Page 553 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 9 .15 Dry Gas Seal Best Practices
B.P. 9.14. Supporting Material pressure across 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
Separation systems
design maximum. In addition, if carbon contact seals use
cryogenic N 2 , the best practice is to condition the N 2 .
Regardless of the type of seal configuration (double or tandem), Experience shows that in the case of a catastrophic seal fail-
the function of the separation system is to prevent process gas ure, there is a possibility that process gas could enter the bearing
from entering the bearing housing in the event of a seal failure, housing through the separation seal. For this reason, the best
and oil from entering the seal cartridge. Entrance of process gas practice is to individually vent each of the bearing housings to
into the bearing housing exposes the plant to catastrophic a safe location.
consequences and extended downtime. The method of separation gas control depends on the type of
There are several types of separation seals. The choice seal selected. For labyrinth and abradable labyrinth seals, the
depends on the availability of the separation gas (usually N 2 ). best practice is to use differential pressure control e seal supply
The alternatives, arranged in order of highest usage of separation
pressure minus secondary vent pressure e to each seal. For
gas, are:
carbon ring seals, pressure control could limit the maximum
- Labyrinth seals differential pressure across the carbon rings.
- Abradable labyrinth seals The condition of each separation seal can be determined by
- Non-contact carbon seals monitoring and alarming on low differential pressure for laby-
- Segmented carbon contact seals rinth and abradable labyrinth seals. For carbon ring seals, mon-
itoring and alarming on low pressure is recommended. These
The best practice is to use labyrinth or abradable labyrinth parameters should be used as permissive signals to prevent
separation seals, if sufficient N 2 is available. This recom- starting the oil system if N 2 gas is not being supplied to the
mendation is based on the reliability of labyrinth-type seals separation seals.
compared to carbon seals, and the fact that the differential

Best Practice 9.15Practice 9.15
Best
Color code the following dry gas seal sub-systems, using Lessons Learned
an OME team (operations, engineering and maintenance) Lack of operator, maintenance and engineering awareness
to ensure complete understanding of the system functions of DGS system function and operation has led to reduced
and optimize safety and reliability: dry gas seal system safety and low seal MTBFs (less than
Seal gas system 12 months). Using a site OME team to color code these
Primary vent system systems will correct these issues.
Intermediate nitrogen system
Secondary vent system Benchmarks
Separation seal system
This best practice has been recommended since 2000, initially on
Dry gas systems are critical to compressor safety and plant revenue a project that used the first dry gas seal system in an existing chemical
(since a dry gas seal failure can impact plant safety and daily revenue). plant that previously had used oil seal systems. The color coding of the
Enabling the plant personnel DGS system function understanding dry gas seal system by operations and engineering greatly increased
(operations, maintenance and engineering) significantly contributes to awareness and understanding of this system.
increased plant safety and reliability.
Using a ‘team’ approach to color code each DGS sub-system
(paint e new units or color tape e existing units) will lead to
understanding and ownership of these important systems.

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