Page 451 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 7 .24 Lube, Seal and Control Oil System Best Practices
collected or directly piped to an oil reclamation unit. Consid- 1. Care should always be given to the inspection of site
ering the cost of typical mineral oil (approximately $25 per glasses.
gallon), a standard compressor with two seals can use $1,000 of 2. Oil samples to determine adequate degassing (flashpoint
oil per day if oil cannot be returned to the reservoir. determination) should be taken periodically.
Figure 7.23.9 shows a typical oil reclamation unit which has 3. Leakage rates from the drainer should be regularly measured
capability to degas all oil entering the unit. In large installations, to determine the condition of the seals.
this unit may be justified for direct installation downstream of 4. The atmospheric side seal drain should incorporate site
the drainers taking suction from the degassing tank and returning glasses as a means of monitoring flow visually to determine if
oil to the seal oil reservoir. For smaller systems, the purchase of flow quantities have significantly changed.
one unit should be considered for the site. Gas entrained liquids 5. It must be remembered that control valves can be used as
can then be collected and transported to the unit for reclamation rough flow meters when pressure across the valve and valve
at specified intervals or the unit can be temporarily installed travel are known. A control valve can give an indication of
between the degassing tank and the seal oil reservoir. change of flow rates in the system and will indicate any
change in seal oil flows. It must be remembered that the
atmospheric side seal oil flows will vary with changing with
System reliability considerations
reference pressure. However, in most installations once units
are on-line, reference pressure is relatively constant.
The contaminated oil drain system is one of the most neglected Therefore, significant changes in seal oil control valve
subsystems in a compressor seal oil system. It is out of the way, position will indicate deterioration of seals.
its function is not fully understood, and it is not usually moni-
tored with accuracy. Following are a few suggestions concerning This system will be discussed further when the subject of
drainer system reliability: buffer gas systems is studied in subsequent chapters.

Best
Best Practice 7.24Practice 7.24Practice 7.24Practice 7.24
Best
Best
Replace mature plant switches with TMR transmitters in all Considering the current (2010) high daily revenue values, it is easy
trip circuits for optimum oil console and serviced unit to justify the installation of TMR smart transmitters for all trip circuits in
reliability. critical equipment installations.
Oil systems installed as late as the 1980s use single switches for
pressure and temperature protection of machine components. Benchmarks
These old devices expose the plant to unscheduled shutdowns. This best practice has been in use since the late 1990s, and clients that
Triple modular redundant (TMR) smart transmitters (two-out-of- we have audited are recommended to replace all trip switches in
three voting for a trip) ensure accurate and reliable operation, and auxiliary systems with TMR two-out-of-three voting logic. Where our
prevent spurious trips e which expose the plant to millions of USD in recommendations were followed, optimum machine reliabilities were
revenue losses. achieved (99.7 % þ).
Lessons Learned
Many plants have registered low machine reliability and
corresponding revenue losses because of the malfunc-
tioning of old instrumentation.

B.P. 7.24. Supporting Material on-line but were usually ignored because they had caused unit
trips in the past.
Prior to the use of smart transmitters, single switches were used The use of TMR logic for all oil system shutdown logic en-
for shutdown logic on critical equipment trains. PM schedules sures accurate response for shutdowns and prevents spurious
were usually at 3 month intervals to check switch settings unit trips due to transmitter malfunction.

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