Page 484 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Lube, Seal and Control Oil System Best Practices Best Practice 7 .35
bodies in series, or six seal assemblies. Usually each compressor gas being compressed has a tendency to change the specification
body is maintained at the suction pressure to that body, therefore of the seal oil to off specification conditions, one of two possi-
three discreet seal pressure levels would be required and three bilities remain:
differential pressure systems would be utilized. The concepts
discussed in this section follow through regardless of the amount - Introduce a clean buffer gas between the seal to ensure
of seals in the system. Sometimes, the entire train, that is, all the proper oil conditions
seals referenced to the same pressure. In this case, one differ- - Dispose of the seal oil leakage
ential seal system could be used across all seals. In most cases, the first alternative is utilized. Once the seal oil
In conclusion, remembering the concept of an orifice will is in the drainer, a combination of oil and gas are present. A vent
help in understanding the operation of these systems. Re- may be installed in the drainer pot to remove some of the gas, or
member, the gas side bushing is essentially zero flow, the at- a degassing tank can be incorporated.
mospheric side bushing flow varies with changing differential This concludes the overview section of seal oil systems. As
across the seal and any seal chamber through flow will change can be seen from the above discussion, it is evident that the
either as a result of differential across a fixed orifice or the design of a seal oil system follows closely to that of a lube
repositioning of the control valve. system. The major difference is that the downstream refer-
ence pressure of the components (seal) varies, whereas in the
Seal liquid leakage system case of a lube system it does not. In addition, the collection of
the expensive seal oil is required in most cases and a down-
This seal system sub-system’s function is to collect all of the stream collector, or drainer system, must be utilized. Other
leakage from the gas side seal and return it to the seal reservoir at than these two exceptions, the design of the seal system is
specified seal fluid conditions. Depending upon the gas condi- very similar to that of a lube system and the same concepts
tion in the case, this objective may or may not be possible. If the apply in both cases.

Best
Best Practice 7.35Practice 7.35
Do not use dynamic gas side (inner side) bushing seals Lessons Learned
unless absolutely necessary since these types of seals The MTBF of dynamic bushing inner oil seals is less than
typically have lower MTBFs than higher seal oil leakage straight though bushing seals and the cost of unscheduled
straight bushing seals and can cause gas leakage to the shutdowns during a continuous critical service compres-
atmosphere through the atmospheric bushing (outer side). sor run will usually exceed the savings gained by having
Dynamic gas side seals employ devices to act as reverse flow less oil leakage to the drainers.
pumps to minimize inner seal oil leakage to the seal oil drainers. Note that if the reason for change-out to a dynamic bushing is the
These devices are speed and clearance sensitive and operate at entry of oil into the compressor, especially in refrigeration applications
higher temperatures than straight through bushing seals. This reduces where entrained oil in the process will affect chiller cooling effective-
the life of these seals, to less than 12 months in some cases. ness, there is another modification that can be made to the seal oil
In many cases the reverse pumping action will bring gas into the system to correct this problem (and that option is not to use dry gas
port between the inner and outer (atmospheric) bushing seal, resulting seals). See B.P. 7.42.
in gas entering the outer seal and being transported outside the
compressor. Benchmarks
Vendors and non-OEMs offer dynamic inner bushing seals as
This practice has been used since the mid-1980s to maximize the
modifications to reduce seal oil leakage into the drainers and/or the
MTBF of oil bushing seals from all vendors. The use of straight through
compressor.
or static inner bushing seals has resulted in oil seal MTBFs in excess of
It is recommended that prior to accepting a dynamic inner seal
120 months and control of inner seal oil leakage to manageable values
modification, end users perform a life cycle cost analysis. This should
where clean buffer gas and proper system modifications are
compare the savings of the reduced requirement for seal oil to the
implemented.
drainers with the cost of one shutdown for a seal change-out during
a run (4e5 years). A typical seal change will take a minimum of five days
resulting in lost revenue for critical service compressors.
B.P. 7.35. Supporting Material

See B.P: 7.34 for supporting material.

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