Chemical Engineering May 2012 - 63

the compressor. Figure 4 shows an example
of a steam turbine driver.
Electric motor drivers require, with
FIGURE 6. This gas turbine
is a cold-end drive
machine. Gas turbines are
relatively standardized,
even though they cover a
wide range of power and
speed. They are not custom
engineered to the specifi c
application for a speed
match.
some exceptions, that a step-up gear
is used for speed match. Because fossil
fuel can be more efficiently converted
to electricity in large combined-cycle
power plants (currently with efficiency
more than 60%), the costs of electrical
energy for electric motors become sufficiently
low to displace the more convenient
steam turbine drivers. Large
electric motor drivers (currently up to
70 MW) using variable frequency conversion
to provide for variable-speed,
are very popular.
Gas-turbine drivers are common in
power. In the past, " isothermal compressors "
with many cooling surfaces
and intercoolers were popular. However,
due to their high capital cost and
excessive maintenance, they are not
currently in production. By keeping
the gas temperature low, significant
amounts of power can be saved. Mechanical
considerations usually limit
the discharge temperature to about
250°C. But the commonly used sealing
elements made from elastomers
(O-rings for casing, or similar components)
usually limit the top temperature
to 170°C. Some gases must be
kept at lower temperature based on
process requirements (for instance, to
avoid decomposition, reaction, polymerization
or similar process reasons).
Hydrocarbons are usually limited to
around 120-130°C maximum temperature.
Permissible gas temperatures for
acetylene, chlorine, ammonia and carbon
monoxide are around 60, 100, 160
and 175°C respectively. If the machine
operates relatively close to the surge,
a higher discharge temperature could
occur. If the gas temperature is too
high, inter-cooling must be provided.
Pressure margins. For applications
where the pressure ratio can be defined
fairly precisely, a 10% margin should
be applied to capacity. For applications
where the pressure ratio is heavily
dependent on flow (for instance, with
a recycle duty) a 5% margin should
be applied to both capacity and head.
Higher margins than these can only
be justified if the operation and capital
cost increase is acceptable. In some
cases, extra margins are included in
the anticipation of future debottlenecking.
This may be economic where
variable-speed drives are employed.
Performance. The characteristic (performance)
curve of a centrifugal compressor
is a plot of the head against the
flow (capacity). For reliable operation,
the head-capacity characteristic curve
should rise continuously from the certified
operating point to the actual
surge point (usually 5-10% increase
is specified). For compressors operating
in parallel, the head is at the same
specific flowrate needed within certain
limits (most often 2%) at any flowrate
on the compressor curve.
At flows greater than the design
flow, the characteristic is limited by a
rapid fall of head. This is due to the
high losses, particularly in the frontal
stages of the compressor, caused by
the high gas velocities and incidence
angles at the entry into the impellers.
Compressor drives
Historically, the most popular drive
for the centrifugal compressor has
been the steam turbine. A steam turbine
can readily be speed matched to
some CPI applications, such as very
large compressor units, remote areas,
where cheap fuel is readily available
or similar situations. The operating
speed range of a gas turbine is standard
for a given model. Sometimes the
output speed of the gas turbine can be
considered to design an efficient centrifugal
compressor. Usually it is not
possible, however, and so an intermediate
gear unit is needed. As an indication,
gas turbine drivers are employed
in 5-140-MW drive power range for
various CPI compressor applications.
Figures 5 and 6 show examples of gas
turbine drivers.
■
Edited by Gerald Ondrey
References
1 Bloch, H. P. A " Practical Guide to Compressor
Technology, " 2nd ed., John Wiley, N.J., 2006.
2 Bloch, H. P. " Compressor and Modern Process
Application, " John Wiley, N.J., 2006.
3 Brown, R. N. " Compressors Selection and
Sizing, " 3rd ed., Gulf Publishing Co., Houston,
pp. 120-220, 2005.
Author
Amin Almasi is a lead
rotating equipment engineer
at WorleyParsons Services
Pty Ltd. in Brisbane,
Australia (amin.almasi@
worleyparsons.com). He previously
worked in Technicas
Reunidas (Madrid) and Fluor
(various offices). He holds a
chartered professional engineer's
license from Engineers
Australia (MIEAust CPEngMechanical),
and a chartered engineer certificate
from IMechE (CEng MIMechE), RPEQ (Registered
Professional Engineer in Queensland). He
also holds M.S. and B.S. degrees in mechanical
engineering. He specializes in rotating machines
including centrifugal, screw and reciprocating
compressors, gas and steam turbines, pumps,
condition monitoring and reliability. He has authored
more than 45 papers and articles dealing
with rotating machines.
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