Hydrocarbon Processing - April 2021 - 75

Maintenance
and Reliability
S. ZARDYNEZHAD, CNRL-Canada,
Calgary, Alberta, Canada

Improve the reliability of a CO2 compressor
in a urea synthesis/granulation plant
The carbon dioxide (CO2 ) compressor is one of the key major equipment
in a urea production plant that produces
fertilizers. The compressor is normally a
between-bearings, single-shaft centrifugal compressor design based on API 617.1
Without the compressor, plant production
would be stopped. FIG. 1 shows a typical
CO2 compression process in a urea plant,
including a steam turbine, low-pressure
casing, gearbox and high-pressure casing
with suction knockout drum, inter-stages
piping, coolers, reactor and separators.
The production cost of a urea plant
producing fertilizer can be optimized by
maximizing the compression system operation time, reducing unplanned shutdowns and keeping operating costs to a
minimum under operational conditions.
The CO2 compressor is the heart of the
urea plant's synthesis process and its performance and reliability will impact the
overall plant operating cost, availability
and reliability.
This article addresses parameters that
impact the performance and reliability of
the CO2 compressor, based on the author's
past experience working in large-scale ammonia and urea production plants.

* Integrally geared compressors
impellers run at different speeds
* Integrally geared compressors
impellers run at higher speeds
than the single-shaft compressor
* Integrally geared compressors
are normally a package design,
which limits ease and safe access
for operation and maintenance
* The number of dry gas seal
cartridges is higher than the
single-shaft compressor
* They are more susceptible to
vibration compared to single-shaft
compressors due to the overhung
design of the impeller and a higher
shaft and impeller velocity
* Their efficiency is lower than a
single-shaft compressor
* Their height is normally higher
than a single-shaft compressor
* Integrally geared compressors need
an internal guide valve (IGV)
* Single-shaft compressors can be
direct driven with a steam turbine.

Steam turbine

LP casing
compressor

However, a detailed study is required,
and other factors such as the vendor's experience, power, efficiency, etc., should
be considered when selecting the compressor type.
Material selection. CO2 corrosion re-

sults when CO2 gas dissolves in water, liquid or process condensate to form carbonic acid [(H2CO3 ) siderite], especially with
higher pressure. This type of corrosion can
happen either during normal operating
conditions or when the compressor is in
standby and under pressure. Material selection is the key activity of the compressor,
upstream equipment/piping, downstream
equipment, piping and instrumentations.
Note: It is highly recommended to use
the API 617 format datasheet and clearly
mention the gas composition, hydrogen
content, liquid content, water content, etc.
Increasing the level of chromium in the
wetted steels offers no major improvement
in resistance until a minimum of 12% is
reached.2 Critical factors that impact CO2

Gearbox

HP casing
compressor

Pool reactor
145 barg-150 barg

Compressor type selection. Depend-

ing on plant production capacity and the
licensor's past experience, both singleshaft, between-bearings compressors or
integrally geared compressors are possible
options. However, single-shaft, betweenbearings centrifugal compressors are normally selected by many licensors and are
recommended by the author because:
* The number of bearings and
shafts are higher than the
single-shaft compressor

H2
removal
reactor
Inter-stage cooler
and separator

Inter-stage cooler
and separator

Suction
knockout drum

CO2 from
Process air
ammonia plant
ASV

Inter-stage cooler
and separator
ASV

FIG. 1. Simplified process for CO2 compression in a urea plant. The dotted line shows the
boundary of CO2-wetted, stainless-steel wetted material.
Hydrocarbon Processing | APRIL 2021

Hydrocarbon Processing - April 2021

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