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Balancing Piston

Balancing Piston

Balancing Piston

How important is the pressure monitoring on the balancing chamber of a multistage compressor (through flow)?Thanks...

RE: Balancing Piston

I consider balance chamber pressure (or balance line DP or balance line flow) to be important for both large pumps and centrifugal compressor; but for different reasons.  With the pumps, a failure of the balance piston could result in a catastrophic thrust bearing failure of the most destructive sort.  Because of the high differential pressure in a pump, a thrust bearing failure can result in a complete seal failure with a high potential for fire.

In a compressor, the differential pressures are probably lower and the risk of catastrophic thrust failure is lower. It is also likely that the compressor has thrust position monitoring which should be armed to trip to protect against this failure.  So, the value in a compressor may be different.  The flow across the balance piston is an indication of the clearance of the piston which is an indication of the condition of all of the internal clearances within the machine.  Knowing the balance line flow allows for better troubleshooting and performance evaluation.  It can help drive repair strategies and overhaul frequencies.  If you only have a turnaround window every 6 years, you need as much information as possible to determine if you need to overhaul the compressor or if you can expect it to run for 6 more years.  Balance line flow is another piece of information to help with that decision.  

One of the last compressors that I overhauled had severe damage to the balance piston. We did not have appropriate monitoring for balance line flow so we did not know that this problem existed.  In order to properly plan the overhaul and to justify the cost to overhaul a machine that the operators thought was running fine, it would have been nice to have data to show that the balance piston had failed.  I would rather have too much information rather than not enough.

Johnny Pellin

RE: Balancing Piston

Thank you very much Mr. Johnny, for your informative comments. Based on API 617. A pressure monitoring on the balancing chamber of a multistage compressor is mandatory. However, the balancing line delta p is a recommendation since API stated "if specified". What is then the better monitoring practice, to have the pressure monitoring on the balancing chamber, the delta p monitoring on the balancing line or both.

RE: Balancing Piston

It depends.  If the suction conditions are stable and predictable, it might be adequate to monitor balance chamber pressure. If the suction pressure is variable and inconsistent, then balance line DP would be a better indication.  Of course, if you have balance chamber pressure and suction pressure in your DCS system, you can calculate the DP and alarm accordingly.  For the machines that I work on, balance chamber pressure is adequate.

Johnny Pellin

RE: Balancing Piston

For a charge gas compressor in an ethylene cracker plant on which we are constructing, where the suction pressure depends on the top pressure of the quench tower and the suction pressure of the compressor will control by the speed of the condensing steam turbine then as per your recommendation / experience balance line DP would be a better indication. Thank you very much for this technical knowledge you share...I have already talk with our vendor and they will provide the balance line DP monitoring....

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