Minimum recycle line for reciprocating compresor

[AMXX]

Hi;
I have been requested to consider a minimum recycle line for a variable speed electrical reciprocating compressor train which it should be used to protect the system for the train blocked discharge case.

For a centrifugal compressor, it is normal to consider a minimum recycle line but for a reciprocating compressor I believe if it is necessary to consider a recycle line, it should be designed for full compressor capacity.

The compressor train has three stages each stage has:
1) A recycle line to maintain the suction pressure of compressor
2) High high pressure trip.
3) Pressure relief valve as ultimate protection of the system.

The overall train recycle line can protect the compressor for the following cases:

1) Manual valve at the train discharge shut close- this can be avoided by considering LO valve.
2) Check valve at the train discharge shut close- this can be happened during compressor start up. As the check valve cannot be fixed quickly, in this case the compressor should be tripped and standby train should be used.
3) Tran discharge shutdown valve (SDV) fail close- if the discharge SDV is shut closed again the compressor should be tripped as the SDV should be repaired.

So in my case, I am thinking that the recycle line is not necessary and it just make the system more complicated.

I appreciate if I can have your valuable experience in this matter.

 

[georgeverghese]

Since you already have a recycle valve on each stage, you can use any one of them to double up in this service. Suggest you use the one stage 1. Setup a PIC on stage 3D and send its output to a high or low signal autoselect which also gets the signal from stage 1S low suction PC. The selector will pick the appropriate signal to feed the stage 1 recycle CV. If you are not sure how to do this, ask your plant process controls engineer to help set this up.
Capacity of this control valve would be full flow seen at max normal stage 1S pressure and max normal stage 1D pressure ( coincident with high pressure on stage 3D). But check to see if:
1)The recycle flow stream temp is not higher than the normal stage 1S temp.
2)Specify a low noise valve for this recycle service if noise is estimated to be high.
The basic idea here is to recycle gas around stage 1, and thereby reduce net forward flow of the compressor in an attempt to reduce the final discharge pressure on stage 3.

 

[AMXX]

Hi georgeverghese;

Thank you so much for your kind reply. So you also believe that an additional recycle line is not necessary.

 

[georgeverghese]

Yes, not required. The new PIC on stage 3D should be a high setpoint controller ie. it only acts to open the stage 1 recyle valve when stage 3D pressure is high and approaching high high trip pressure.

 

[AMXX]

thanks georgeverghese