Sorry dcasto, I didn’t understand your questions, but I will try to be more specific. One of my client asked me if the PSV that we sized for them were size for spillback, which I don’t know what it that means. What I am looking is for a definition of spillback sizing. The application is a reciprocating compressor for natural gas, 1 stage PSV. The PSV is located at the discharge of the cylinder.
The spill back must mean the recycle on the compresor. for a reciprocating compressor, the volume through a cylider is a function of cylinder displacement and suction pressure. There is some effect of the discharge pressure. Sound simple.
The problem is what suction pressure do I select and discharge pressure to calculate volume (or mass). The discharge is easy, its the MOAP of the piping and the relief pressure. Ok, suction pressure. If the normal pressure is P1 and the cylinder can move V1, then the relief valve is sized for V1. But what happens if P1 is increased to 2 times P2, then the volume will be slightly more than 2 times P1 (more because volumetric effiency should be going up). OH, but at V2 the horspower almost doubles and the engine or motor will trip and the volume will go to zero, OR the spill back valve will only allow a volume of .5 V1 plus the original V1.
The relief valve must sized relieve the greater of a volume that will not overload the driver (pick a value of maybe 120% of maximum avaiable HP) or the volume available plus the volume that the spillback valve can pass.
I guess, spillback PSV sizing means a reciprocating compressor discharge is recycled back to compressor suction drum and The PSV installed on this vessel should be sized for two cases; 1) External Fire Case and 2)Spillback Control Valve Failure.
So proceed doing sizing like we do for control valve fail open.
Can it be anti-surge control valve (ASCV) failed close when it is demand to open ?
Or
ASCV on 2nd stage compressor (which also work as recycle / spillback on high discharge pressure) failed open causing tremendous flow to PSV on 1st stage discharge ?
