Centrifugal Compressor Thrust Bearing Jacking Oil

[DLite30]

Working on an overhung centrifugal compressor, and come to find out the OEM has a "jacking oil" system (giggity), which feeds high pressure (2000 psi), low volume ( 0.5 gpm) lube oil to, what looks to be just one side of the thrust bearing, and only during start-up and shutdown.

We have a couple different other brands of overhung compressors which are quite mechanically similar to this one, and they have just normal lube oil supply at pressures less than 100 psi.

The only thing I can figure is that with out the aid of a balance piston in the overhung design, that the high pressure oil is there to supplement the oil film at low shaft speeds until such speed is reached that the hydrodynamic oil film can develop with normal oil supply conditions.

However none of the other designs has this system, and am wondering if this most likely a knee-jerk design consideration after a incidident involving or due to a failed thrust bearing.

Anyone have any real knowledge about why such a system may have been put in service?

 

[JJPellin]

None of our overhung machines have this feature. But, all of ours run with relatively low suction pressure. If your machine has a high suction pressure, then the thrust load on the inactive thrust bearing could be quite high until the rotor builds pressure and begins normal thrust. Keep in mind, I am not referring to a high compressor differential pressure. All that matters is suction pressure acting over the area of the seal. A net thrust is produced that thrusts away from the impeller. If this thrust is high, then the breakaway torque to get the machine spinning might be too high for the driver. This could be especially true if the driver is a synchronous motor which has limited breakaway torque. But, without more details about you machine, driver, and process conditions, this is just speculation.

Johnny Pellin

 

[DLite30]

Details of this particular machine:

22,000 hp, VFD driven synchronous motor, 1800 rpm, with speed increasing gear box, with compressor speeds up to about 8300 rpm.

process conditions are generally 500 to 600 psig suction pressure, and 700 to 800 pisg discharge pressure, with 0.6 SG natural gas.

We have several other machines by two other manufactures, which are very similar to this one and operate at pretty much the same process conditions, and none of them have this jacking oil system either.

Whatever the reason, we're going to re-install and re-use this system.