Chemical Engineering November 2012 - 53

Examples
Example 1. The first example is presented
for a two-process-stage centrifugal
compressor. The two process stages
are low pressure (LP) and high pressure
(HP). The compressor is a variable-speed
synchronous electric-motordriven
unit (using gears) to compress
natural gas from around 10 barg to approximately
100 barg. Each compressor
stage has an anti-surge valve.
The calculated Cv
for the LP and
HP compressor casings (at the surge
flow of the high-speed curve) are 215
and 81, respectively. The selected
anti-surge valve's Cv
values for the
LP and HP stages are 385 and 169,
respectively. In other words, the Cv
factors (the anti-surge valve Cv / the
calculated Cv at the high-speed surge
flow) are approximately 1.79 and 2.09
for the LP and HP loops, respectively.
The selected anti-surge valves are
globe valves. The opening time is
2 s and the closing time is 5 s. The
valves were purchased with a NACE
(National Association of Corrosion
Engineers)-compliant carbon-steel
body, a stainless-steel (grade 17-4-PH)
trim and special PTFE packing. Grade
17-4-PH stainless steel is a precipitation
hardened (a chromium-copper
precipitation hardened) stainless
steel, employed for applications requiring
high-strength and a high level
of corrosion resistance.
Example 2. The second example is
presented for a petroleum-refinery
type hydrogen-service centrifugal
compressor. It is a steam-turbinedriven,
hydrogen-recycle centrifugal
compressor for a hydrocracker unit to
compress the hydrogen from around
140 to 190 barg. The calculated Cv for
the surge point at the high-speed curve
(on the compressor map) is 179. The
recommended range for the anti-surge
Cv is 1.7-2.1 times the calculated Cv
for the maximum-speed surge flow.
Applying the 1.7 factor (the minimum
factor) results in an anti-surge valve
Cv above 305.
The selected anti-surge valve is an
8-in. globe valve, 1500# class, RTJ (ring
type joint) facing, with a Cv
of 308,
an opening time of 2 s and a closing
time of 6 s. The anti-surge valve was
selected with a carbon-steel body (a
NACE grade), a stainless-steel (grade
17-4-PH) trim and a special graphite
packing. Since the compressor train
inertia is low (it is a steam-turbine,
direct-driven compressor train), the
anti-surge loop is taken from the immediate
downstream of the compressor
(upstream of the cooler).
Based on experience, for a compressor
configuration like this, a relatively
low Cv (around or lower than 1.7) is
sometimes specified by vendors. Some
vendors used Cv factors (the anti-surge
valve Cv divided by the calculated Cv at
the high-speed surge flow) even as low
as 1.55 or 1.6 to keep the anti-surge
valve size small. This is not recommended,
however, since a sufficiently
large anti-surge valve is necessary to
handle emergency rapid shutdown situations
and unscheduled trips at uncommon
(off-design) operating cases.
In other words, 1.7 is the minimum
acceptable factor. This hydrogen compressor
train is expected to compress
nitrogen during the initial startup
of the unit. The calculated Cv for the
nitrogen operation (the surge flow
at the high-speed curve) is around
260. In other words, a surge could be
expected in case of an unscheduled
rapid trip during the operation with
the nitrogen, if a small anti-surge
valve is selected.
■
Edited by Dorothy Lozowski
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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