Backflow preventor for pilot operated relief valves

[georgeverghese]

In API 520 part II section 5.4 (5th edition, Aug 2003) and the GPSA, a backflow preventor feature for a pilot operated RV is recommended for scenarios where the RV exit backpressure could exceed the "inlet pressure " on the RV.

The design standard for a reputable supermajor oil/gas operating company states that the backflow prevention device is only required for scenarios where the exit backpressure could be higher than the design pressure (or setpressure of the RV where the RV SP = protected equipment design pressure) of the upstream protected equipment. It also quotes some risk in the failure of the RV to reclose if there is a malfunction of the backflow prevention device.

Pls comment / advise

 

[The Obturator]

A backflow preventor on a POSRV, is an option provided by manufacturers where the POSRV design is such that the main valve may open where the outlet pressure is greater than inlet* pressure. This can be in cases of system start up. The set pressure has not been reached, but the pressure existing at valve outlet at that time is greater than the inlet (system upstream) pressure. *Inlet pressure not set pressure

Per ISO, only the term Safety Valve is used regardless of application or design.

 

[georgeverghese]

So what would be the reason for not using this backflow preventor when backpressure is less than design pressure ( = set pressure on the pilot op PSV in most cases) for the connected equipment with this oil company ?

 

[don1980]

If the POSRV is routed to the atmosphere then there's no need for a backflow preventer. The only time to consider a backflow preventer for such cases is when there's a possibility that the vessel could go under a vacuum and you want to prevent air from backflowing through if/when vacuum occurs.

My suggestion is to call for a backflow preventer whenever a POSRV is routed to any closed system (anything other than atm). For example, say you have a POSRV routed to a flare header. Although the flare header backpressure may be low, you can get backflow into the vessel (through the POSRV) when/if the vessel is shutdown and depressured to atm pressure.

Without a backflow preventer, backflow (through the POSRV) into the vessel can occur anytime the outlet system pressure exceeds the pressure in the protected vessel. It's a small inexpensive accessory, thus it's a good idea to have one whenever the POSRV discharges to any location other than atm.