Maximum Allowable Working Pressure

The maximum pressure allowed in a vessel at its designated temperature. This is the basis for the

set pressure of the relief devices that protect the vessel.

Nozzle Area

The cross sectional flow area of a nozzle at the minimum nozzle diameter.

Opening Pressure

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The value of an increasing inlet static pressure at which there is a measurable lift of the disc or at

which discharge of the fluid becomes continuous.

Overpressure

The pressure increase over the set pressure of the relieving device, expressed in pressure units or

as a percentage. It is the same as Accumulation when the relieving device is set at the maximum

allowable working pressure of the vessel.

Pilot Operated Pressure Relief Valve

A pressure relief valve in which the main valve is combined with and controlled by an auxiliary

pressure relief valve.

Popping Pressure

The pressure at which the valve disk rapidly moves from a simmer position to a full open

position.

Pressure Relief Device

A device actuated by inlet static pressure and designed to open during an emergency or abnormal

condition to prevent a rise of internal fluid pressure in excess of a specified value. A Pressure

Relief Device can also be designed to prevent an excessive internal vacuum.

Rated Relieving Capacity

The proportion of the measured relieving capacity permitted by the applicable code or regulation

to be used as a basis for the application of a pressure relief device.

Relief Valve

A spring loaded pressure relief valve actuated by the static pressure upstream of the valve. The

valve normally opens in proportion to the pressure increase over the opening pressure. Used

primarily with incompressible fluids.

Relieving Conditions

The inlet pressure and temperature on a pressure relief device at a specific overpressure.

Relieving Pressure

The valve set pressure plus overpressure.

Required Discharge Area

A nominal, or computed, area of a pressure relief valve used in recognised flow formula to

determine the size of the valve. It will be less than the actual discharge area.

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Rupture Disk Device

A non re-closing differential pressure relief device actuated by inlet static pressure and designed

to function by bursting the pressure containing rupture disk.

Safety Relief Valve

A spring loaded pressure relief valve that may be used as either a safety or a relief valve

depending on the application.

Safety Valve

A spring loaded pressure relief valve actuated by the static pressure upstream of the valve and

charachterised by rapid opening or pop action. Normally used with compressible fluids.

Set Pressure

The inlet gauge pressure at which the pressure relief valve is set to open under service

conditions.

Simmer

The audible, or visible, escape of compressible fluid between the seat and disk at an inlet static

pressure above the set pressure and at no measurable capacity.

Spring Loaded Pressure Relief Valve

A pressure relieve device designed to automatically re-close and prevent the further flow of

liquid. Reclosure supplied by spring force.

Stamped CapacityThe rated relieving capacity that appears on the device nameplate. It is based on the set pressure

or burst pressure plus the allowable overpressure for compressible fluids and the differential

pressure for incompressible fluids.

Superimposed Back Pressure

The static pressure that exists at the outlet of a pressure relief device at the time the device is

required to operate. It is the result of pressure in the discharge system e.g. flare system, and

therefore may be constant or variable.