Hydrocarbon Processing - October 2021 - GP-40

MAINTENANCE AND RELIABILITY
removed from the compressor casing, disassembled,
cleaned carefully and then reassembled
and reused without any problem.
It was unnecessary to send the cartridges
back to the vendor for cleaning, inspection,
balancing, assembly and testing. The
case would be different if the compressor
was started with contaminated DGSs. In
that case, the cartridges should be sent to
the vendor, as they would be damaged after
compressor startup due to high shear and
massive heat generated due to the lube oil.
Causes of mating rings contact. There
are two main causes of mating rings contact:
no lift and forced contact. Lifting the
mating rings requires minimum gas pressure
between the faces and minimum shaft
speed. Leakage in static and dynamic conditions
may occur.
The lift of speed is different between
unidirectional (2.3 m/sec) and bidirectional
(3.5 m/sec) DGSs. In static conditions,
such as settle-out condition, slow-run and
shutdown conditions, lift off between the
rotary/mating face and stationary face is
provided by gas pressure only. In dynamic
condition, the seal requires less pressure
because the shaft rotation sucks the gas
into the grooves and generates a high-pressure
area between the faces.
During the design stage, the minimum
speed for the faces to lift off is calculated
based on compressor operating conditons
by the seal manufactuerer. This calculation
ensures that lift off hapens before/at
reaching slow roll speeds. FIG. 7 shows a
slow roll diagram of the DGS based on gas
pressure and speed.
Forced contact is where two faces have
been forced to contact each other during
normal or transient conditions (e.g., startup
or shutdown). The result can be three
types of contacts:
* Outer diameter contact of
the mating rings: In this case,
the lift-up may be loosened
due to the high risk of groove
damage in the rotating ring.
FIG. 7. Typical slow roll diagram (DGS).
* Inner diameter contact of the mating
rings: In this case, the lift-up may
be possible due to a lowered risk of
grove damage in the rotating ring.
* Full face contact of the mating
rings: In this case, the lift-up may
be loosened due to the high risk of
groove damage in the rotating ring.
FIG. 8 shows some causes of the no lift,
and FIG. 9 shows some causes of forced
contact.
FIG. 8. Causes of no lift.
Recommendations. The following considerations
improve DGS reliability and
prevent failure.
Slow run. If the compressor train reFIG.
9. Causes of forced contact.
quires a slow run to complete a cooling
or warming cycle-mainly steam or gas
turbines as per driver/compressor manufacturer
requirements-the specific requirements
should be communicated to
the DGS vendor. A slow run normally
takes 24 hr-36 hr with a speed lower than
the lift off speed of the DGS and a lower
discharge pressure, which impacts the primary
gas seal pressure of the DGS. The
seal may require special modifications
on the grooves or engineered seal. In this
case, temporarily disconnecting the compressor
from rotation at low speed may be
40 SEPTEMBER/OCTOBER 2021 | GasProcessingNews.com
a solution with the help of a clutch mechanism,
as a slow run is mainly required for
steam and gas turbines.
Seal gas analysis. Regular seal gas analyses
are highly recommended as part of a
preventive maintenance plan, preferably
on a monthly basis. A provision for sampling
from the gas is highly recommended.
Trend analysis. Monitoring seal leakage,
flowrates and vent pressure is very
important. Out-of-specification leakage
and flowrates are indicators of a faulty seal.
Other observations of bearing vibration
and abnormal temperatures are important
indicators of DGS condition.
Oil atmospheric drain. Adding an oil
atmospheric drain to the DGS cartridge
will help notify the operator if the oil has
migrated from the bearing housing to the
dry gas seal. Monitoring the trend of separation
gas pressures and flowrates are also
kety indicators if oil has been leaked to
the DGS.
Choose the correct seal faces material.
When the rotary and stationary seal
faces touch due to a lack of gas pressure
or enough shaft speeds, a rubbing and
frictional contact simultaneously generates
high heat. Selecting the correct seal
rings ensures reliability-the seal rings
must be hard, rigid, chemically resistant,
thermally conductive, tough and wear resistant.
Tungsten carbide is a popular seal
ring material, and is made by heating tungsten
carbide powder and a metallic binder
(i.e., cobalt and nickel) to 1,500°C to melt
and fuse the binder. Check the chemical
compatibility of nickel and cobalt when
using tungsten carbide in a DGS. Strong
acids can attack binders, causing cobalt
formation at the seal faces and premature
seal failure. Use proper seal gas with the
gas conditioning unit and change the seal
material to silicon carbide if a risk of cobalt
formation exists between the seal faces.
Shaft lifting. During installation or
removal of the DGS cartridges, it is necessary
to lift the compressor shaft a few
times. This may damage the DGS. Review
the procedures and tools with the
compressor and DGS manufacturers.
Minimize the shaft lifting during installation/removal
of the DGS cartridge by using
specially engineered components and
parts. For example, check if split metallic
rings might work instead of solid rings to
fix the cartridges to the casing.
Filter cleaning. Do not clean the filter
elements in a DGS panel with solvent
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Hydrocarbon Processing - October 2021

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