Page 549 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 9 .9 Dry Gas Seal Best Practices
monitor the condition of the secondary or outer seal by mea- - Labyrinth seals
suring one of the following: - Abradable labyrinth seals
- Non-contact carbon seals
- High pressure in the secondary vent e suggested setting: 1e2 - Segmented carbon contact seals
kpag (5e10 inches water column)
- Low pressure differential between the separation seal inlet The best practice is to use labyrinth or abradable labyrinth
and secondary vent pressure (if the separation gas is separation seals, if sufficient N 2 is available. This recommen-
controlled at a fixed pressure) dation is based on the reliability of labyrinth-type seals com-
- Low pressure in the primary seal vent e this assumes that pared to carbon seals, and the fact that the differential pressure
a pressure of 30e40 kpag (5 psig) is normally maintained across labyrinth seals is not limited, as is the case for most
between the primary and secondary seals. carbon ring seals.
If carbon ring seals are used, the control system must limit
The decision to alarm or trip will depend on the application the differential pressure to the design maximum. In addition, if
and the potential daily revenue loss of the plant. Since this vent carbon contact seals use cryogenic N 2 , the best practice is to
can also contain oil or oil mist in the event of a separation seal condition the N 2 .
system malfunction, the best practice is to monitor the effec- Experience shows that in the case of a catastrophic seal fail-
tiveness of the separation seal by locating the vent in the seal ure, there is a possibility that process gas could enter the bearing
chamber at the low point (6 o’clock position), and installing housing through the separation seal. For this reason, the best
a device to indicate oil contamination (level glass as a minimum), practice is to individually vent each of the bearing housings to
with a drain valve to a safe location in the vent line.
a safe location.
The method of separation gas control depends on the type of
Separation systems seal selected. For labyrinth and abradable labyrinth seals, the
best practice is to use differential pressure control e seal supply
Regardless of the type of seal configuration (double or tandem), pressure minus secondary vent pressure e to each seal. For
the function of the separation system is to prevent process gas carbon ring seals, pressure control could limit the maximum
from entering the bearing housing in the event of a seal failure, differential pressure across the carbon rings.
and oil from entering the seal cartridge. Entrance of process gas The condition of each separation seal can be determined by
into the bearing housing exposes the plant to catastrophic con- monitoring and alarming on low differential pressure for labyrinth
sequences and extended downtime. and abradable labyrinth seals. For carbon ring seals, monitoring
There are several types of separation seals. The choice depends and alarming on low pressure is recommended. These parameters
on the availability of the separation gas (usually N 2 ). The alter- should be used as permissive signals to prevent starting the oil
natives, arranged in order of highest usage of separation gas, are: system if N 2 gas is not being supplied to the separation seals.

Best Practice 9.9Practice 9.9Practice 9.9
Best
Best
Require a seal gas conditioning unit (cooler, separator and Lessons Learned
heater) when there is any possibility that seal gas can be Failure to include a seal gas conditioning unit when the
saturated and an external clean dry seal gas source is not seal gas can be saturated has resulted in low seal reliability
available. (less than 12 months MTBF in some cases).
If the search for an installed source of clean dry external seal gas The failure to design a gas conditioning unit into the seal system,
has not been successful, there is proven industry experience (since especially in oil and gas applications, has eventually required modifi-
2000) with the use of a seal gas condition unit that has the following cation to a GCU (gas conditioning unit) that was justified by multiple
features: seal failures and significant product revenue losses.

A seal gas cooler that reduces the seal gas to a minimum of 15 C
below saturation temperature at the lowest pressure experienced in Benchmarks
the primary vent This best practice has been used since 2000 for field modifications, to
A properly sized separation vessel with automatic level control and result in seal MTBFs in excess of 48 months where the previous system
a demister (seal gas sent directly to the seal gas filters) could not yield seal MTBFs
A seal gas heater that raises the seal gas to a minimum of 15 C above 12 months.

above saturation temperature
A dual coalescer filter with a continuous drain to ensure only clean,
dry gas enters the seal chamber
Note: If the seal gas contains C6þ components, each gas
component must be individually included in the gas analysis
calculations to ensure the correct saturation temperature at
operating pressure conditions.

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