Page 177 - Valve Selection Handbook

164 Valve Selection Handbook

StarHo-open pressure: See "set pressure."

Popping pressure: The pressure at which a safety or safety relief valve
pops open on gas or vapor service.

Opening pressure difference: The difference between set pressure and
the popping pressure.

Cold differential test pressure: The pressure at which the pressure relief
valve is adjusted on the test stand. The cold differential test pressure
includes corrections for service conditions of back pressure or tempera-
ture or both.

Relieving pressure: The set pressure plus overpressure.

Overpressure: The pressure increase over the set pressure, expressed as
a percentage of the set pressure or in pressure units.

Back pressure: The pressure that exists at the outlet of a pressure relief
valve as a result of the pressure in the discharge system. It is the sum of
built-up and superimposed back pressures, expressed 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 pressure relief valve opens,
expressed as a percentage of the set pressure or in pressure units.

Superimposed back pressure: The pressure that is present at the valve
outlet when the valve is required to open, expressed as a percentage of
the set pressure or in pressure units. The superimposed back pressure
may be constant or variable with time.

Secondary back pressure: The back pressure component due to flow
within the valve body to the valve outlet, normally a function of the
valve orifice area compared to the valve outlet area and the shape of the
body interior downstream of the nozzle.

Reseating pressure: The pressure at receding overpressure at which the
valve disc reestablishes contact with the nozzle.