Did you know, under certain circumstances, you need to verify if the Cold Differential Test Pressure (CDTP) is properly compensated for the superimposed backpressure from the flare header?
Have you come across a conventional pressure relief valve relieving to a flare header? Did you know that, under certain circumstances, you need to verify if the Cold Differential Test Pressure (CDTP) is properly compensated for the superimposed backpressure from the flare header?
API 520 Part I, Ninth Edition, Section 5.3.2.1 says, "Superimposed backpressure at the outlet of a conventional spring-loaded PRV acts to hold the valve disc closed with a force additive to the spring force. The actual spring setting can be reduced by an amount equal to the superimposed backpressure to compensate for this."
It further goes on to say in Sections 5.3.2.2 & 5.3.2.3, "Balanced spring-loaded or pilot-operated PRVs should be considered if the superimposed backpressure is variable. However, if the amount of variable superimposed backpressure is small, a conventional valve could be used provided:
- The bench set pressure (CDTP) has been appropriately compensated for superimposed backpressure, and
- The maximum pressure during relief does not exceed the code-allowed limits for accumulation in the equipment being protected.
For example, conventional valves are often used when the outlet is piped into a relief header without compensating the set pressures for the superimposed backpressure caused by other relieving devices. This approach can be used provided the allowable accumulation is not exceeded during the release.