Reciprocating compressor

[AMXX]

Hi;

The capacity of a reciprocating compressor is controlled by "variable speed drive " and "pocket clearance". The compressor train has a recycle valve to control the pressure at the compressor suction scrubber ( transmitter is located at the compressor suction scrubber). This is required to control quality of the separated condensate as it should be send back to the condensate tower.

In the stop sequence of the compressor, the step after "closure of the inlet/outlet valves" is :
"Place the recycle valve in manual and fully open the recycle valve (one shot)"

My question is:

1) As per my experience the control of the recycle valve always should be part of the stop sequence. So is it acceptable to consider opening of the recycle valve manual?
2) The recycle valve is FO so what is the benefit of "One shot" type valve ? Does this type of the valve help to minimize the possibility of high pressure at the suction of the compressor ( valve outlet) as it is limiting flow of the valve in case of high suction pressure?

Thanks

 

[georgeverghese]

The recycle valve should be gradually opened to full open at the controller on manual mode first, then stop the compressor, and then close the inlet / exit valves.

Not familiar with this term " one shot ".

 

[AMXX]

Hi georgeverghese;

Many thanks for your kind response.
Why should it be opened on manual mode not as part of the compressor sequence?

About sequence of the train valve closure and recycle valve opening, I agree with you. However, in my compressor logic diagram, first the inlet/outlet valves should be closed then the transmitter should be placed in manual mode and fully opened.

FYI, one shot is in the bracket after the valves tag number. I am not sure if it means one shot opening of the valve ( do not know what can be the logic to close the valves after some times) or it is the type of control valve ( I cannot find anything in the valve datasheet).

cheers

 

[georgeverghese]

This shifting of the controller to manual could as well be incorporated in the compressor shutdown sequence. Alternatively, you can leave the controller on auto and slowly reduce controller setpoint.

Agreed, the exit valve could be closed first, but then the closure has to be done slowly especially as the exit valve approaches full close - this is to allow some time to enable the recycle controller to react to the rise in discharge pressure. If the exit valve closing speed cannot be controlled, then you have no choice but to close this valve last, else you will trigger the compressor discharge PSV.

 

[pmover]

to supplement previous post . . .
objective is to maintain flow through the compressor while minimizing any temperature rise in compressor discharge. if the compressor discharge piping has a check valve, then opening the recycle valve first, dP signal across check valve subsequently closes discharge valve. once the discharge valve is closed, a possible cool/slow rpm down period (need to monitor compressor Td), close suction valve, shutdown unit and then open vent to atmosphere or otherwise.

 

[AMXX]

Many thanks georgeverghese and pmover for your kind response.