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If the pressure drop in the inlet piping to a pressure relief valve is too high, there is the possibility the valve will cycle or chatter when the pressure at the inlet to the valve goes below its blowdown pressure (reseat pressure). Therefore, inlet lines that are too small, too long, or contain too many fittings or valves need to be avoided. PRV chatter is a phenomenon where, for various reasons, the valve opens and closes in a rapid fashion. This can result in reduced PRV capacity and/or high piping vibration/loads. There have been incidents in industry where PRV chatter has led to PRV damage, flange leaks, and piping failures.
For gas, vapor, or flashing-liquid service, it is recommended that the inlet piping between the protected equipment and the inlet flange of the pressure relief valve be designed so that the total pressure loss in the line shall be the sum total of the inlet loss, line loss, and valve loss and shall not exceed 3 percent of the set pressure, in pounds per square inch gage, of the valve.
Where inlet pressure drop is calculated to be in excess of 3% of the valve’s set pressure, the Owner/User is left with several options. Costly inlet piping modifications are one way of addressing the problem. However, further Engineering Analysis,” as presented in API 520 Part II, can now be performed that will assist the user in determining which installations truly require costly capital expenditures and which ones can be mitigated by further analysis.
The 6th Edition of API 520 Part II released in March of 2015 provides more details on this engineering analysis. It consists of a thorough review of past inspection and operating records to determine if there is any historical evidence of chatter. A new Force Balance method (incorporating inlet pressure drop, outlet pressure drop, and PRV opening and closing characteristics) has been added that shows excellent correlation with the flow test data when predicting whether a valve is stable or not. Also, an acoustic interaction check is now recommended, particularly for liquid lines where the majority of actual chattering valves have been experienced.
E2G Can Help Before any capital dollars are spent rerouting or replacing inlet piping to PRVs to mitigate a 3% issue, Owner/Users should be performing an engineering analysis to determine if modifications are necessary.
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