Hydrocarbon Processing - October 2021 - GP-41

MAINTENANCE AND RELIABILITY
or air, and reuse them (when possible)
to save money. Always follow the OEM's
recommendation.
API 692. Ask the vendor to follow
API 692. While API 6143
contains 35
pages of information regarding DGS
systems, API 692 has almost 100 pages
in four parts. Part 3 was written to specifically
address design issues related to
a DGS support system and includes predefined
P&ID modules to build systems
to suit the application. Annex D of API
692 provides criteria for the selection of
different types of DGSs. This standard is
becoming more popular with new projects
compared to API 614.
System cleanliness. Check the cleanness
of all components within the DGS
panel, including piping material. It is recommended
to use stainless-steel piping to
connect the panel to upstream and downstream
systems (flare, vent, compressor
discharge pipe, etc.) whenever possible.
Check internal surface cleanliness of the
piping and gas channels in compressor end
walls with a professional borescope and
perform chemical cleaning where needed.4
Risk of condensation. Check and
monitor the risk of condensation during
all operating conditions, especially standby
conditions under settle-out pressure.
To avoid condensation, a gas conditioning
unit with a gas heater should be used
to keep the gas temperature above the
dewpoint in all conditions. Considering
low ambient temperatures, the need for
winterization with electrical heat tracing
should be studied. A provision for using
warm nitrogen may be required, as well.
Consult with the seal vendor as to whether
a double-seal DGS has an advantage
compared to a tandem-seal DGS to mitigate
the risk of condensation. Doubleseal
DGSs use nitrogen as seal gas, so the
gas temperature may not an issue. The
use of a coalescing filter will help to separate
liquid from the gas; however, liquid
can be dropped out after passing the gas
inside the cartridge from the intermediate
labyrinth or between seal faces due to
the Joule-Thomson effect.
Restriction orifices. The primary gas
vents contain orifices that keep the gas
film between the faces. Ensure that they
are installed before the initial compressor
startup or after turnaround. Without
those orifices, the primary seal will fail.
The location and elevation of those orifices
are important compared to the compressor
shaft center.4
GP
NOTE
The recommendations outlined in this article are
based on the author's experience and are not related to
any company.
LITERATURE CITED
Complete literature cited available online at
www.HydrocarbonProcessing.com.
SHAHAB ZARDYNEZHAD is a
registered Senior Mechanical/
Pipeline Engineer in Alberta and
British Columbia with more than
29 yr of experience working in
the world's largest oil, gas and
petrochemical projects. He has
experience in engineering, procurement services,
manufacturing, installation, commissioning, startup,
reliability, maintenance and operation of pumps,
compressors and turbines. He holds a BS degree
in mechanical engineering from the University of
Petroleum of Iran, an MS degree in industrial
engineering from IUST Iran, and MS degrees in
mechanical engineering and project management
from the University of Calgary in Canada. He is also
a certified API inspector for rotating equipment.
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Gas Processing & LNG | SEPTEMBER/OCTOBER 2021 41
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