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Use abradable type separation seals for reliability and process advantages.
The main reasons for use of abradable seals are as follows:
At the present time (2010), most compressor vendors offer abradable (fluorisint or equal) seals as an option for separation seals.
Regardless of the type of seal configuration (double or tandem), the function of the separating system is to prevent process gas from entering the bearing housing in the event of a seal failure, and oil from entering the seal cartridge. Entrance of process gas into the bearing housing exposes the plant to catastrophic consequences and extended downtime.
There are several types of separation seals. The choice depends on the availability of the separation gas (usually N2). The alternatives arranged in order of highest usage of separation gas, are:
The best practice is to use labyrinth or abradable labyrinth separation seals, if sufficient N2 is available. This recommendation is based on the reliability of labyrinth type seals compared to carbon seals and the fact that the differential pressure across the labyrinth seals is not limited as is the case for most carbon ring seals.
If carbon ring seals are used, the control system must limit the differential pressure to the design maximum. In addition, if carbon contact seals use cryogenic N2, the best practice is to condition the N2 to ensure sufficient moisture is present for optimum carbon life.
Experience shows that, in the case of a catastrophic seal failure, there is a possibility that process gas could enter the bearing housing through the separation seal. For this reason, the best practice is to individually vent each of the bearing housings to a safe location.
The method of separation gas control depends on the type of seal selected. For labyrinth and abradable labyrinth seals, the best practice is to use differential pressure control -- seal supply pressure - secondary vent pressure -- to each seal. For carbon ring seals, pressure control could limit the maximum differential pressure across the carbon rings.
The condition of each separation seal can be determined by monitoring and alarming on low differential pressure for labyrinth and abradable labyrinth seals. For carbon ring seals, monitoring and alarming on low pressure is recommended. These parameters should be used as permissive signals to prevent starting the oil system if N2 gas is not being supplied to the separation seals.
The nitrogen consumption of conventional labyrinth separation seals is too great to justify their usage, and floating carbon ring seals have not proven to be totally effective in preventing oil migration into the secondary seal vent chamber.
This best practice has been used since early 2000 when I was involved with a mega ethylene project to optimize the effectiveness of the separation seals and reduce nitrogen consumption.