Page 505 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 8 .4
Be st Practice 8 .5 Pump Mechanical Seal Flush Best Practices
these are general, and should not be taken as being consulted when selecting the seal and flush plan; the more
definitive for every application; however they can aid in input they have, the higher seal reliability your plant should
flush plan selection. Ultimately, the seal vendor should be see as a result.

Best Practice 8.4Practice 8.4
Best
All cartridge seals should be statically pressure tested to installation will more than justify the cost of a static seal
prior to installation to ensure that the seal has not been test facility in your plant.
damaged in transit.
To ensure maximum cartridge seal MTBF, the plant should have Benchmarks
a static seal test facility to ensure that the seals are in ‘as manufactured This best practice has been recommended since 2000 to ensure
condition’ at the time of installation. maximum cartridge seal MTBFs for new projects, as well as in existing
This best practice is analogous to the low speed balance of a rotor facilities that have been increasing their cartridge seal population.
prior to installation to ensure proper operation.

Lessons Learned
Cartridge mechanical seals can be damaged in transit
and/or at customs! Static testing of all cartridge seals prior

B.P. 8.4. Supporting Material

See B.P: 8.3 for supporting material.

Best Practice 8.5

Use pressurized dual seals whenever possible in applica- (pressure that is being sealed) and the pressure of the barrier fluid. This
tions that can potentially be hazardous to personnel safety will ensure that the proper barrier fluid to seal fluid pressure is main-
and/or the environment. tained, and that the pressure of the barrier fluid is not high enough to
The use of pressurized dual seals, which require a safe, non-toxic force the inner seal open.
barrier fluid at a higher pressure than the process fluid at the seal face,
will positively ensure that the process fluid will be contained, since Lessons Learned
a positive barrier fluid to process fluid differential pressure will be The use of unpressurized seals in hydrocarbon and/or
maintained by the flush system. toxic applications has resulted in flammable, toxic and/or
Confirmation of compatibility of the barrier and process fluids is sour fluid releases to the plant environment, which has
required. exposed the plant and personnel to hazards.
The barrier fluid can be pressurized by any of the following means:
Plant nitrogen header pressure e if the lowest value of header Benchmarks
pressure is sufficient This best practice has been used since the mid-1990s when multiple,
Nitrogen bottles and a regulator unpressurized (tandem) seal failures and process fluid releases were
A dedicated barrier fluid seal system
experienced, due to improper operational and monitoring procedures.
It is recommended that a differential pressure gauge be installed to Since that time this best practice has resulted in optimum plant safety
record the differential pressure between the reference fluid pressure and seal MTBFs (greater than 100 months in some cases).

B.P. 8.5. Supporting Material arrangements in which dual seals can be used, namely dual un-
pressurized (previously called tandem) and dual pressurized
(previously called double).
Dual unpressurized vs. dual pressurized Refer to the diagram in Figure 8.5.1,which showsthe dual
seals unpressurized arrangement. This type of seal uses a buffer
fluid at or near atmospheric pressure to lubricate the
Today (2010), dual seals are being selected for more and more atmospheric side seal. The buffer fluid pressure is signifi-
applications, due to environmental restrictions. There are two cantly less than the seal chamber pressure, so any leakage

476