Variable Speed Centrifugal Compressor Operating Point

[Rha257]

Hello Everyone,

I was wondering how to predict the location of the operating point of a variable speed compressor changes based on control set point.

For example, consider a compressor running at its rated point and controlled via discharge pressure control. Assuming the operator decides to reduce the discharge pressure control set point from 100 bar to 95 bar. How will the compressor operating point shift considering that inlet conditions do not change initially?

Will the compressor speed reduce thereby reducing pressure and flow hence the point will travel down and to the left? Or will the operating point go straight down mainting same flow but reducing discharge pressure? Or will the compressor speed not change at all but the operating point will go to the right side along the performance curve?

When to know if the compressor speed will change and when to know that speed will not change but the point will travel along the curve?

Thanks in advance!!

 

[georgeverghese]

If feed flow remains constant, it is obvious that speed will drop till discharge pressure is met. On the Q-poly head graph, it will be a vertical drop down (constant flow - am3/hr) to the new reduced speed.
If flow changes at the new Pd, find the new flow line on the graph, then move up vertically to the new polytropic head.

 

[goutam_freelance]

If you change the pressure the flow will change depending on system resistance, provided flow in not controlled. If pressure drop across the discharge pipe is less, the flow through pipe (and compressor) will also be less. So the operating point should shift to the left and down.

Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[Rha257]

georgeverghese, thank you for your response. How can I predict if the operating point will move vertically up/down or if it will move diagonally (e.g. up and right or down and left)? For example in the case where the feed flow stays the same, wouldn't a reduction in the compressor speed reduce the flow as well and vice versa? Granted that the point will move down to meet the new discharge pressure setpoint, however, why would it move directly down and diagonally down and to the left? Wouldn't running at a slower speed increase the suction pressure as well?

For the case where feed flow is changing, I would have guessed that the compressor speed wouldn't change at all and the operating point would move along the same compressor curve. Assuming the discharge pressure set point reduces, the operating point should move to the right of the curve towards the stonewall region. Am I missing something?

 

[georgeverghese]

Q. For example in the case where the feed flow stays the same, wouldn't a reduction in the compressor speed reduce the flow as well and vice versa?
A. (i) If speed is reduced, and discharge pressure is maintained, then flow has to drop, assuming suction pressure is controlled constant.
(ii)If speed is reduced, and discharge pressure then also decreases, but only by a little, flow will decrease, but not as much as in (i). If discharge
pressure decreases a lot, then flow may even increase.
In most cases, you will find compressors are operated at constant suction pressure by varying compressor speed.
If speed is decreased, and the operating point has moved down and to the left, obviously flow has decreased, and in this case Pd also. So this tells me this compressor normally operates at a constant controlled suction pressure by varying speed. In many machines, there is also a a discharge high pressure override controller also that may kick in if Pd exceeds discharge controller SP.

Post us a PFS with overall and local control loops shown for this compressor unit, and pls reword your questions with only one scenario per query. Right now you have multiple scenarios per query and it causes confusion.