Page 520 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Pump Mechanical Seal Flush Best Practices Be st Practice 8.8

Fig 8.8.12 Double mechanical seal

Fig 8.8.10 Single mechanical seal

Fig 8.8.13 Liquid/gas tandem seal combination
Fig 8.8.11 Tandem seal arrangement
seal configuration is used in low specific gravity applications
where the pumped fluid is easily vaporized. Using a gas seal as
Double mechanical seal applications
the back-up has the advantage of eliminating the vessel, cooler
Double mechanical seals (refer to Figure 8.8.12) are used in and pumping ring necessary for conventional tandem liquid
applications where the pumped fluid is flammable or toxic, and seals.
leakage to atmosphere cannot be tolerated under any circum- This application is well proven, and has been used success-
stances. Typical process applications for double seals are H 2 S fully for natural gas liquids, propane, ethylene, ethane and
service, hydrofluoric acid alkylation services or sulfuric acid butane pump applications.
services.
Leakage of the pumped fluid to the atmosphere is positively Double gas seal applications
prevented by providing a seal system, whose liquid is compatible Before leaving this subject, a relatively new application utilizes
with the pumped liquid, which continuously provides a safe two (2) gas seals in a double seal configuration, and uses N 2 or
barrier liquid at a pressure higher than the pumped fluid. The air as a buffer, maintained at a higher pressure than the
seals are usually identical in design with the exception that one pumped fluid to positively prevent the leakage of pumped
seal incorporates a pumping ring to provide a continuous flow of fluid to atmosphere. This configuration, like the tandem
liquid to cool the seals. Typical double seal system components liquid/gas seal mentioned above, eliminates the seal system
are: reservoir, cooler, pressure switch and control valve.
required in a conventional liquid double seal arrangement.
Note however, that the pumped product must be compatible
Liquid/gas tandem mechanical seal applications
with the small amount of gas introduced into the pumped
In this configuration (refer to Figure 8.8.13) a conventional fluid. This configuration cannot be used in recycle (closed
single liquid mechanical seal is used as the primary and a gas seal loop) services.
(non-contacting faces) that can temporarily act as a liquid seal in An excellent resource for additional information covering
the event of primary seal failure serves as the back-up seal. This design, selection and testing criteria is API Standard 682.

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