centrifugal compressor response to a block discharge line
centrifugal compressor response to a block discharge line
(OP)
Hello,
Does centrifugal compressor behaves similar to centrifugal pump as it realtes system resistance? The question is if a valve on the outlet line of a centrifugal compressor is closed, will the new operating point move to the left towards surge on the performance curve? Pls help explain.
Does centrifugal compressor behaves similar to centrifugal pump as it realtes system resistance? The question is if a valve on the outlet line of a centrifugal compressor is closed, will the new operating point move to the left towards surge on the performance curve? Pls help explain.





RE: centrifugal compressor response to a block discharge line
Come on! You said yourself that the compressor's discharge is closed - i.e., there is NO FLOW (a "dead head" condition is reached). -- So how can there be a "surge"?
Obviously there can be no surge since there is no flow. At this condition you're simply wasting horsepower by converting it into discharge heat in your gas fluid and into your equipment.
Both the centrifugal pump and compressor work on the same dynamic principles - the only difference between them is that the latter handles a compressible fluid and the former handles a relatively incompressible fluid. Dynamic machines produce energy by increasing the velocity of the fluid in the impeller or blades and reducing the fluid velocity in the diffuser or stator vanes.
RE: centrifugal compressor response to a block discharge line
RE: centrifugal compressor response to a block discharge line
I thought I had written my post clear and direct enough for you to understand what I meant to say. You continue to insist on using the term "surge" to a system where there is no surge -- it's blocked in, closed, shut off. The term "surge" implies movement - albeit small. One definition of centrifugal compressor surge is:
"A disturbance occurring in gas compressors, especially centrifugal compressors, due to excessive pressure across the compressor in which the compressor blades stall and air flow suddenly drops. The flow can fluctuate and the compressor be damaged. It is the point of instability at which a compressor surges across, trying to reestablish its point of stability."
That is not my definition; it's an expert's definition. You will note that it refers to flow fluctuation (movement). In your case, you said you had NO MOVEMENT. The flow is zero. Therefore, there can be no surge. There can certainly be a lot of other serious problems created, but there is no surge. You are literally dead headed.
If you impose an outside, higher pressure (higher than it's max. dead head) on the discharge of a centrifugal machine you will certainly force it to "stall" or shut down. The driving force across the machine is now contrary to what was designed and the impellers will try to rotate backwards -- you will be having a turboexpander effect instead of a compression effect. You will certainly ruin or smash the compressor affected. Don't go there.
I believe that this is basic, common fluid mechanics and should be well understood.
RE: centrifugal compressor response to a block discharge line
RE: centrifugal compressor response to a block discharge line
RE: centrifugal compressor response to a block discharge line
frogcurry:
You forget to say: If the compressor has a anti-surge recycle line and if there is no block valve between the compressor discharge and the point at which the recycle line branches off. Not every compressor installation was designed correctly.
When Mucuor asked what would happen if the discharge were blocked, the logical assumption is that he meant what he said and that there was a block valve which could indeed shut off the flow ... just as Art Montemayor assumed. I am sure that most of us assumed the same thing.
Milton Beychok
(Visit me at www.air-dispersion.com)
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RE: centrifugal compressor response to a block discharge line
Johnny Pellin
RE: centrifugal compressor response to a block discharge line
It gave me serious worries to understand this condition because all along surge is always interpreted as condition resulting from low suction flowrate where a stall result due to flow reversal. I then posed the question on Eng-Tips forum and I was tongue lashed and since I did not know too, which was why I asked I had to take it in my strides.
Please find a tutorial article (link copied below) from the 35th turbomachinery conference that I came accross. Read from line 5 under introduction, the man from Solar Turbines confirmed the explanation given by JJPellin (Thanks). This is the correct explanantion.
The scenario is a credible one. A centrifugal compressor can experience a high imposed back pressure on the discharge or an inadvertent valve closure or a check valve stuck in place. The scenario was not for flow control. Flow control in centrifugal compressor is achieved by speed, an upstream control valve throttling, inlet guide vane.
I stand to be tongue lashed! But know offence bcos man continues o learn.
The link is:
http://turbolab.tamu.edu/pubs/Turbo35/T35pg123.pdf
RE: centrifugal compressor response to a block discharge line
RE: centrifugal compressor response to a block discharge line
RE: centrifugal compressor response to a block discharge line
I agree with JJPellin. I know of a centrifugal compressor damage where the first stage surged because the second stage suction valves were opened too fast and the first stage suction was not opened sufficiently. This was equivalent to closing the discharge for the first stage. The first stage was the first two impellers and the second stage consisted of the rest 5 impllers in a single barrel. The flow occured from second stage to first stage and there was severe surge. Normally the flow from a second compressor discharge backing into the first is prevented by providing non-return valves on the individual discharges. However, this was not possible in the example sited above. However, the SOP was changed so that the second stage suction was to be opened only after the first stage suction was fully open. We have not faced any problem after this.
Best wishes