Suction throttling valve method

[Jack Nicholson]

Hi everybody!
I have a question, would you please help me?
We have a centrifugal compressor with fixed electric motor speed. The capacity control method of this compressor is suction throttling valve (PV-063) (as the picture has been attached).

How this valve would control the operating of compressor? For example if inlet flow decrease whereas suction pressure is constant, what would be the action of throttle valve (PV-063)?
If both inlet flow and pressure decrease, what would be the action?
What is the relation between operating of PV-063 and anti surge control system?
Thanks in advance.

http://s8.picofile.com/file/8314303284/PV.png

 

[georgeverghese]

The control scheme doesnt show PIC 063 acting in the function of compressor capacity control by suction throttling. It shows PIC 063 acting as a min set backpressure controller - it allows gas to go forward to the compressor only when pressure upstream of PCV 063 is in excess of PIC 063 setpoint. Which is why there is no process control link from PIC063 to the compressor antisurge controls.

 

[LittleInch]

George as usual has it right.

How this valve would control the operating of compressor? For example if inlet flow decrease whereas suction pressure is constant, what would be the action of throttle valve (PV-063)?
It wouldn't move, but how in your system can the flow change but the pressure stay constant? In most system flow and pressure are interlinked.

If both inlet flow and pressure decrease, what would be the action?
The valve you've circled as GV states is controlling on the upstream pressure, presumably to try and maintain a constant up stream pressure, so in this instance the valve would startto close to try and maintain that pressure.

What is the relation between operating of PV-063 and anti surge control system?
Only that if the pressure reduces to the extent that the PV 63 reduces flow or inlet pressure to below the surge point of the compressor, then the A/S system would kick in. You don't show the A/S loop on your diagram but I assume it's somewhere outside the loop. I don't like the look of the valves on the RHS of the drawing. - close them whilst the machine is running and you'll have surge.

Without seeing what the pressure range and flows are it's v difficult to judge what is going on here.

What is your concern?
Is there some operational thing going wrong?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Jack Nicholson]

Dear all
Thanks for your helping me out. I have another question.
Is it possible that inlet flow to the compressor decrease, but inlet pressure be constant???Thanks in advance.

 

[georgeverghese]

Yes, provided there is a PIC on compressor suction for this purpose - in a fixed speed application, this PIC would modulate the compressor capacity recycle valve, while in a variable speed application, it will normally modulate compressor - driver speed.