Page 444 - 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 .23

Best
Best
Best Practice 7.23Practice 7.23Practice 7.23Practice 7.23
Best
Monitor oil flash point in combined lube/seal systems to Recent experience (2008) found that all four (4) of the bushing seals
determine if the seal oil drainers and/or the atmospheric in a two-body compressor train were passing gas to the oil reservoir
seal bushing is passing gas which will expose the plant to through the bearing oil return header and to the compressor deck.
a safety hazard. Indications of low viscosity in the oil sample led to taking flash point
sampling and gas sniffing. The root cause was failure to ever clean the
Oil flash points below approximately 150 C (325 F) in the oil res-

seal oil overhead tanks. Debris from the reference gas lines (over 20þ
ervoir indicate gas entrainment in the oil.
years of operation) had entered all compressor seal oil internal supply
Combined lube/seal oil systems have the following possible paths
ports, which caused them to plug and render approximately zero oil to
of gas entry into the oil reservoir:
gas differential at the atmospheric bushing seal face, even though the
Failure of the oil drainers to completely close, which allows oil and
external differential pressure gauges registered acceptable differential
gas to enter the reservoir.
pressures.
Blocked degassing tank vents prevents the venting of the seal oil
entrained gas mixture.
Passing of gas through the atmospheric bushing due to seal oil Benchmarks
internal compressor port blockage resulting in zero seal oil to gas This best practice has been used since the late 1980s, when a flash
differential at the atmospheric bushing seal face. point sample taken after a glass oil sample bottle broke indicated that
the flash point was 35 C (95 F) in a combined lube/seal oil console

Monitoring the flash point along with all other parameters during reservoir. Since that time, it has been our best practice to always
periodic oil analysis will alert personnel to possible seal system com- recommend that flash points be taken in all oil samples in combined
ponent malfunctions before a safety hazard occurs. lube oil/seal oil consoles.
Lessons Learned
There have been plant explosions and fires that have
resulted from excessive gas in an oil reservoir that was not
properly monitored.
B.P. 7.23. Supporting Material - The contaminated oil drain line to a seal oil drainer for each
seal in the unit.
The return side of a seal oil system consists of three distinct - The drainer.
return lines: - The vent connection or reference connection from the top of
the drainer.
- The atmospheric drain back to the seal oil reservoir. - The drainer level control device (internal or external) which
- The pressurized drain back to the seal oil control valves (if automatically drains upon reaching a preset level.
installed). - The degassing tank (optional).
- The contaminated seal oil drain. - The oil reclamation device (optional).
All liquid seals are designed such that a small amount of oil The following system design alternatives are available
leakage (38e76 liters, 10e20 gallons per day per seal) enters depending upon the operating conditions of the compressor and
the drain between the inboard gas side seal and the compressor the process gas.
internals. This drain is commonly known as the contaminated
oil drain. To ensure that gas does not leak into the seal, a small
amount of liquid is designed to continuously flow through the Sweet hydrocarbon or inert gas service
seal and into the drainer. Depending on the nature of the
process gas, the oil may or may not be recirculated to For sweet or inert gas service, the seal oil drain can be returned
the degassing tank and the seal oil reservoir. The function of directly to the reservoir, provided the drainers are sized for
the contaminated oil drain system is therefore to collect all adequate residence time and the seal oil leakage is reasonable
contaminated (gas entrained) oil and direct this oil to the de- (less than one gallon per hour per seal). A sweet hydrocarbon gas
sired location. This may be directly to the seal oil reservoir, to is defined as a gas that does not contain hydrogen sulfide (H 2 S).
a degassing tank or other oil reclamation device, or to a con- The vent line on top of the drainer can be routed to a lower
taminated oil drain. We will now examine basic contaminated pressure source, atmosphere or back to the compressor suction.
oil drain system configurations and discuss the function of each If routed back to the compressor suction, a demister should be
component. installed to prevent oil from entering the compressor case. The
sizing of the orifice in the vent line of each drainer is critical in
that it ensures that all contaminated oil flow will enter the
Basic system configuration drainer. Too low a velocity will allow contaminated oil to enter
the compressor. Too high a velocity could cause oil to enter the
Refer to Figure 7.23.1 which shows a basic contaminated oil compressor via the vent or reference line. Typical velocities in
drain system. The system consists of: this line should be 4.5e6 m/sec (15e20 ft/sec).

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