Hydrocarbon Processing - October 2021 - GP-38

MAINTENANCE AND RELIABILITY
medium- and high-pressure applications
(e.g., > 82 barg) where leakage of process/
seal gas to the atmosphere is unacceptable.
Seal gas is injected between the process
side seal and the primary seal faces.
The majority of the seal gas flows into
the compressor and a small amount flows
across the primary seal faces. The seal gas
pressure is reduced across the primary
seal to primary vent pressure. The intermediate
labyrinth provides a restriction
between the primary and secondary seal.
Secondary seal gas is supplied between
the intermediate labyrinth and secondary
seal to provide a barrier and prevent
primary seal leakage from reaching the
secondary seal. Leakage from the primary
seal is diluted with secondary seal gas and
routed to a vent system. Secondary seal
leakage is reduced across the secondary
seal to atmospheric pressure and typically
routed to the atmosphere. In case of
primary seal failure, the secondary seal is
designed to operate at primary seal conditions,
which prevents uncontrolled leakage
to the atmosphere and achieves a safe
shutdown of the compressor.
Double-seal DSGs are used for highly
toxic or abrasive process gases or where
there is a very low suction pressure. Normally
double-seal DSGs work at low pressures
with a nitrogen seal gas supply.
Dry gas seal failure statistics. According
to a John Crane webinar training DGS
failure course, DGS failure statistics are
shown in FIG. 3, which indicates:
* 80% of DGS failures are due
to significant contamination
* 50% by hydrocarbon (either liquid
from the process gas or lube oil
from the bearing housings)
* 10% of failure related to solid/
particulates contamination
due to dirty pipework, incorrect
or poor filtration of the primary,
or secondary gases supply to
the faces
* 4% by chloride content
of the process gas
* 2% due to free water
from the process gas
* 14% due to unknown
contaminations.
Data collection. In any root cause analysis,
data collection is vital. An effective
root cause analysis cannot be conducted
without searching, interviewing, going to
the site, conferring with various departments,
etc. Reliability engineers should
be aware that the opportunity for data
collection will be lost when the DGS is removed
from the compressor, so data collection
must begin as soon as a work order
is generated. Reliability engineers should
collect the full details of " actual " operating
data at the time of failure. The following
information should be collected:
* Start/stop condition and history
* Vibration trend of the machine
* Cartridge pressure test
* Cold standby duration
* Hot standby duration
* Dynamic failure (i.e., failure below
the 1-hr initial running time)
* Operating hour
* Sudden failure or progressive
leakage increase
* Alarm/shutdown setting
* Any Management of Change (MoC)
* Storage time
* Any change in operational condition
* Dimensional checking and change
* DE/NDE condition
* Any unusual condition
* Any deviation from normal
procedure.
Dry gas seal components analysis. The
causes of DGS failure are identified to help
reliability and maintenance teams mitigate
and avoid future failures. Although wear is
a potential cause, the author's industry experience
indicates that this is only the case
in *10% of mechanical seal failures.
To correctly determine the root cause
FIG. 3. DGS failure statistics. Source: John Crane webinar training course, July 2020.
of a mechanical seal failure, reliability engineers
should utilize evidence to form an
accurate seal failure analysis. FIG. 4 includes
some questions that should be asked when
reviewing a mechanical seal failure.
FIG. 4. Five questions for DGS component analysis.
38 SEPTEMBER/OCTOBER 2021 | GasProcessingNews.com
Contamination by process gas. FIGS. 5
and 6 illustrate ways in which the DGS cartridge
can be contaminated by process gas,
particles or liquid. If it is assumed that the
compressor gas is clean, the question may
arise as to why the primary seal gas supply
is necessary. The compressor gas can leak
from the inter-stage labyrinth toward the
DGS. Can that leaked gas be used instead
of the primary seal gas, and why is spending
required for a dry gas seal condition skid?
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Hydrocarbon Processing - October 2021

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