For a Reciprocating compressor skid -

If recycle valve is fail open, system settle to settle-out pressure. Based on which blowdown calculation is done (calculate blowdown rate and restricted orifice size by depressuring the system to typically 6.9 barg (100 psig) within 15 minutes.)

Now I have a system where recycle valve is fail close and compressor is a four stage compression with blowdown at the final stage

1. As there is no possibility for settle out in overall system, do we need depressure from the fourth stage pressure till third stage pressure and successively till we attain 100 psig?[list=1]

2. If we are depressuring successively, do we need to consider settle-out in individual stages?

Montu,

You made me excited when your recycle valve is failed close (FC) type. Is FC recycle valve is really SAFE for your system ? Any specific reason you go for FC type recycle valve ?

Controller associate to your recycle valve may fail and cause recycle valve full open and subsequent leads to settle-out condition. Have you envisaged this issue ?

JoeWong

Iam not sure what could be the advantage in using FC type Recycle Valve.
Probabily, We can design the suction equipments of compressor skid below settle-out pressure.

In my case it is client spec.

If you are not sure about the reason behind, i am strongly request you to establish it for safe operation. I have seen many recycle control valves for compressor surge and/or pressure control were designed to failed open.



As i mentioned earlier, even though your recycle control valve is FC type, but your compressor suction system may still potentially reach settle condition on the following reasons :

i) recycle control may not FULLY tight Shut-off (TSO). Leakage and passing on control valve may leads to overpressure of suction system.

ii) Controller associate to recycle valve may fail and cause recycle valve full open and subsequent leads to settle-out condition (mentioned earlier)

Please assess these scenarios as minimum.
If the system is intended to install a PSV for overpressure scenario, please assess the potential of inventory loss for above events.



Many engineers said this kind of statement when they do not have very strong reasons to against it (this also happened to me). My only advice to you is NEVER 100% rely on client spec. PLS ASSESS it base on you professional judgment where you are paying for.

JoeWong