Suction bottle safety for a reciprocating compressor

[Chem1482]

I am trying to verify the safety of suction bottles of a reciprocating compressor. The compressor is a two stage. First stage discharge has PSV at 350 psig. Second stage discharge has PSV at 525 psig and this line goes to a discharge header as there are three parallel units. There is check valve on this line before the discharge header. There is also a recycle line from second stage discharge to first stage suction bottle which is good for only 250 psig ( mawp = 250 psig). As per operation people they use recycle line only during the start up of compressor. I am confused whether the recycle line will over pressure the suction bottle during start up or not. Shall i recommend a PSV or not. Anyone having experience in this system or knows guidelines please help me.

Thanks

 

[JLSeagull]

Consider the situation when operations initiates a compressor shutdown by opening the bypass valve from discharge to suction. It sounds like this situation with the compressor running could overpressure the first stage suction bottle.

 

[dcasto]

most startup recycle lines from the discharge to the suction are upstream of the compessor check valve. so no gas could flow back into the suction bottle to overpressure. The worse case would be if you have the compressor suction valve closed, the check valve is broken and someone opens the startup recycle valve. In this case, you'll need a PSV rated for the total flow of every compressor on the system.

I've taken the stance that the startup recycle valve is under admistrative controls and as of the unit start up proceedures, the valve cannot be opened without a permit or under a lock tag try condition. With this administrative proceedure in place it keeps from putting a huge relief valve on the suction bottle.

 

[Chem1482]

Thanks alot dcasto & JLSeagull for the information

 

[JoeWong88]

Is the recycle valve controlled by suction or discharge pressure or pure manual control ? If without any control, is the compressor stop when discharge is blocked during normal operation ? Is there double check valve provided on the 2nd stage compressor discharge ?

During start-up the recycle valve will open to allow gas backflow from discharge to suction. During start-up period, the check valve on compressor discharge could failed and lead to excessive flow passing check valve, flow to suction drum via the recycle line. The PSV will need to take the excessive load.

Nevertheless, API 521 has recommended provision of double check valve. Yet to consider check valve passing with reduce load...