Chemical Engineering December 2010 - 43

der), can contribute to significant
pressure drops.
To gain a better understanding of
reciprocating compressor performance
and track ongoing operation, the following
performance curves should
be developed:
* Suction pressure versus load
* Suction pressure versus flow
* Discharge pressure versus load
* Discharge pressure versus flow
* Suction pressure versus discharge
pressure, per load step (that is, for
each flow-reduction step using unloaders,
typically 50%, 75%, 100%
flow; 25% is rarely used because of
the potential for reliability and loadreversal
issues)
Such flow curves typically plot the
minimum achievable flowrate to the
maximum achievable flowrate in specified
increment steps (for instance, in
10% steps). (Flow-versus-dischargepressure
plots of specific suction pressures
may be an acceptable alternative
when suction-pressure variations
are limited). A review of the steepness
of the proposed load curves can help
the engineer to quickly identify which
load curves (and where) are too steep.
In these situations, small changes in
pressure can have significant changes
in load and flow. In general, compressors
with steep load curves are hard to
automate and tune. Thus, steep load
curves usually indicate improper sizing
of cylinders.
Optimum conditions
When scoping a reciprocating compressor
system, it is absolutely necessary
to have a minimum of two
technically accepted proposals from
qualified vendors. Small and medium
compressors should be delivered fully
fabricated as one skid-mounted package.
Larger compressors are typically
delivered as a prefabricated system
(including the crankcase, distance
pieces, and so on) with dismantled cylinders.
Assembled cylinders are typically
delivered to the site separately
and installed later. It is common for
the vendor to provide site-supervision
work for cylinder installation at a negotiated
lump-sum price.
n
Edited by Suzanne Shelley
Author
Amin Almasi is lead rotating
equipment engineer for
WorleyParsons Services Pty
Ltd (60 Albert St.; Brisbane
QLD 4000 Australia; Phone:
+61 (0)7 3319 3902; Email:
amin.almasi@worleyparsons.
com). He holds a chartered
engineer certificate from IMechE
(CEng MIMechE) and a
chartered professional engineers
license from Engineers
Australia (MIEAust CPEng-Mechanical). He
also holds M.Sc. and B.Sc. degrees in mechanical
engineering. Almasi specializes in rotating
machines including reciprocating, centrifugal
and screw compressors, gas and steam turbines,
process pumps, condition monitoring and reliability.
He is an active member of IMechE, Engineers
Australia, SPE, IEEE, ASME, CMVI,
the Vibration Institute, SMRP and IDGTE, and
has authored several papers and articles dealing
with rotating machines, condition monitoring,
offshore operations and reliability.
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Chemical Engineering December 2010

Table of Contents for the Digital Edition of Chemical Engineering December 2010