The query is related to safety valves which are not mounted directly on equipment's but on process lines.
For such cases as per API-520 Part II, Section 4.2.2:
“When a pressure relief valve is installed on a process line, the 3 percent limit should be applied to the sum of the loss in the normally non-flowing pressure relief inlet pipe and the incremental pressure loss in the process line caused by the flow through the pressure relief valve.”
There is a confusion between our colleagues for calculation of inlet line losses based on the interpretation of the above API statement.
These are the current steps used to calculate "incremental pressure loss" in the process line.
1. Add normal process flow rate and rated relief flow rate.
2. Calculate pressure drop for this total flow rate using relieving properties eg. relieving density etc.
3. Calculate incremental pressure drop by subtracting normal ∆P from ∆P calculated in step 2.
The confusion is for step 2, with these two interpretaions:
Interpretaion 1:
calculate ∆P using relieving density as stated above for total flow rate (Normal + PSV relief)
Interpretaion 2:
Do not use reliveing density for total flow rate, instead take weighted averages of density of normal flow rate properties and relief load properties and then calculate pressure drop for total flow rate using these new density.
This new interpretation has come based on a practical example in which the normal flow rate has density of around 200 kg/m3 and relief flow rate has density of 2.0 kg/m3 (H2 gas blowthrough case). So if pressure drop is calculated using interpretation 1, it calculates using a density of 2.0 kg/m3 for total flow rate and gives a very high ∆P.
Interpretaion 3:
calculate ∆P using relieving density as stated above only for PSV rated relief rate. do not take normal flow into account.
If anything is not clear, kindly ask..
Thanks in advance.
Edited by MI_Sidd, 12 March 2013 - 05:51 AM.