Hi; For a reciprocating compresso 1

[AMXX]

Hi;
For a reciprocating compressor, the recycle valve failure is one of the suction PSV sizing scenarios. As during the reciprocating compressor start-up, the discharge Shutdown valve (SDV) should be kept open ( not same as centrifugal compressor which it should be closed), so failure of both recycle valve and SDV will be a double jeopardy. I just wanted to understand in which case this SDV can be closed so we can define the recycle valve failure scenario for PSV sizing?

thanks

 

[georgeverghese]

The only time when a recycle valve failing open can lead to overpressure of the compressor suction components is when the suction side process - mechanical design pressure is not designed to handle worst case settle out pressure. This can also happen if the discharge SDV is wide open, since the discharge side check valve ( which would be downstream of the recycle valve piping takeoff) will act as a stop valve when the recycle valve goes fail open.
In my experience, there were many cases when compressor packages were not designed to handle worst case settle out pressure when there is loss of control of the recycle valve. It is not possible to set up a PSV to protect the suction side in this case. Engineering support to Operations end up having to provide half baked measures such as trip and blowdown of the compressor unit on detection of high pressure on compressor suction. Some folks may add a PSV on the suction side to handle some limited overpressure flows, but this cannot be called a protection measure.

 

[KEVINZY]

At first, for the reciprocating compressor, the discharge shutdown valve is not common item. We normally use non-return valve at discharge side. Sometimes we need a blowdown valve to release the pressure at discharge side.
And your question is when the SDV can be closed. Of course the SDV can only be closed when the compressor stop.
For the suction PSV sizing, we generally consider the maximium flowrate of the compressor.

 

[AMXX]

Hi georgeverghese ;

You are right about settle out pressure and the PSV sizing is not for settle out condition. The settle out pressure is considered as design pressure of suction side to avoid PSV pop up during compressor trip which it could be happened frequently.

The PSV should be sized to protect the system from over-pressure during unexpected situation.

For the centrifugal compressors one of the relieving scenarios is :
“ Failure of the recycle valve during the compressor start up”.
In this scenario, the discharge valve is closed so PSV should be able to release the recycle valve fail open rate as compressor is still running .

My question is about reciprocating compressors, I found in some projects the same relieving rate was calculated for PSV sizing however I am not sure which case can lead to this scenario as discharge valve of the reciprocating compressor is not closed during start up.

 

[AMXX]

Hi KEVINZY;

The check valve reliability is very low so pending on the project , deletion of the SDV can be investigated. As per API 521:
1) In case of two dissimilar check valves if the check valves are maintained, 10% leakage needs to be considered for the check valves
2) Complete check valve failure is assumed for other cases ( check valves in series that are not inspected and maintained and for a single check valve regardless if it is inspected and maintained)

Maximum compressor throughput is one case for PSV relief rate calculation.
As reciprocating compressor has suction and discharge bottle so they can work as extra reservoir during recycle valve failure for extra relieving rate. But I think this scenario can be applicable if the compressor train discharge is blocked ( e.g. SDV closed). As I found in some projects the recycle valve failure scenario was considered for the PSV relieving rate calculation so I am not sure which case can lead to this scenario as discharge valve of the reciprocating compressor is not closed during start up.

 

[georgeverghese]

The scenario is no different for a recip compressor, as long as you have a discharge check valve located downstream of the recycle line takeoff. This check valve behaves like a stop valve when the recycle CV fails open: it goes closed when the pressure upstream of the check valve becomes less than that downstream of the check valve.

 

[KEVINZY]

Hi, AMXX,
As Georgeverghese said, the problem is the pressure downstream of the check valve or SDV.
If the downstream of the compressor is a storage for emergency condition, it means the downstream pressure is always nearly the rated discharge pressure of compressor.
In similar application(the downstream pressure is not ambient), if only SDV but no check valve, the SDV should be closed when the compressor start up.
However, we usually use check valve for this application.

 

[AMXX]

Dear georgeverghese
You are right. I missed this point.

Thanks for your kind respond.

 

[AMXX]

Dear KEVINZY;

The downstream of the compressor is high pressure system. However after recycle valve fail open, for a short period of time (before system reach to the steady state condition) the check valve can be closed so in this basis the recycle valve failure for the PSV relieving rate calculation can be justified.

As you mentioned, the SDV status always is checked before compressor start up so the discharge SDV closure during recycle valve failure will be a double jeopardy and I cannot address for it.

Thanks for your kind respond.