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PaulR600 (Petroleum) (OP)
23 Mar 13 21:53
Hi,
I am just starting on a new instrumentation project that involves compressor oil-seal system. What I can't seem to determine is what the operating pressure of the system should be. Assuming there is 1000 psi on the gas side, a dp of 45 psi, then the seal oil system would operate at 1045psi, for example. Is this correct? Also, in this example, what would the typical pressure drop be across the bearing?
Thanks
Paul
JJPellin (Mechanical)
24 Mar 13 18:42
Each seal design has a required diferential pressure. Iso-carbon seals might require 35 to 45 psi. Iso-sleeve seals might require 5 psi. Cone seals might require about 30 psi. The last part of your question is unclear. What does bearing pressure have to do with seal oil pressure?

Johnny Pellin

PaulR600 (Petroleum) (OP)
27 Mar 13 19:38
Hi,
Thanks for the reply. I mis-stated my question. I meant, what is a typical dp, which you answered.
As I understand it, then, the seal oil system must operate at high pressures when the process is high pressure, how is that pressure regulationed in the seal-oil system?
Thanks
Paul
JJPellin (Mechanical)
28 Mar 13 7:34
If the process pressure is variable, then the seal oil pressure needs to be controlled with a differential pressure controller to maintain the required differential pressure above the process pressure. This can be done with differential pressure control valve that has process pressure on the bottom of the diaphragm and seal oil pressure on the top. A better method is to use a differential pressure transmitter. The signal from the DP transmitter is processed in a PLC or in DCS and sends the necessary signal to a positioner on the seal oil control valve.

If the process pressure is very stable, it may be possible to control the seal oil pressure with a simple pressure controller. This requires more attention on the part of the operators make sure that the set point of the pressure controller gives an acceptable differential pressure above the process gas pressure.

For iso-sleeve seals, a different method is commonly used. The seal oil is directed into an overhead vessel that is level controlled. The process gas is directed to the top of that vessel. The height of the level in the vessel is pre-determined to provide the needed differential pressure by virtue of elevation. As long as the level in the vessel is correct and the process gas pressure above the oil is the same as the process gas pressure on the seals, the seal oil differential pressure will be perfect. There are other problems with this method. The process gas is usually in contact with the oil which would contaminate the oil if the process is dirty. A rubber bladder can be used to solve this. But, a bladder failure introduces another failure mode that can have very serious consequences.

Johnny Pellin

Airsporter1st (Mechanical)
6 Apr 13 11:58
Just to add to the excellent response above, some mechanical seals require quite a narrow control of the pressure differential across the seals for optimum performance longevity. A typical example might be 3 bard minimum, 5 bard maximum, The chosen seal oil system needs to take this into account.

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