To understand the concept of Cold Differential Test Pressure (CDTP), it is important to first understand the role of back pressure in case of opening of conventional, balance bellows and pilot type relief valves.
Superimposed back pressure
Superimposed back pressure is the static backpressure that exists on the outlet of the pressure relief valve, when the valve is closed. This pressure can be constant or variable depending on conditions downstream to the relief valve, before the relief valve can discharge.
Built-up back pressure
Built-up back pressure is the backpressure generated due to pressure losses at the outlet of an open relief valve when it is discharging. This pressure depends on the pressure of the vent header downstream to the relief valve and the relieving flowrate which is being discharged. The built-up backpressure is the pressure in the vent header plus pressure drop in the line from relief valve to vent header, when the valve is discharging at full capacity.
When the relief valve is discharging, effects of superimposed and built-up back pressure exist together and felt as the combined back pressure.
For certain relief valve designs, the backpressure on the valve acts as a closing force and can affect the opening pressure for the valve. ‘Conventional’ valves are highly susceptible to this effect and hence not used in applications where high backpressure is expected. ‘Balanced Bellows’ and ‘Pilot Operated’ relief valves relatively shielded from effects of high backpressure.
Cold Differential Test Pressure (CDTP)
For some relief valves (especially conventional type relief valves), the opening of the valve is affected by backpressure seen by the valve. If the backpressure seen by such relief valves is higher than atmospheric, then it has to be designed to open at a lower differential pressure value than the relief valve set pressure minus atmospheric pressure. However when the relief valve is tested before installation, it only sees atmospheric pressure as backpressure. Hence to open this relief valve at same differential pressure value, the set point pressure for opening the valve has to be lower than original design set point pressure. This set point value is known as cold differential test pressure (CDTP).
For pilot operated and balanced bellows type relief valves, effect of backpressure on valve opening characteristics is very low and hence CDTP is the same as the original design set point pressure value.
