cdtp safety valve quotation

The Cold Differential Test Pressure (CDTP)of thePressure Relief Valve (PRV) is a set pressure that is adjusted to be used for the PRV (Bench Testing). Because in actual use conditions, the PRV may be affected by the Backpressure. And the operating temperature. Therefore, when testing at room temperature and without backpressure, it is necessary to adjust the set pressure to compensate for these factors. In other words, the shop test pressure = CDTP.

An example of aCold Differential Test Pressure (CDTP)with abackpressure (superimposed)effect. In this case, CDTP is equal to Set Pressure minus Backpressure.

An example of aCold Differential Test Pressure (CDTP)with different temperature compensations for use and testing. And an example ofCDTP that has been adjusted to compensate for both Backpressure (Superimposed) and temperatureeffects.

Test Stand or Test Bench is a set of equipment used for Pressure Relief Valve (PRV) in Pop Test (Set Pressure Test), Blowdown and Seat Tightness Test (Leakage Test)

As for theBalanced Bellow Pressure Relief Valve, which has a bellowfor reducing the impact of theBackpressure, the Cold Differential Test Pressure (CDTP) of the Balanced Bellow Type PRV will compensate only for the impact of different temperatures between the Shop Test and In-service.

Further for Convetional Valve API-521 paragraph 4.2.3.3 A convetional PRV Operating with a constant superimposed back pressure normally required a correction factor to compensate for the back pressure. In this case the required set pressure minus superimposed back pressure is equal to CDTP. This change account for the addtional closing force exerted on the valve disk by back pressure.

Now, we have to add this pressure for back pressure consideration or valve is tested based on CDTP, But if we tested based on CDTP than this is less that the set pressure as CDTP is Set Pressure - Superimposed 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.

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.

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.

If you read API-520 5.3.3.1.3. carefully towards the end, it states that for superimposed constant back pressure "...the spring load is reduced to compensate...". This means that for a Conventional Pressure Relief Valve, the shop Spring Setting is = Set Pressure - Constant Back Pressure.

There is effectively no limit on constant back pressure within reason. The most extreme I have seen is 95% constant back pressure. For liquids the size of the valve is affected in the calculation which takes the pressure differential into account (DeltaP). For balanced bellows PRV"s with variable back pressures, gas/vapours and liquids, API-520 states maximum back pressures at 50% but this is a guide only. Many manufacturers have possibilities to go higher and have their own corrections factors, but generally follow the established limitations.

Your PRV nameplate should have a Spring Setting of Set Pressure 2500 - Constant Back Pressure 2100 = 400 kPag (it may be prefixed CDTP = Cold Differential Test Pressure and have a small temperature correction factor applied for ambient testing).

​As we had discussed the backpressure that relief valve faces in a closed system would not be present during testing in the shop and this needs to be compensated for conventional relief valves.

However as discussed above the parameter of Temperature correction factor would still be applicable to Bellows and pilot-operated relief valve if the PRV temperature is significantly different from that of ambient temperature.

Project Scenario : A Relief valve of Dresser 1900 Series (Conventional relief valve) valve is required to open at 800 Psig where the service temperature is 400°F and backpressure is 100 Psig.