Safety valve backpressure issue
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I have been working on calculations for safety valves recently, and I have the following question regarding how to determine the back pressure of a safety valve during such calculations: Assuming gas is being released, according to API520 it is necessary to determine whether the flow is critical or non-critical before deciding which formula to use for the calculations. However, since the safety valve discharges into the flare system, in my understanding the back pressure is not a fixed value. So how can it be determined whether the flow is critical or non-critical? Should we still calculate it based on critical flow? I would be very grateful if experts in valve safety calculation could offer their guidance. Thank you so much!
First, calculate based on critical flow, as this results in the maximum buildup of back pressure; then add the additional back pressure to verify whether it is critical flow.