Page 442 - 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 .21
not employ lifting jacks when transferring valves. This type of
valve can become bound and even break at the stem, necessi-
tating critical equipment shutdown. Prior to transferring flow
from one bank to another, the bank to be transferred to must be
full of auxiliary system fluid and properly vented. If it is not, the
capacity of coolers, filters and piping in the bank to be trans-
ferred to act as a vessel, and consequently reduce flow and
pressure to the critical equipment, which will shut down the
unit. Also, improperly vented components containing air or
gases can cause control valve instabilities which can also lead to
equipment shutdown.

Fig 7.20.2 6 way (double, 3 way, flexibly coupled) (Courtesy of Hycoa)

Best
Best Practice 7.21Practice 7.21
Arrange critical machinery protection systems to have only Overspeed trip
one trip signal for each component to prevent spurious High suction vessel level trip
trips.
Most new PLC/DCS based protection systems installed today
Lessons Learned
(2010) incorporate alarm and trip signals for each instrument.
Excessive unit trips lower the reliability of critical machines
Machine reliability is a function of simplicity.
and expose the plant to prolonged downtime and corre-
Over instrumented trip circuits expose a critical machine unit to
spurious trips. sponding loss of revenue.
FAI has followed the best practice of minimizing trips while totally
protecting the machinery unit and all associated support systems by Benchmarks
using only one trip signal per component or system. This best practice has been used since the late 1990s, when two cli-
A typical protection scheme for a steam turbine driven compressor ents were starting a new plant that was essentially a duplicate of their
train with dry gas seals and oil lubrication is as follows: past plants. Neither client could get their respective units commis-
Radial vibration trip e each radial bearing sioned and started on schedule and requested our help. In both cases,
Axial displacement trip e each thrust bearing the root cause of the problems was ‘over-instrumentation of trip cir-
Dry gas seal primary vent trip e each seal cuits’. Existing (“old”) successful plant trip logic was copied, excessive
Low lube oil pressure trip e train trip trips deleted and both plants achieved smooth trouble free start-up
Low control oil pressure trip (integral in trip valve) and optimum machine reliabilities.
High compressor discharge temperature trip (if applicable)

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