Page 437 - Compression Machinery for Oil and Gas

416 SECTION III Applications

or coupling guard temperature calculations appear high. API 671 [9] recom-
mends maximum coupling guard temperatures of 71°C.
This is a test that has to be done on all compressor bodies regardless of them
being identical as the rotordynamic/vibration behavior is a function of the close
clearances, and assembly procedures including balancing.

Performance Tests
The PTC-type tests can be of several classes, but the commonly applied tests are
similitude tests [10] (Type 2) where the compressor is driven by a shop driver
(typically a steam turbine or a variable speed motor) and then aerodynamic
similitude is derived between the test conditions and the site conditions. Details
on shop testing are provided in Colby [11]. This is a test always specified
(unless the compressor is identical to a pretested design). It is typically a type
test conducted on one compressor of a specific service used in a project.
For LNG compressors, the test will be a Type 2 test, on a substitute gas. For
performance testing a multistage compressor, volume reduction is a critical
parameter. The test volume reduction must match the design volume reduction
to be representative of the compressor performance on the specified gas. The
head coefficient and efficiency of an impeller vary with the machine Mach
number. Therefore, performance testing is required to be conducted at a Mach
number relatively close to the design Mach number for each stage. The Reyn-
olds number relates to the boundary layer and frictional losses of the medium in
the flow path. Type 2 in-shop performance tests are almost always conducted at
a lower Reynolds number than under specified conditions. The lower the Reyn-
olds number the greater the frictional losses and the lower the efficiency. Most
users do not allow the manufacturer to correct the test data for the differences in
Reynolds number. Minimum straight lengths of inlet pipe are required for an
accurate performance test, due to the establishing laminar flow into the impeller
and depending on the type of pressure, temperature, and flow devices. Equiv-
alent arrangements can be studied by the compressor manufacturer and reduce
the straight lengths. A minimum of five points, including surge and overload,
shall be taken at normal speed. For variable-speed machines, additional points
may be specified for alternate process operating conditions and two alternate
speeds to define the slope of the surge line and help the supplier in determining
where, in which stage, surge initiated. If the three surge points are very close in
flow coefficient, surge is initiated early in the compressor, in the first or second
stage. A greater separation between the three surge points would indicate surge
to be initiated in one of the later stages.
Compressors with intermediate specified process pressures (i.e., side loads)
should have individual sectional head (pressure) tolerances as mutually agreed.
Many compressor suppliers have a standard tolerance of 2% on side-load head
(pressure), so if the owner’s process would prefer that a side-load pressure (tem-
perature) be lower or higher than design, this direction should be furnished to