Chemical & Process Technology: Use of conventional type PSV with back pressure exceeded 10% set pressure

Friday, August 10, 2007

Use of conventional type PSV with back pressure exceeded 10% set pressure

Back pressure is seriously affects the operational and capacity of Pressure Relief valves (PRV/PSV). As the backpressure to spring loaded direct acting PSV (conventional type) is increased, the driven force which is a function of differential pressure across a PSV will drop and reduce the ability to flow across the PSV. impact on flow capacity is minimal as long as the back pressure is below it critical pressure.

Another phenomenon which common understood by everyone is that as the backpressure is increased, additional force apply on the PSV disc and reduce disc lift, it reduce discharge capability. This clearly explained in figure 22 in API RP 521 Part-I.

However, the impact is insignificant at low backpressure. API 521 Part-I has recommended that in a conventional PSV application, built-up backpressure SHOULD NOT exceed 10% of the set pressure at 10% allowable overpressure (refer 3.3.3.1.3.). This statement is recommendation instead of mandatory requirement per API. Those conventional type PSV still can be used if the built backpressure > 10% of set pressure for specific events and selected PSVs.

A studies <<Back pressure effects on safety valves operating with compressible flow>>, conducted by Vincenzo Dossena has clearly proved above statement.

This report has showed that
i) ratio of measured discharge coefficient (Kd,m) and theoretical discharge coefficient (Kd,t) (at 10% overpressure) is in the range of 0.95 to 1.02 at 10% built-up backpressure

ii) ratio of measured discharge coefficient (Kd,m) and theoretical discharge coefficient (Kd,t) (at 10% overpressure) maintain at / above 1 for some PSVs at >10% built-up backpressure

Point (i), with the minimum ratio of 0.95, normal Kd factor for PSV is 1, the actual Kd may be conservatively 0.95.
API 520 Part-I has recommended Kd factor of 0.975. SHELL DEP has taken conservative approach where Kd factor of 0.9 to be used.

Point (ii), this implies that conventional PCV may be used even the back pressure is exceeded 10% built-up back pressure. Designer/engineer shall take EXTRA attention if conventional PSV is used when the back pressure > 10% set pressure and shall always seek advice & confirm with PSV vendor.

Reference :
i) API RP 520 Part-I, Sizing, Selection and Installation of Pressure Relieving Devices in Refineries, Seven Edition, an 2000

ii) Vincenzo Dossena, “Back pressure effects on safety valves operating with compressible flow”, published in VALVE WORLD.