Hydrocarbon Processing - April 2021 - 76

Maintenance and Reliability
corrosion rates include CO2 partial pressure, pH, gas velocity and temperature.
CO2 compressors are unique in terms
of material selection. All wetted metallic material and parts with CO2 gas in a
compressor (i.e., casing, inlet guide valves,
diaphragms, impellers, shafts), upstream
piping material, upstream equipment,
inter-stage piping material, inter-stage
equipment, metallic gaskets and instrumentations should be stainless steel, except for labyrinths, shown in FIG. 1.
This requirement will impact the
compressor's CAPEX, so vendors or engineering companies sometimes do not
use stainless-steel material for the wetted
metallic parts, which can cause major difficulties for the end user. It is highly recommended to add stainless material for
the wetted parts in the API 617 datasheet
in case the gas nature is wet or a possibility exists of wet gas during operation or
standby condition under pressure.
If cost constraints are a factor and the
intention is to use carbon steel material
(which the author does not recommend for
wet gas services), cost-effective options to
mitigate the risk of CO2 corrosion include:
* Applying electro-less nickel plating
on the wetted carbon-steel material
* Using stainless-steel cladding on
the wetted carbon steel material

* Evaluating where a CO2 gas
condition may change to wet gas
inside the compressor in each stage
(i.e., impeller, diaphragm, diffuser
and casing) and checking the gas
condition at any temperature and
stage above its water dewpoint
during compressor operation or
standby under pressure. Note that
the variation in gas composition can
change the water dewpoint of the gas.
FIG. 2 shows examples of severe CO2
corrosion in a CO2 compressor with carbon-steel, stationary wetted material.
Process air. Process air is added to the

CO2 stream in ammonia and urea plants
for the following main reasons:
* As combustion air for hydrogen
(H2) removal from CO2 in an
H2 removal reactor
* As an anti-corrosion agent for the
synthesis equipment-the oxygen
in the air keeps the synthesis
equipment passive for corrosion.
Cleanliness and dryness of the process
air supply plays an important role in the
reliability and performance of the CO2
compressor. Proper sizing of the strainer
and separator close to the tie point must
be considered for the process air. Careful de-watering, flushing, drying and

FIG. 2. Severe CO2 corrosion of carbon-steel casing.

76 APRIL 2021 | HydrocarbonProcessing.com

commissioning of the process air piping
should be considered.
Knockout drum. Liquid in the gas
stream is generally harmful to any compressor and should be avoided by proper
inlet system design. Where risk of liquids
is present, compressor inlet and inter-stages should be provided with properly sized
liquid separators or knockout drums. The
location, orientation and elevation of the
gas inlet and outlet nozzles in the separator should be carefully placed to prevent
risk of liquid carryover to outlet gas toward the compressor inlet. In addition, if
the separators are furnished with process
condensate pumps for drainage and the
minimum flow lines of those pumps are
connected to the separators, the connection point will be in proper orientation
and elevation to prevent liquid carryover
or liquid spray in the flowing gas.
Level transmitters, level indicators and
level switches are necessary to monitor
and control the liquid level in the knockout drums and the separators. It is recommended to validate their operation daily
and confirm their nozzle elevation and
location in the vessels before initial field
calibration. The use of high-efficiency
demister pads and internals are necessary
to prevent liquid from migrating to the

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Hydrocarbon Processing - April 2021

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