Chemical Engineering May 2013 - 56

Feature Report
Compressors:
Pursuing Lowest
Cost of Ownership
Proper specification, purchase, mechanical-run
and shop performance testing, installation,
operation and maintenance are all important
Amin Almasi
Rotating Machinery Consultant
A
variety of technologies, methods
and best practices are available
to obtain the lowest total cost of
ownership for compressors used
in plants throughout the chemical process
industries (CPI). This article discusses
several critical areas in which
concerted effort can help to reduce the
overall cost of ownership.
Specifying compressors
The ability to carry out proper shopbased
performance tests and mechanical
run tests - that is, testing that is
carried out by the vendor, before the
unit is delivered to the CPI facility
- can provide tremendous value for
users. Shop-based performance testing
is of particular necessity for any
CPI compressor, because it allows for
careful analysis of the compressor
under conditions that closely simulate
those of the actual installation, to help
detect potential malfunctions and
other problems before the compressor
has been delivered. This is important
because corrections and modifications
at the CPI plant site tend to be costly.
A solid compressor specification
outlines for the vendors what the compressor
package must have, and how
reliable the unit must be. Key aspects
to be addressed in the specification include
the following:
Proper sizing. Before the start of any
compressor specification, the equipment
must be sized, at least in a preliminary
manner. For example, specifying
a single-stage compressor when
a multi-stage compressor is needed
will have inappropriate consequences
down the road. And, there is usually
more than one compressor type that
may be well suited for any given CPI
application. For example, many modern
CPI applications can use either the
integrally geared centrifugal compressor
or the conventional-type centrifugal
compressor. Thus, the reliability
and efficiency of the various competing
compressor types should be evaluated
early in the specification process.
Know the operating scenario. The
first cost (that is, the initial purchase
price) is an important factor that
should always be considered during
compressor selection. However, ongoing
costs related to operation and
maintenancemust also be evaluated to
assess the total cost of ownership.
The operating conditions (as described
in the compressor data sheet
and specification) should be divided
into two types - those that characterize
normal operation, and those
that characterize operating conditions
that deviate from normal. The entire
anticipated range of operating conditions
should also be defined, either by
range limits or by alternative operating
conditions.
A common example of off-design
conditions is the scenario in which
the compressor encounters deviations
in the molecular weight of the compressed
gas, as might be encountered
in a CPI plant during startup. Another
situation that could lead to off-design
conditions is the equipment holding
period when a part of a process is not
yet in operation. Such anticipated scenarios
must be evaluated and noted
during the specification stage.
54 CHEMICAL ENGINEERING WWW.CHE.COM MAY 2013
FIGURE 1. This centrifugal compressor
is an electric-motor-driven machine
that uses a gear unit for the speed match
Unusual operating conditions -
even insignificant items - should also
be mentioned. Examples include the
presence of hydrogen sulfide or chloride
in the gas, even if levels may be
in the ppm range. Keep in mind that
the compressor vendor's knowledge of
your process will be limited. Keep the
following facts in mind during compressor
specification:
1. The compressor manufacturers specialize
in compressor design and
manufacturing and they are not
focused on the intricacies of CPI
processes.
2. If engineers at the vendor company
have experienced a problem in another
customer's process that is similar
to the current job, he or she may
be able to share that experience (by
raising questions and seeking solutions)
during the development of your
specification. However, considering
vendor-staffing policies, the vendor's
overall philosophy in that regard,
the fact that vendor engineers often
only know a small part of any process
(that is, the part of the process that is
related to their machine), and many
different processes (and process licenses)
that exist for most CPI units,
the ability to rely on the vendor for
process-specific input tends to be the
exception rather than the rule.
As a result, it is important that the
compressor purchaser use its own deep
knowledge of the process - in terms
of the potential scenarios that could
lead to unusual conditions or process
upsets that could impact compressor
operation during the life of the unit
- during compressor specification. A
good example is the gas-temperature
runaway potential in hot-gas units.
The specification should set forth the
expected maximum temperatures, and
ask the vendor to verify the maximum
temperature the unit can handle.
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Chemical Engineering May 2013

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