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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 is mechanical or process engineers who specify pressure relief 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.
 
Blowdown
The 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
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 Capacity
A 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
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.
 
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 Capacity
The 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.
 

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Further Reading

Relief Valves
 
Pressure Relief Valves
 
Pressure Relief Valves - A Quick Guide
A Quick Guide to Pressure Relief Valves (PRVs)
 
Guidlines for Pressure Relief
Guidelines for Pressure Relief and Effluent Handling Systems
 
Pressure Systems Safety Regulations
Pressure Systems Safety Regulations: A Quick Guide
 
Pressure Systems Casebook
Pressure Systems Casebook: Causes and Avoidance of Failures and Defects
 
Valve Selection Handbook
Valve Selection Handbook: Engineering Fundamentals for Selecting Manual Valves, Check Valves, Pressure Relief Valves, and Rupture Discs
 
Relief Systems Handbook
Relief Systems Handbook
Typical P&ID Relief Valve Symbol

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