Centrifugal Compressor in Continuous Blowdown.

[waross]

We have a question in the motors forum as to the efficiency/inefficiency of a 600 HP centrifugal compressor running with continuous blowdown.
It may be that blowdown is being used for capacity control.
From an electrical perspective it seems wasteful to control the capacity by blowing off compressed air.
We need some input from someone familiar with the operation of a centrifugal compressor.
This may be more of a mechanical issue that an electrical issue.

2300v motor efficiency
thread237-446148



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rotw]

Hi,

I saw the posting on the other section (electrical).

Main worry here if I am correct, is: operator has tweaked the power settings, and noticed that even at lower min. current the compressor is not surging, the operator concluded that the compressor is blowing off unnecessarily. Is this correct?
Corrollary: is the compressor continuously blowing off during normal operation or does it happen on certain occurences (change of inlet conditions in general, ambiant air temperature ?) If it is blowing continously during any operation - assuming no drift or deterioration of machinery performance (fouling, wear, etc.) - then yes obviously something has not designed properly. Need more details.

Okay, some educated guess regarding the configuration:

- This seems to be an air centrifugal compressor, equipped with three stages.
- Attached documentation indicates HS and LS pinion, thus it seems to an integrally geared type of compressor. we need to know if it is built to API617 or to other API672.
- Going from atm. pressure to approx. 8.3 barg means there must be intercooling / aftercooling. Otherwise temperature will be excessive. We need to know the type of intercooling, I expect water type and the location of the coolers in the process (PFD diagram basic). Especially, if there is air cooler, the performance of compressor (power) may be affected by the ambiant air temperature.
- It is needed to understand how surge controls is done. Need a basic control diagram. It seems that the surge control is based on motor power. If the power goes beyond a certain level, the compressor is assumed to be approaching surge and then blow off valve opens. Needs to look at the diagram. The blow off valve located at the final discharge (if this is the case) will not protect each section individually. It will protect the overall, thus this solution costs less but is less energy effective - this as general note.
- The triggering for surge protection is the control line, not the surge line, typically there is 10% or more margin between these. This means that even if the flow goes lower than control line it does not mean that the compressor effectively surges. Keep this in mind.
- It is needed to know if the compressor has been surge tested or not to tune the parameters. There can be a big gap between the actual/predicted and real maps; surge testing is for this purpose
- Need to know if the operating parameters are monitored, which ones and is there an historian. In particular has there been a drift or degradation of performance that prompted an investigation or is this prompted by "operator curiosity"?
- What type of inlet air filter is used ? self-cleaning? is the delta pressure accross the filter within normal operation limit.
- Finally, I noticed the presence of the suction throttle valve. What is the purpose of this valve? how / when this interfer with the blow off. This is not clear.

 

[waross]

The issue is that the demand for compressed air has been considerable reduced. The compressor is blowing off excess air.
The OP is concerned about wasted power.
His concerns may be well founded.
There could be a lot of dollars being blown out with the air.
Management is unconcerned. His attempts to save money are going nowhere with management.
We have advised him to return the settings to the original settings to cover his assets.
Thank you for your efforts to help.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[georgeverghese]

There are typically 2 modes of capacity control on these packaged air compressors (a) continuous suction throttling (b) start stop or load/unload. Both of these modes operate off a feed signal from a PIC on the compressor discharge. For (b) to be effective at low demand, there must be a well sized wet air receiver. Check the air compressor operating manual for the procedure to invoke either or both of these control modes.