Chemical Engineering December 2010 - 39

Feature Report
Engineering Practice
Optimizing Reciprocating
Compressors for CPI Plants
Follow this guidance to
improve the design, performance
and reliability of these widely
used machines
Amin Almasi
WorleyParsons Services Pty. Ltd.
R
eciprocating compressors -
the most commonly used type
of compressor throughout the
chemical process industries
(CPI) - are flexible and efficient,
and they can generate high head
(from several bar to several thousand
bar) independent of gas density.
Worldwide, the installed reciprocating
compressor horsepower is
approximately two times that of centrifugal
compressors.
However, the maintenance costs associated
with reciprocating compressors
are approximately three times
greater than those for centrifugal compressors
(due to valve, unloader and
packing-maintenance requirements).
This article provides practical recommendations
for users to consider in an
effort to improve the selection, operation
and maintenance of reciprocating
compressors in CPI applications.
Compressor design
Figure 1 shows the basic design of a
reciprocating compressor. Close attention
to the selection of the piston
rod packing can improve performance,
because this is a common source of
reliability problems associated with
reciprocating compressors, and is a
common path for the leakage of potentially
hazardous process gases.
Experience shows that packing life
can be extended by as much as a factor
of three by adding the proper coating
(tungsten carbide is a widely used
coating material for piston rods).
Interstage cooling is required when
the machine or gas being compressed
has a temperature limit. In this case,
as the gas cools, any liquid that may
form is separated in interstage facilities
and then the gas is returned
to next compressor stage for further
compression. Each compressor stage
may consist one or more cylinders.
Vendors usually offer a range of interstage
pressures. The ability to optimize
interstage pressures can help to
minimize the total cost of ownership
for the compressor and interstage facilities.
This optimization can be done
by evaluating the initial cost and
operating costs of compressors and
interstage facilities for various interstage
pressures.
During operation, interstage pressures
will increase during part-load
operation (that is, operation at lower
flow that results when an unloader
device is used; this is discussed below)
combined with variation in pressure
at the suction inlet.
In a typical reciprocating-compressor
design, the first stage may contain
one or more cylinders and a clearance
pocket. An additional bottle may be
added to the cylinder with an actuated
on/off valve. To avoid unwanted
interstage pressure increases, users
may consider installing additional
clearance pocket(s) on the first-stage
cylinder(s) and using part-load operation
via the compressor control logic.
By selecting the right interstage design
pressure, users can ensure proper
operation in the face of part-load operation
and variation in suction pressure.
In general, the interstage design
pressures should be around 15%
higher than the interstage basic design
values for applications that are
working with common part-load steps
(such as 25%, 50%, 75% and 100%
capacity) and are expected to experience
a variation in suction pressure of
around +/- 7% during operation.
In some applications, reciprocating
compressors must be designed to operate
reliably in the face of considerable
suction pressure variations while
still providing full design flow at the
desired discharge pressure. These
operating requirements will have a
direct impact on compressor sizing,
especially the frame rating and motor
power required for the unit.
Figure 2 shows load curves for the
connecting rod of a reciprocating compressor
in petroleum refining service.
Variation in suction pressure (in this
case, a roughly 7% reduction in suction
pressure) results in a higher load
on the rod. As a general rule, the compressor
should be designed so that
the maximum-anticipated rod load
ChemiCal engineering www.Che.Com DeCember 2010 39
Connecting rod
Crankshaft
Crosshead
Distance
piece
Piston rod
packing
Cylinder
Figure 1. Reciprocating compressors compress air or other
gases using a piston that is driven by a crankshaft. Shown here
are the main parts of a reciprocating compressor
Piston rod
Piston
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Chemical Engineering December 2010

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