Page 438 - Compression Machinery for Oil and Gas

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the supplier upfront before they begin their detailed impeller design. Perfor-
mance testing is especially important where there are two process services
on the same train since any speed change to improve the performance issue
of one compressor service will also impact the performance of the other com-
pressor service. Performance testing is also very important when there is a con-
stant speed driver, since the compressor speed cannot be increased or decreased
to make up for performance problems. This performance testing will be
conducted typically using only one contract rotor.
For some large LNG compressors, in-shop performance testing is not pos-
sible, due to the size of available shop drivers. In this situation, performance
testing in a string test configuration is employed. A disadvantage of this
arrangement is that any performance problems discovered during the string test,
may cause schedule problems with the compressor and the string test stand since
the compressor may need to be sent back to the shop for impeller or other mod-
ifications. Performance testing in the string test configuration is also conducted
in accordance with ASME PTC 10-1997 [10]. There are two methods for deter-
mining the efficiency of the compressor. A torquemeter may be used to measure
the input into the compressor. The second and most common method is the heat
balance method where the work input is measured via the enthalpy rise from
inlet to discharge.

String Tests
String tests are specified when there is concern about the overall performance in
terms of the turbomachinery train under consideration. The test if conducted
under full load, full speed conditions can replicate actual operating conditions.
As the job driver and control system are used, many of the field start-up prob-
lems can be minimized. The string test verifies that the entire job components
are correctly designed and manufactured. The string test can be expensive and
time-consuming test, adding several weeks to the schedule.
This testing would include any applicable compressors, gas turbines, gear
units, gas seals and associated buffer gas systems, lube oil systems, and
helper/starter motors. For large helper/starter motors and variable frequency
drives (VFDs), where two identical motors are purchased, consideration should
be given to a back-to-back motor test at the motor supplier’s facility. Once
again, any problems discovered during the back-to-back test can be solved more
efficiently and timely at the supplier’s factory.
The primary advantage of the string test is the ability to perform a full load
mechanical running test and measure equipment vibration and bearing temper-
atures at full load and full speed. Machinery vibrations can be investigated in
both the steady-state and transient conditions. Equipment train alignment capa-
bility can also be established. Besides the equipment, many auxiliaries are used
during the string test including inlet filters/ducting, lube systems, couplings,
coupling guards, control systems, electrical motor systems, exhaust systems,