centrifugal compressor response to a block discharge line 2

[mucour]

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.

 

[Montemayor]

Mucour:

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.

 

[mucour]

Montemayor, thanks. At closed discharge , a centrifugal pump attains its shut-in pressure, which is the maximum pressure that the pump can reach. The question is that what happens in the case of centrifugal compressor. Does the system resistance due to blocked out moves the operating point close to surge? Or in another way, supposing the centrifugal compressor discharges to a common pipeline outlet where other compressors discharges into and the pressure at this outlet imposes an increased back pressure on the compressor, how does the compressor behaves in this instance?

 

[Montemayor]

Mucour:

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.

 

[mucour]

Montemayor, now it is very, very clear. Thank you. I do not mind the abuse as long as I get the support. Thanks again

 

[frogcurry]

If a centrifugal compressor has its discharge blocked in then it will recycle at its anti-surge control point. Flow isn't zero.

 

[mbeychok]

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)
.

 

[JJPellin]

It may be a question of terminology, but I have a different interpretation of this question. If a centrifugal compressor is blocked in on the suction or the discharge it will surge. Because it is a compressible fluid, there can be movement even with a blocked valve. I have seen this many times with a compressor blocked on the suction side. I have never seen this with a compressor blocked on the discharge since that is a dangerous thing to do. But in either case the situation would be the same. As the valve is closed, the differential head across the compressor will increase which pushes the compressor back up the performance curve the same as a pump. When the differential pressure gets too great for forward flow to continue, the flow will stall and reverse, which reduces the pressure at the outlet and increases the pressure at the inlet. At this point, the differential pressure is low enough for flow to be reestablished and forward flow resumes. The process repeats until the resistance is removed. Even at a completely blocked condition, there will be a surge. Since the gas is compressible, an amount of gas can move forward through the compressor until the pressure at the outlet gets high enough (or the inlet pressure low enough for blocked inlet) that flow stalls and reverses. All of my relevant experience is with multi-stage gas compressors in the range of 1,000 to 18,000 HP. A low power "blower" or "fan" might not be able to produce a sufficient compression ratio to result in surge. But a large machine can definitely surge. As noted above, I cannot overemphasize the fact that blocking in the discharge of a compressor is dangerous in the extreme. If the flow to a compressor must be throttled, it should always be throttled at the inlet, not the outlet.

Johnny Pellin

 

[mucour]

Thanks JJPellin, for your better explanantion. In centrifugal pump, a scenario of higher system resistance due to e.g. blocked outlet or pumps installed in parallel imposes higher resistance or back pressure on another parallel pump causes the operating point to shift to the left on the performance curve. The maximum head for the given speed (assume constant speed) is the shut-in/off head. At this point, energy in terms if heat is dissipated on the pumped liquid and pump casing/components whereby temperature will build up, and if run in this condition for a long time can damage the pump. This led to my thread since it is understood that a centrifugal pump characteristics is the same as centrifugal compressor, I then asked: will surge occur which is the condition that will be created as the operating point moves beyond the surge control line and towards the surge limit. But the reaction from the forum is that the gas is compressible and since there is no movement of flow in a blocked valve scenario the pump will not surge. This I cannot argue bcos I have not tried or experienced it in the plant but it negates fluid mechanics. A compressible gas means that if the outlet is blocked or a higher back pressure is imposed, the gas molecules will to come closer together and by this the pressure will increase. If the pressure increases, the compressor differential polytropic head increases. The consequence of this is that the operating point moves towards the left on the compressor performance curve. The only thing I see towards the left of this curve that is damaging is surge.

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

 

[mucour]

In my last posting on line 9, I meant to say compressor not pump. Surge condition does not happen in pumps.

 

[lastone]

Mucour, I think this was an interesting question posed, but I think its more academic/theoretical than practical. If you work in an industrial situation where you have a reasonable potential for a cent. compressor to run blocked in; or some other part of the process has a reasonable potential to create a back pressure the machine can not overcome, you should be informing your management of a serious situation that needs to be corrected and act on it.

 

[reena1957]

Dear Mucour,
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