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Hi! We have six oil flooded screw compressors installed at our plant for instrument and plant air unit. All four have rated pressure of 8.5 barg however four of them having 42 m3/min flowrate while one has 12.5 m3/min and last one has 7 m3/min. During parallel operation recently, we are experiencing motor overload tripping for both small compressors. It it due to back pressure of big compressors having high flowrate causing small compressors to overload or there can be other reason for this?

 

 

Tags: Motor, Compressor, overload

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All of the compressors should be seeing the same pressure at discharge if all valves are aligned correctly. I have a few questions to help troubleshoot. 1. Are all of the compressors set to the same discharge pressure set point? If so, do the pressure transducers or gauges all read the same? 2. Are these compressors variable speed or variable capacity? If so, are they staggered to have load picked up and shed in a sequential manner as system demand changes? 3. Do you have a system receiver tank to help minimize load fluctuations on the system? Discrepancies in any of these areas could be causing the problems you are seeing. If the smaller compressors are being used to "trim" the system, they could be cycling frequently and tripping because multiple starts in a short time are overheating the thermal overloads from repeated inrush current cycles in a short time period. If you can give me a little more information I believe that I can help you resolve the issue.

RM

Hi! Michael. First of all thank a lot for providing your input.

I have attached schematics of system which I will be frequently referring in answering your queries below:

First of all, please understand normal operation first.

During normal operation, 3 out of 4 C-4202 ABCD compressors are put in operation to maintain receiver tank pressure near 7 bar. If one compressor out of these four is out of service for maintenance and we are struggling to maintain header pressure near 7 bar then C-4201 & C-4203 are put into operation.

1- Are all of the compressors set to the same discharge pressure set point? If so, do the pressure transducers or gauges all read the same?

No they are not set at same discharge pressure. There is 0.3 to 0.4 Bar variation in it.

2. Are these compressors variable speed or variable capacity?

There are variable capacity type guided by an inlet flow capacity valve and are operated on Direct On Line System.

3.If so, are they staggered to have load picked up and shed in a sequential manner as system demand changes?

Controlled through loading and unloading set points

4- Do you have a system receiver tank to help minimize load fluctuations on the system? 

Yes, we have as shown in attached image.

Let me know if you require any further info.

I look forward to your response.

Thanks.

 

Attachments

Images (1)
  • IAPA Schmatics
RM

Farhad, 

  Apologies for the delay in replying. I have a couple of more questions and some things to possibly check for potential problems. 

1. Is the Oil removal unit strictly for removing oil, or also a dryer? (I would recommend a dryer be installed if one isn't there)

2. Have you considered a receiver upstream of the oil removal unit (dryer)? 

3. Does your receiver have a timed drain to remove any condensate buildup? 

  I would check the receiver for possible condensate buildup. Verify that your timed drain, if installed, is working correctly. If working and still getting condensate buildup, you may need to change the timer to drain more frequently, especially in summertime. 

  Check your smaller compressors and make sure the variable flow capacity valves are modulating properly to unload the compressors when appropriate. 

  Use a clamp meter to verify the current on the smaller compressor motors is actually exceeding the trip setpoints. Check the integrity of the electrical connections to the smaller compressors from the Motor Control Center to the motor terminal boxes. Preferably use an infrared camera and inspect for hot connections. If you do not have an infrared camera and cannot obtain one, shut down and lock out the compressors one at a time and inspect the connections manually and clean and tighten to proper torque as required. Make sure you insulate them properly when completed. 

  I hope this helps. If you have any additional questions, please reach out. 

RM

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