Chemical Engineering May 2013 - 61

the shop performance test. For example,
data with an uncertainty of 2%
cannot yield conclusions requiring an
accuracy of 1.5%. Thus, if 1.5% accuracy
is required (for instance, by the
CPI operator) for assessment of a shop
performance test, then the entire shop
test installation and test procedure
should reflect the uncertainty goal.
The polytrophic efficiency (that is,
the efficiency corresponding to theoretically
reversible compression process)
depends on many details, such
as the impeller specific speed, but this
is usually limited to approximately
74-80% for traditional 2D impellers,
and about 80-87% for modern 3D
impellers (usually with vaned diffusers).
The most important performance
checkpoint is a comparison between
the driver power (compressor drivergenerated
power) and driven power
(compressor absorbed power).
If the difference between the driver
power and driven power is small, and
the calculated head and efficiency are
also lower than expected, then the
shop performance data are likely correctly
measured and the compressor
likely has a performance problem.
However, if either the head or the efficiency
is off, in terms of having large
differences compared to the theoretical
or expected values, but the powers
(that is, the driver power and compressor
power) do not agree, this often
indicates inaccurate test data.
Mechanical run tests
A pressurized run test is always preferred
(one that is pressurized usually
with nitrogen), except for very special
cases for which testing under vacuum
conditions may be required. As far
as practically possible, all job components
should be used in a shop test,
and the use of shop facilities should be
minimized. If the use of a job coupling
is not practical, the shop test can be
carried out using a coupling simulator
(to simulate the weight and the bending
effects of the job coupling).
The shop mechanical run-test procedure
is straightforward. The compressor
is started and the speed increases
to the maximum continuous speed.
The operation continues until bearing
metal temperatures, lubrication
oil temperatures and shaft vibrations
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ChemiCal engineering www.Che.Com maY 2013 59
are stabilized. Then the compressor
should be operated for 15 minutes
at the trip speed. After that, the unit
should be operated continuously for
four hours at the maximum operating
speed. The main focus of the shop run
test is to evaluate vibration.
During the shop test of the lubrication
oil system, the oil temperature
rise through the bearing should not
exceed certain limits (as an indication,
oil temperature rise could be
around 30°C). Vibration readings and
bearing temperatures at the end of
the four-hour run should essentially
be the same as those recorded at the
beginning of the test. After the test is
complete, all bearings should be re
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