Definitions
SAFETY VALVE-An automatic pressure relieving device actuated by the static pressure upstream of th valve, and characterized by rapid full opening or pop action. It is used for steam, gas, or vapor service.
RELIEF VALVE-An automatic pressure relieving device actuated by the static pressure upstream of the valve, which opens in proportion to the increase in pressure over the opening pressure.
SAFETY RELIEF VALVE-An automatic pressure actuated relieving device suitable for use as either a safety or relief valve, depending on application.
PRESSURE RELIEF VALVE-A pressure relief valve is a pressure relief device which is designed to reclose and prevent the further flow of fluid after normal conditions have been restored.

SET PRESSURE- Set pressure, in pounds per square inch gage, is the inlet pressure at which the pressure relief valve is adjusted to open under service conditions. In a safety or safety relief valve in gas, vapor, or steam service, the set pressure is the inlet pressure at which the valve pops under service conditions. In a relief or safety relief valve in liquid service, the set pressure is inlet pressure at which the valve starts to discharge under service conditions.
ACCUMULATION-Accumulation is the pressure increase over the maximum allowable working pressure of the vessel during discharge through the pressure relief valve, expressed as a percent of that pressure or in pounds per square inch.
BLOWDOWN-Blowdown is the difference between actual popping pressure of a pressure relief valve and actual reseating pressure expressed as a percentage of set pressure or in pressure units.
LIFT- Lift is the actual travel of the disc away from closed position when a valve is relieving.
MAXIMUM ALLOWABLE WORKING PRESSURE-Maximum allowable working pressure is the maximum gage pressure permissible in the top of a completed vessel in its operating position for a designated temperature. This pressure is based on carculations for each element in a vessel using nominal thicknesses, exclusive of allowances for corrosion and thickness required for loadings other than pressure it is the basis for the pressure setting of the pressure-relieving devices protecting the vessel. The design pressure may be used in place of the maximum allowable working pressure in cases where calculations are not made to determine the value of the latter.
OVERPRESSURE-Overpressure is a pressure increase over the set pressure of a pressure relief valve, usually expressed as a percentage of set pressure.
DIFFERENTIAL SET PRESSURE-The pressure differential in pounds per square inch between the set pressure and the constant superimposed back pressure, it is applicable only when a conventional type safety relief valve is being used in service against constant superimposed back pressure.
COLD DIFFERENTIAL TEST PRESSURE-Cold differential test pressure, in pounds per square inch gage, is the inlet static pressure at which the pressure relief vatve is adjusted to open on the test stand, This pressure includes the corrections for service conditions of back pressure or temperature, or both.
BACK PRESSURE-Back pressure is the static pressure existing at the outlet of a pressure relief device due to pressure in the discharge system.
CONSTANT BACK PRESSURE- Back pressure which does not change appreciably under any condition of operation whether the pressure relief valve is closed or open.
VARIABLE BACK PRESBURE- Refer to discussion on BalanSeaJ Valves.
OPERATING PRESSURE-The operating pressure of a vessel is the pressure, in pounds per square inch gage, to which the vessel is usually subjected in service. A vessel is usually designed for a maximum allowable working pressure, in pounds per square inch gage, which wiil provide a suitable margin above the operating pressure in order to prevent any undesirable operation of the relief device. (It is suggested that this margin be as great as possible consistent with economical vessel and other equipment design, system operation and the performance characteristics of the pressure relieving device.)
BUILT-UP BACK PRESSURE- Built-up back pressure is pressure existing at the outlet of a pressure relief device occasioned by the flow through that particular device into a discharge system.
SUPERIMPOSED BACK PRESSURE-Superimposed back pressure is the static pressure existing at the outlet of a pressure rerief device at the time the device is required to operate. It is the result of pressure in the discharge system from other sources.