Terminology & definitions in Safety Valve
Terminology & Definitions in Safety Valve
- Most gases exist as molecules,
- Safety Valve:An automatic pressure relieving device actuated by the static pressure upstream of the 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. It is used primarily for liquid service.
- 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 gauge, is the inlet pressure at which the pressure relief valve is adjusted to open under service conditions. In a pressure or safety relief valve in gas, vapor, or steam service, the set pressure is the inlet pressure at which the valve starts to discharge under service conditions.
- 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 gauge, is the inlet static pressure at which the pressure relief valve is adjusted to open on the test bench. This pressure includes the corrections for service conditions of back pressure or temperature, or both.
- Operating Pressure: The operating pressure of a vessel is the pressure in pounds per square inch gauge, 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 gauge, which will provide a suitable margin above the operating pressure in order to prevent any undesirable operation of the relief device.
- Maximum Allowable Working Pressure: Maximum allowable working pressure is the maximum gauge pressure permissible at the top of a completed vessel in its operating position for a designated temperature. This pressure is based on calculations 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.
- 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.
- Back Pressure: Back pressure is the static pressure existing at the outlet of a pressure relief device due to pressure in the discharge system.
- Superimposed Back Pressure: Superimposed back pressure is the static pressure existing 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 from other sources when the valve is closed. Superimposed back pressure can be constant, variable or both.
- Constant Superimposed Back Pressure: Back pressure which does not change appreciably under any condition of operation whether the pressure relief valve is closed or open.
- Variable Superimposed Back Pressure: The pressure existing at the outlet of a pressure relief device, which does not remain constant when the pressure relief valve is closed.
- 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 discharging system.
- Coefficient of discharge: The ratio of the mass flow rate is a value to that of an ideal nozzle. The Coefficient of discharge is used for calculating flow through a pressure relief valve.
- Rated coefficient of discharge: A value used with the actual discharge area to calculate the flow capacity of a pressure relief valve. The rated coefficient of discharge of a pressure relief valve is determined in accordance with the applicable code or regulation.