Stonewall protection

[AMXX]

Hi;
As I know for the centrifugal compressors, the stonewall protection is an optional protection (is not mandatory same as Anti-surge protection). However operating in the stonewall condition also damage the compressor so I am thinking if there is no stonewall protection, maybe it should be just relied to the operator action to trip the compressor. In this basis I have two questions:
1) Does it need to trip the compressors in case of stone wall?
2) if yes and compressor does not have stonewall protection, which alarm will be activated for operation attention ( e.g. : high compressor vibration) ?


Many thanks

 

[rotw]

Not required. Thrust bearings must be sized according to standard (ex. API617). Manufacturer shall verify sizing against:
- operating points
- map limits
- conditions for thrust reversal

Proper alarm/trip settings on high axial displacememt. Trip on axial bearing temperature is to be evaluated according to operator/plant guidelines.

 

[AMXX]

Hi ROTW;

Thanks for your kind reply,. The performance curve just includes the predicted surge and operating point. The map limit ( Anti-surge point and the stonewall) is not defined (see the attached file).

So generally the compressor is protected as if the operating point moves toward Stonewall , first operator will receive high Axial bearing temperature alarm ( which can decide to trip the compressor) and then the ultimate compressor protection is the high axial displacement ( high vibration) which will trip the compressor automatically.

 

[rotw]

The map limit is perfectly defined for your curve attached (fixed speed).
The end of curve is by default your limit in view of stonewall, if we call it so. There is no fixed rule to define stone wall limit. Depends from one manufacturer to next, users standard etc. There is also distinction between choke and stone wall. To define choke:
Some criteria are based on efficiency drop versus max efficiency or versus flow
Some criteria uses drop of head versus flow.
We can even go more deeper and use more sophisticated criteria to set what the limit is.

Thus I would recommend to restrict to manufacturer end of curve point as the criteria to stick to.

The document you should look at in respect to axial thrust, is the axial thrust calculation and curves. Normally is document requested at purchase order which tells you about the calculation vs. operating cases considered.

In regard to monitoring:
There can be other reasons for axial bearing temperature and/or axial displacement high.
So if the compressor is choking but machinery health parameters are OK then I do not see a big problem:
Main drawback is operation at (very) low efficiency plus typically poor predictability of performance in this region.

 

[AMXX]

Dear ROTW;
please find my query below in red:
The map limit is perfectly defined for your curve attached (fixed speed). Yes, the curve limit is defined but the operational limit (Anti-surge and stonewall) was not shown in the curve. I thought at least the anti-surge limit should be defined in the curve. Is not the anti-surge limit important for small compressors?
Thus I would recommend to restrict to manufacturer end of curve point as the criteria to stick to. Is it possible to go beyond the curve limit?

The document you should look at in respect to axial thrust, is the axial thrust calculation and curves. Normally is document requested at purchase order which tells you about the calculation vs. operating cases considered. I searched for the "Axial thrust " calculation but I could not find it in the vendor documents. I am not a mechanical engineer so I do not have any idea how this calculation can help me to determine the compressor trip at the stonewall.

There can be other reasons for axial bearing temperature and/or axial displacement high. Big compressors always have high bearing and vibration trips. Are these trips optional for the small compressors? If they are optional so the only safeguard will be operator intervention.


So if the compressor is choking but machinery health parameters are OK then I do not see a big problem: I have to add that:
1) The compressor is recycling 10% of the plant gas (working in a closed loop)
2) We do not want to run the compressor at higher flow-rate.
3) I have a case-study to determine the system safeguard in case of some control failure. If that is happened:
• the compressor flow-rate will be increase dramatically
• The suction pressure will be increased to the discharge pressure.
• The compressor cannot increase the discharge pressure as the downstream pressure is limited by other systems so the compressor head will be dropped.
To protect the system, I need to find a compressor trip function otherwise the only safeguard will be the operator act.
Main drawback is operation at (very) low efficiency plus typically poor predictability of performance in this region. Do you mean that the compressor is not damaged if it is working at stonewall condition? As operation in the stonewall condition will increase the compressor vibration and high bearing temperature so if there is no auto trip function, how long can the compressor work in this condition before operator decide to trip the system?


Many thanks for your time and help.

 

[rotw]

Yes, the curve limit is defined but the operational limit (Anti-surge and stonewall) was not shown in the curve. I thought at least the anti-surge limit should be defined in the curve. Is not the anti-surge limit important for small compressors?
Response: The anti-surge line is not showed for fixed speed machine. The controller monitors the deviation to surge flow versus fixed point (surge flow) on the left. The curve limits are the operational limits, except for the surge line where a control line in between is applied for anti-surge control. But here again this is fixed speed machine so there is no such a line to show. Moreover even in variable speed machine, it is not custom to show the control line. Roughly it is known that this line is situated at 10% from surge line.
It does not matter for the size as far as anti-surge control is concerned.

I searched for the "Axial thrust " calculation but I could not find it in the vendor documents. I am not a mechanical engineer so I do not have any idea how this calculation can help me to determine the compressor trip at the stonewall.
There is no compressor trip at stonewall. Never heard of. Thrust calculations will tell which operating scenario have been considered for sizing the thrust bearing, including choke limit if the job has been done to good standard.

Big compressors always have high bearing and vibration trips.
Not true. Many big operator wants no trip on bearing's temperatures for example because spurious trips can be very costly.

To protect the system, I need to find a compressor trip function otherwise the only safeguard will be the operator act.
I have the impression that a fixed speed compressor has been selected and it is lacking operating flexibility. If that is the case, I doubt there is a quick fix.

Do you mean that the compressor is not damaged if it is working at stonewall condition? As operation in the stonewall condition will increase the compressor vibration and high bearing temperature so if there is no auto trip function, how long can the compressor work in this condition before operator decide to trip the system?
If you are operating within the performance envelope (in this case, it is a fixed speed curve) provided by the manufacturer and supposedly the design has been executed properly by a knowledgeable vendor, and the machine is not deteriorated from its clean state, then I do not see why not operate at the end of the curve (choke), except like I said incur a drop of performance and poor efficiency continuous operation. If you process conditions are upset or off specification then in my opinion it would be another problem to address.

 

[georgeverghese]

As far as I know, there is no need for any instrumented protection for stonewall region , since startup at low dp is in this region also, and all centrifugal compressors I've come across work okay in this region. The compressor goes into this region also during shutdown with blowdown as the compressor decelerates.

If you are running at high internal recycle rates, check that the recycle cooler cools the gas adequately so that suction temp is still within the normal operating range, else the compressor will trip on discharge high temp. Industry standard thermal design practice for the discharge cooler is to enable full recycle, with no net fresh gas supply. But dont take this for granted - run a thermal rate check on this cooler just to be sure, with the necessary fouling resistances included.

 

[AMXX]

Dear rotw and georgeverghese

OK it seems I need to think differently and resolve my issue in another way . Thank you so much to share your valuable knowledge.

 

[AMXX]

Dear georgeverghese;

There is two coolers to cool-down the temperature so I do not have any issue with the gas recycling.