Page 480 - Compression Machinery for Oil and Gas

460 SECTION III Applications

changing the calculated power and capacity. Refer to Chapters 5 and 11 for a
more detailed discussion on gas pulsation.

Screw Compressors

Performance testing is typically carried out on screw compressors at the factory
for new machines. ISO 1217 [7] is used as a guide but only to a limited extent.
ISO1217 [7] defines allowable deviations in speed, pressure, pressure ratio, and
gas properties (gas constant and isentropic exponent) between the job and the
test conditions. If the deviations are not larger than a few percent, ISO1217 [7]
gives a calculation method for recalculating the test results to design conditions.
Power shall be measured using reaction mounted driver, torque meter, indi-
rectly determined from electrical measurements, or from a certified driving
prime mover. Power consumption, volumetric flow rate, and other parameters
computed from the test are corrected and then compared with specified values.
For process compressors allowable deviations are usually not fulfilled and
therefore the calculation per ISO1217 [7] cannot be used. Manufacturers have
developed their own recalculation methods.
Mechanical run tests are also usually carried out at the factory, at the same
time as the performance test or immediately before or after to the extent pos-
sible. Screw compressors are normally tested to meet the intent of API 619
[8] and ISO 10440 [9], but practical limitations sometimes make it difficult
to meet all requirements. One limitation is that mechanical testing is typically
done with air and for screw compressor designed for light or heavy gases may
not be able to run at their maximum speed, differential pressure, or horsepower
without a more appropriate fluid.
A performance test on site is sometimes requested by customers, but rarely
is this test performed once the compressor is up and running. When required, the
purpose of a field performance test is to: (a) prove that the results/calculation of
the factory test (with ambient air) hold true for the field gas and conditions, and
(b) show that the complete package delivers the required performance. Any
deviations between the factory and field test must be accounted for. For exam-
ple, if the pressure drop across a silencer is too high, then the power consump-
tion might also be too high, but this is part of the package performance and not
necessarily the compressor itself. In addition, a correct performance test
requires certain preparations such as required pipe lengths and precisely defined
orifices for flow measurements, an online gas analyzer to determine gas quality,
etc. It must be emphasized that the exact conditions and acceptance criteria for
such a test must be agreed upon in advance, and the challenge is usually for the
customer to replicate the guaranteed case. It is normal for the field service per-
sonnel to check for high vibration over the entire package after start-up and take
reference vibration measurements, but dynamic testing is not typically
carried out.