Reciprocating compressor blocked discharge protection

[AMXX]

Hi;
I have a 2-stage (tandem) reciprocating compressor train. Each stage has a recycle valve to control the suction pressure.
The compressor capacity is control by:
• Speed control (only by the 1st stage suction pressure)
• Clearance pocket
During the HAZOP of the project, it was requested to review requirement of the minimum recycle arrangement for the compressors to protect the train against a blocked compressor discharge. I have the following questions:
1) I think if a recycle line is required it should be designed for full compressor capacity. For a centrifugal compressor the minimum recycle is clear but what can be minimum recycle for a reciprocating compressor?
2) What is the advantage /disadvantage of an additional recycle line to protect the compressor against blocked discharge?
3) Can a separate transmitter be considered instead of recycle line to increase the protect level of the compressors against blocked discharge?
4) Can a discharge pressure control be added to the existing control valve to override the suction pressure control in case of high discharge pressure condition?

It should be noted:
• For blocked discharge protection each stage has one pressure transmitter for alarm/trip (common transmitter) and a PSV.
• The recycle valve maintains the suction pressure so in case of blocked discharge, the valve is closed as the suction pressure is increased (it is opened only in case of low suction pressure)
• The SIL level of the transmitter cannot meet the plant safety target so an additional layer of protection should be added to the system.

 

[pmover]

1) I think if a recycle line is required it should be designed for full compressor capacity. For a centrifugal compressor the minimum recycle is clear but what can be minimum recycle for a reciprocating compressor?
Not going to disagree about recycle pipe size at compressor capacity. Think temperature rise; higher resistance implies higher Td's. Also, at higher ratios, flow diminishes through the compressor such that heat removal becomes a problem. Always want the heat of compression carried through & out the compressor at all times.

2) What is the advantage /disadvantage of an additional recycle line to protect the compressor against blocked discharge?
No need for an additional bypass pipe, size the one pipe for ample capacity based on max Ps/settle out pressure and minimize flow resistance to keep Td's from rising too quickly.

3) Can a separate transmitter be considered instead of recycle line to increase the protect level of the compressors against blocked discharge?
Yes. Never want blocked discharge as that will quickly result in high Td's and eventually, zero flow through compressor.

4) Can a discharge pressure control be added to the existing control valve to override the suction pressure control in case of high discharge pressure condition?
If added, higher Td's are the result & need further evaluation. Why add/ & complicate Hazop when not necessary.

 

[AMXX]

Hi pmover;
thanks for your respond.
1) The recycle line is taken from air cooler outlet so the temperature rise is limited. Still I am not sure what is the minimum recycle for a reciprocating compressor?
2) My question was about operational advantage / disadvantage to have a spill back line ( recycle form discharge of the 2nd stage to suction of the 1st stage) .
3) I am not sure if an additional transmitter can be considered as replacement of the recycle line for safety of the system during blocked discharge scenario.
4) The recycle line is taken from cooler outlet. Also the recycle valve has to drop the discharge pressure to the suction pressure so the compression ratio is not increased. Therefore temperature rise should not be an issue. I am thinking about operational difficulty for this arrangement.