Hydrocarbon Processing - April 2021 - 78

Maintenance and Reliability
nation. Insulation material on austenitic
stainless-steel equipment should be analyzed for chlorides, while insulation material on carbon steel should be analyzed for
nitrates due to the risk of external stress
corrosion cracking. Do not forget to reinstall insulation completely after turnaround, outage, maintenance and inspection of the compression unit.
Liquid trap. Adding a liquid trap at the
inlet piping of a compressor is highly recommended, as is the regular examination
of leakage and operation of the traps.

Moisture sensor/dewpoint sensor.
The addition of a moisture sensor in the
inlet piping to the compressor is recommended, particularly when carbon material is selected.

technically and economically possible. If
the hydrogen content is high and/or the
operation is a high-pressure application
(e.g., high-pressure casing), a tandem dry
gas seal is highly recommended.

Dry gas seal. A risk of suffocation exists
when gas escapes through a leak. Sometimes, a labyrinth seal with nitrogen purge
gas or carbon ring are used as a last line of
defense at the compressor casing to seal
the shaft. However, as the compressor is
normally located inside the building, it is
recommended to use a dry gas seal when

Piping gaskets. Normally, ring-type

FIG. 5. Corrosion and erosion on the gas passage will cause local stalling in a compressor.

joint (RT) gaskets are used between piping and compressor flanges. RT gaskets
have a lower gas leakage rate compared
to spiral wound gaskets. However, if the
flange bolts are overtightened, the gasket
may damage the flange grooves and cause
CO2 gas leakage. Repairing grooves in an
installed condition may not be possible
and may require flange replacement.
As all wetted piping and equipment in
the compression system are stainless steel,
gaskets should be stainless steel, as well.
Using carbon steel material for the gasket
carries the risk of galvanic corrosion and
can result in gas leakage. Additionally, carbon steel, RT joint gaskets can easily be
corroded if liquid, water or process condensate comes in contact with CO2 gas.
For galvanic corrosion to occur, three elements are required:
* Two metals with different
corrosion potentials
* Direct metal-to-metal
electrical contact
* A conductive electrolyte solution
(e.g., water).
In addition to these three elements, the
relative surface area (not mass) of the exposed metals is also an important factor.
If the area of the cathode (noble metal,
here stainless-steel piping and flange) is
very large, and the anode (active metal,
here carbon steel material) is very small,
the current produced is likely to be very
high and the anode (i.e, carbon steel RT
gasket) will corrode quickly. FIG. 4A shows
a new carbon steel RT gasket installed between stainless-steel flanges, and FIG. 4B
shows that same carbon steel gasket corroded after less than 2,000 hr in service
with CO2 gas and process condensate.
Inlet pipe to compressor. Inlet piping to

FIG. 6. Corrosion products and scale on the return wall of a CO2 compressor (A),
and catalyst carryover entering the compressor (B).

APRIL 2021 | HydrocarbonProcessing.com

each stage of the compressor should be designed for ease of cleaning and inspection.
Inlet piping from below the compressor
casing is recommended with the provision for insertion and removal of temporary strainers close to the compressor inlet
without upsetting compressor alignment.
A T-type welded strainer is highly recommended compared to cone or Y-type strain-

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Hydrocarbon Processing - April 2021

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