Terminology
- Operating pressure (working pressure)
is the gauge pressure existing at normal operating conditions within the system to be protected. - Set pressure
is the gauge pressure at which under operating conditions direct loaded safety Valves commence to lift. - Test pressure
is the gauge pressure at which under test stand conditions (atmospheric backpressure) direct loaded safety Valves commence to lift. - Opening pressure
is the gauge pressure at which the lift is sufficient to discharge the predetermined flowing capacity. It is equal to the set pressure plus opening pressure difference. - Reseating pressure
is the gauge pressure at which the direct loaded safety Valve is re-closed. - Built-up backpressure
is the gauge pressure built up at the outlet side by blowing. - Superimposed backpressure
is the gauge pressure on the outlet side of the closed Valve. - Backpressure
is the gauge pressure built up on the outlet side during blowing (built-up backpressure + superimposed backpressure). - Accumulation
is the increase in pressure over the maximum allowable working gauge pressure of the system to be protected. - Opening pressure difference
is the pressure rise over the set pressure necessary for a lift suitable to permit the predetermined flowing capacity. - Reseating pressure difference
is the difference between set pressure and reseating pressure. - Functional pressure difference
is the sum of opening pressure difference and reseating pressure difference. - Operating pressure difference
is the pressure difference between set pressure and operating pressure. - Lift
is the travel of the disc away from the closed position. - Commencement of lift (opening)
is the first measurable movement of the disc or the perception of discharge noise. - Flow area
is the cross sectional area upstream or downstream of the body seat calculated from the minimum diameter which is used to calculate the flow capacity without any deduction for obstructions. - Flow diameter
is the minimum geometrical diameter upstream or downstream of the body seat. - Nominal size designation
of a safety Valve is the nominal size of the inlet. - Theoretical flowing capacity
is the calculated mass flow from an orifice having a cross sectional area equal to the flow area of the safety Valve without regard to flow losses of the Valve. - Actual flowing capacityis the flowing capacity determined by measurement.
- Certified flowing capacity
is actual flowing capacity reduced by 10%. - Coefficient of discharge
is the ratio of actual to the theoretical discharge capacity. - Certified coefficient of discharge
is the coefficient of discharge reduced by 10% (also known as derated coefficient of discharge).
- Blowdown (reseating pressure difference) –
difference between actual popping pressure and actual reseating pressure, usually expressed as a percentage of set pressure or in pressure units. - Cold differential test pressure
the pressure at which a Valve is set on a test rig using a test fluid at ambient temperature. This test pressure includes corrections for service conditions e.g. backpressure or high temperatures. - Flow rating pressure
is the inlet static pressure at which the relieving capacity of a pressure relief device is measured. - Leak test pressure
is the specified inlet static pressure at which a quantitative seat leakage test is performed in accordance with a standard procedure. - Measured relieving capacity
is the relieving capacity of a pressure relief device measured at the flow rating pressure. - Rated relieving capacity
is that portion of the measured relieving capacity permitted by the applicable code or regulation to be used as a basis for the application of a pressure relieving device. - Overpressure
is a pressure increase over the set pressure of a pressure Relief Valve, usually expressed as a percentage of set pressure. - Popping pressure
is the value of increasing static inlet pressure of a pressure Relief Valve at which there is a measurable lift, or at which the discharge becomes continuous as determined by seeing, feeling or hearing. - Relieving pressure
is set pressure plus overpressure. - Simmer
is the pressure zone between the set pressure and popping pressure. - Maximum operating pressure
is the maximum pressure expected during system operation. - Maximum allowable working pressure (MAWP)
is the maximum gauge pressure permissible at the top of a completed vessel in its operating position for a designated temperature. - Maximum allowable accumulated pressure (MAAP)
is the maximum allowable working pressure plus the accumulation as established by reference to the applicable codes for operating or fire contingencies.