Balance line in centrifual compresor seal gas system 2

[lahori]

Hello,
I am working with Centrifugal compressor gas seal system. Seal are of Dry Gas type. I can see a Balance line on the P&ID of seal system. Can anybody please explain what is the purpose of this balance line?

Regards
Yasir

 

[JJPellin]

The balance line on a centrifugal compressor serves two purposes. First, it balances both seal chambers to the same pressure, which should be very close to inlet pressure for the compressor. Second, it allows for the use of a balance piston, balance disk or balance drum to balance out the thrust loads on the rotor. For the sake of the dry gas seals, it is most important that both seals operate at the same pressure. Since there can be a small pressure drop across the balance line, the seal system would be controlled to hold a differential pressure across the seal on the upstream side of the balance line which could be at slightly higher pressure than the other seal.

Johnny Pellin

 

[JJPellin]

I wrote an FAQ on the Pump Engineering forum that describes the purpose and function of a balance piston / balance line. The concept is very much the same for a compressor as it is for a pump.

Johnny Pellin

 

[lahori]

Hi Johnny,
Thanks for your reply. What is the reason behind to balance both seal chambers to the same the pressure.
Regards
Yasir

 

[JJPellin]

The control system for the gas seals is much simpler if both seals are at the same pressure. From a safety standpoint, it is desirable to have the seals exposed to the lowest pressure possible. In the event of a catastrophic failure, a lower pressure will result in a smaller release.

Johnny Pellin