What is CDTP in Relief valve
What is CDTP in Relief Valve ?
CDTP in relief valve standards for Cold differential test pressure (CDTP).
The conditions at which PRV is set to operate on a test stand could be different from the actual service conditions under which a PRV is required to open in the plant.
We need to compensate for this effect.
Thus CDTP is specified for adjusting the set pressure of the relief valve on the test stand.
The CDTP in relief valve is the correction for actual service conditions of backpressure and/or temperature. (As per API 520 4.2.3.1)
An example to understand the concept of CDTP in relief valve
In control valve we have come across the concept of Inherent valve characteristics and installed valve characteristics.
Inherent valve characteristics (Ideal valve characteristics) get slightly deviated when a valve is installed in service
We call this as installed valve characteristics.
Similarly concept is that of CDTP in relief valves.
The Relief valve when tested on test stand will have different characteristics as compared to valve that is installed in process conditions.
Few parameters that change are Temperature & Back pressure conditions.
Thus we compensate for this by CDPT (Cold differential test pressure)
Inherent valve characteristics (Ideal valve characteristics) get slightly deviated when a valve is installed in service
We call this as installed valve characteristics.
Similarly concept is that of CDTP in relief valves.
The Relief valve when tested on test stand will have different characteristics as compared to valve that is installed in process conditions.
Few parameters that change are Temperature & Back pressure conditions.
Thus we compensate for this by CDPT (Cold differential test pressure)
Temperature correction factor :-
CDPT consists of Temperature correction factor when the pressure relief valve temperature is significantly different from the ambient temperature.
However note that the magnitude of this correction factor is depended on the type of pressure relief valve.( Conventional, Bellows or Pilot).
The correction factor is usually a multiplier to the set pressure.
However note that the magnitude of this correction factor is depended on the type of pressure relief valve.( Conventional, Bellows or Pilot).
The correction factor is usually a multiplier to the set pressure.
From where do we get this value of Temperature correction factor ?
The correction factor is usually obtained from the pressure relief valve manufacturer.
It is normally mentioned in their IOM manual ( Installation, Operation and Maintenance manual)
It is normally mentioned in their IOM manual ( Installation, Operation and Maintenance manual)
Backpressure compensation :-
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.
Note that for Pilot operated and bellows type relief valve no adjustment is required for backpressure and thus this parameter is not part of CDTP.
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.
Note that for Pilot operated and bellows type relief valve no adjustment is required for backpressure and thus this parameter is not part of CDTP.
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.
Steps to calculate CDTP for PSV :-
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.
Step 1 : Calculate the Differential pressure
Differential Set Pressure = Set pressure – Backpressure
Differential Set pressure = 800 Psig – 100 Psig
Differential Set pressure = 700 Psig
Step 2 : Find the Temperature multiplier from Vendor IOM (Installation Operation Manual)
Step 1 : Calculate the Differential pressure
Differential Set Pressure = Set pressure – Backpressure
Differential Set pressure = 800 Psig – 100 Psig
Differential Set pressure = 700 Psig
Step 2 : Find the Temperature multiplier from Vendor IOM (Installation Operation Manual)
Temperature Multiplier : 1.013
Step 3 : Calculate Cold differential Test Pressure
Cold differential test pressure ( CDTP) = Differential test pressure X Temperature Multiplier.
Cold differential test pressure ( CDTP) = 700 Psig X 1.013
Therefore, Cold differential test pressure ( CDTP) = 709.10
Hope this article has been of value to you !!
Step 3 : Calculate Cold differential Test Pressure
Cold differential test pressure ( CDTP) = Differential test pressure X Temperature Multiplier.
Cold differential test pressure ( CDTP) = 700 Psig X 1.013
Therefore, Cold differential test pressure ( CDTP) = 709.10
Hope this article has been of value to you !!
