PRESSURE RELIEF DEVICE – TERMINOLOGY

                Pressure relief devices are the devices actuated by inlet static pressure and designed to open during emergency or abnormal conditions to prevent a rise of internal fluid pressure in excess of a specified design value.

Figure - PSV cross sectional view

These devices are designed to open and relieve excess pressure and to re close and prevent the further flow of fluid after normal conditions have been restored.

Type of valve \ Property	Loading type	Valve opening	Fluid type
Relief valve	Spring loaded	Proportional to pressure rise over opening pressure	Incompressible fluids (liquid service)
Safety valve	Spring loaded	Rapid or Pop action	Compressible fluids (Gas service)
Safety relief valve	Spring loaded	Used as safety valve or relief valve depending on the application	Multi service applications (foam like) [Two phase mixture at valve inlet is 50% vapour or less]

 

Conventional pressure relief device

Ø  Spring loaded device

Ø  Operational characteristics are directly affected by changes in the back pressure.

Figure - Conventional and Bellow PSV

 

Balanced pressure relief valve

Ø  Spring loaded device

Ø  Incorporates bellows or other means for minimizing the effect of back pressure on the operating characteristics of the valve.

Pilot operated relief valve

Ø  Major relieving device or generally main valve is combined with and controlled by a self actuated auxiliary pressure relief valve called pilot.

Figure - Pilot PSV

Huddling chamber

An annular chamber located downstream of the seat of a pressure relieving device for the purpose of assisting the valve to lift.

 

Pressure

In this contest, Pressure can be mainly divided into two categories.

1.       System pressures &

2.       Device pressures

System pressures

·         Maximum operating pressure is the maximum pressure expected during normal system operation.

·         Maximum Allowable Working Pressure (MAWP) is the maximum gauge pressure permissible at the top of a completed vessel in its normal operating position at the designated coincident temperature specified for that pressure.

Or

It measures the greatest amount of pressure that the weakest part of the vessel can handle at specific operating temperatures.

·         Design pressure of the vessel along with the design temperature is used to determine the minimum thickness or physical characteristic of each vessel component as determined by vessel design rules.

It is selected by the user to provide a suitable margin above the most severe pressure expected during normal operation at coincident temperature.

Design pressure less than or equal to MAWP

MAWP is the highest pressure it could be exposed to.

Design pressure is the highest pressure it should be exposed to, during normal operating conditions.

·         Accumulation is the pressure increase over MAWP of the vessel during discharge through pressure relieving device.

·         Over pressure is the pressure increase over the set pressure of the relieving device allowed to achieve the rated flow.

 

Figure - Pressure level relationships for pressure relief valves

 

Design pressures

·         Set pressure is the inlet gauge pressure at which the pressure relief device is set to open under service conditions.

·         Cold Differential Test Pressure (CDTP) or Cold Differential Set Pressure (CDSP) is the pressure at which a pressure relief valve is adjusted to open on the test bench.

It includes correction for the service conditions of back pressure or temperature or both.

·         Back pressure is the pressure that exists at the outlet of a pressure relief device as a result of the pressure in the discharge system.

Back pressure = Super imposed back pressure + Built up back pressure.

·         Built up back pressure is the increase in pressure at the outlet of pressure relief device that develops as a result of flow after pressure relief valve opens.

·         Super imposed back pressure is the static pressure that exists at the outlet of a pressure relief device at the time of device required to operate.

For pilot and balanced bellows type relief valves, effect of back pressure on valve opening characteristics is very low and hence CDTP is same as the set pressure.

 

For example, if PSV installed in a process system is designed to relieve at 10 bar and the PSV discharge is connected to the flare header having 1 bar, then

 

Set pressure = 10 bar

CDTP = 9 bar

Back pressure = 1 bar

 

·         Blow down is the difference between set pressure and the closing pressure.

·         Simmer is audible or visible escape of compressible fluid between seat and disc which may occur at inlet static pressure below the set pressure prior to opening.

·         Operating ratio = Maximum system operating pressure / Set pressure.

For any industry PSV is considered as one layer of safety protection to the plant. So it is the safety critical equipment of the industry for which no chance has to be taken in the maintenance perspective.