Page 306 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Steam Turbine Best Practices Be st Practice 5.8

Best
Best Practice 5.8Practice 5.8Practice 5.8
Best
Condition monitor steam seals on large turbines by contamination of the oil systems and reduced bearing
trending gland condenser vacuum and taking corrective life.
action when vacuum is less than -5 inches water column to In many installations, vacuum gauges in the steam seal system are
positively prevent contamination of the oil system. either missing or broken.
Most steam turbine vendors use a vacuum gland condenser system
to ensure that air enters the last section of the steam seal, thus Benchmarks
preventing stream from entering the bearing bracket and contaminat- This best practice has been used since the 1990s to prevent excessive
ing the oil system. contamination of the oil system (greater than 200 ppm of H 2 O). When
Monitoring the vacuum of the system will allow determination of the eductors in the steam seal system are properly monitored and
necessary maintenance (eductor nozzle change). maintained, steam seal MTBFs will exceed 100 months.

Lessons Learned
Failure to monitor gland condenser vacuum on special
purpose (un-spared) steam turbines has resulted in gross

B.P. 5.8. Supporting Material
The key to successful shaft end seal operation is to con-
Shaft end seals tinuously maintain a slight (2-4 cm [5-10"] H 2 O) vacuum in
the last chamber of the seal. By maintaining a vacuum at this
location, atmospheric air will enter the seal thus assuring that
Facts concerning shaft end seals and sealing systems for critical
service non-condensing and condensing turbines are shown in steam (moisture) will not enter the bearing bracket and
Figures 5.8.1e5.8.4. contaminate the oil system.

Fig 5.8.1 Expansion turbine shaft end seals (Special purpose (unspared) turbines) Left: Typical exhaust end seal. Right: Typical inlet end seal
(Courtesy of IMO Industries)

Function: prevent steam from escaping to atmosphere along
the shaft and entering the bearing housing
Special purpose turbines usually employ a low vacuum
(2 – 4 cm [5 – 10"] H 2 O vacuum) to buffer atmospheric end
labyrinth with air
General purpose turbines usually do not employ a vacuum
system and do not totally prevent moisture from entering
bearing housing

Fig 5.8.2 Steam turbine shaft sealing systems

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