Page 552 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Best Practice 9 .13
Dry Gas Seal Best Practices Best Practice 9 .14
options are based on the type of seal configuration and secondary - Double seals e trip on ‘high-high’ seal gas supply flow ‘AND’
seal. They are: vent ‘high-high’ pressure
- Tandem seals e trip on ‘high-high’ primary seal flow or For any of the above options, instruments associated with the
pressure ‘AND’ high secondary vent pressure shutdown circuit should be triple modular redundant. The final
- Tandem seals with carbon contact separation seal e trip decision regarding unit trips is of critical importance to plant
on ‘high-high’ primary seal flow or pressure ‘AND’ safety and reliability, and will require a HAZOP review for each
low N 2 separation gas supply minus secondary vent application.
pressure

Best Practice 9.13

Do not connect bearing vents of the machinery to Lessons Learned
a common header in dry gas seal applications. This will A recent (2007) plant fire was started when a total dry gas
ensure that a dry gas seal failure cannot transfer process seal failure in a steam turbine driven compressor train
gas to the bearings of the other machinery in the train. transferred process gas through the common bearing
If a catastrophic tandem seal failure occurs (simultaneous failure of housing vent system to the inlet steam end bearing of
primary and secondary seals) process gas may enter the bearing a VHP (very high pressure e greater than 10,000 kPa)
housing of the compressor. steam turbine.
If the individual bearing housings in the compressor train are
connected to a common header, process gas will be transferred to all Benchmarks
the other bearing housings in the train.
This best practice has been recommended since the reliability issue
If a turbine driver or non-intrinsically safe motor is contained in the
identified above for all new and existing installations that have been
train, exposure to an explosion or fire is present in hydrocarbon
audited by our organization since 2007.
services.

B.P. 9.13. Supporting Material
housing through the separation seal. For this reason, the best
Experience shows that in the case of a catastrophic seal failure, practice is to individually vent each of the bearing housings to
there is a possibility that process gas could enter the bearing a safe location.

Best
Best Practice 9.14Practice 9.14
Control separation nitrogen pressure using a differential releases to the plant environment during tandem seal
pressure transmitter sensing secondary vent pressure to reliability issues.
ensure a nitrogen atmosphere between the secondary seal
and separation seal to positively prevent a hydrocarbon Benchmarks
release to the plant environment. This best practice has been used since 1990 in projects to optimize
Frequently called the ‘disaster bushing’, the separation seal is the plant safety and reliability. (no known process gas releases for the
last item in the dry gas seal chamber that can prevent a gas release to subject projects and seal MTBFs greater than 70 months).
the plant environment.
If the separation gas system (usually N 2 ) is designed with a differ-
ential pressure controller to regulate supplied separation gas above the
secondary seal vent pressure 20 to 40 kPad (3 to 6 psi), positive sealing
will be ensured.
Lessons Learned
Failure to use a differential pressure controller for the
separation seal system has resulted in process gas

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