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In our refinery, we are running 5 no of Air Compressors (of Elliott make) to cater the Plant Air and Instrument Air requirement of the entire refinery. The setup is as follows:
4 nos of Big compressors of capacity 6040 NM3/hr each and 1 no of Small compressor of having 3020 NM3/hr and design pressure of 8.3 kg/cm2
All the compressors' discharge pressures are set at more or less equal pressure of ~7.2 kg/cm2.
Instrument Air pressure is maintained at ~5.8 kg/cm2
Of the 4 big compressors, 2 compressors are older by 12--15 years.
Now the question is the Suction C/V (IGV) of Old compressors is opening only 50 to 70% even if the demand is more. C/V stroke checked and observed to OK.
What could be the reason for less opening of IGV?
 
Answers
02/12/2011 A: Bruce Carr, Suncor Inc, matukaze1@hotmail.com
You may want to check the PLC logic for inputs... You didn't mention the type of compressor, if they are multiple stage centrifugal there is a good possibility they have are designed for constant thruput and control by unloading the to atmosphere.... if so the suction valve is designed to compensate for air density and not flow and will throttle the inlet C/V accordingly....
19/08/2011 A: G KAMALAKANNAN, Foster Wheeler India Ltd, siddhukamal@yahoo.co.in
I suggest you to check the following items.
1. Check whether the performance characteristic curves of all the compressors are same or different. In my opinion, the small compressor with higher discharge pressure may be creating pressure imbalance in the system.
2. If possible, please switch off the smaller compressor for some time (using the Inst air reservoir) and check the IGV opening of other similar compressors.
3. It is better to check the actual performance of each machine separately by doing the following exercise:
a) Resistance of each item (Piping, valve, elbows etc). You can measure the same by the discharge pressure provided the Pressure gauge used is a calibrated one.
b) Terminal pressure (Discharge Pressure - Terminal pressure = System resistance)
c) Collect the data of system resistance for various flow rates and make a plot. Superimpose with the compressor characteristic curve issued by Elliot.
By this exercise, you can find out the actual problem of compressor. Either compressor performance is not up to expectation(due to low efficiency) or the system resistance is too high. Low efficiency problem can be attributed to various reasons. (Poor lubrication, Vibration of machine, Air filter choke etc.) Please talk to Elliot for finding other reasons.
4. If you can't find a solution based on above steps, check the program calculation. DCS or PLC will have a logic block to calculate the % opening of IGV. Try to collect system data (discharge pressure, temp) and calculate the IGV manually and compare whether the program is running alright.
Please give feed back regarding the results of trouble shooting so that the forum can learn.