Page 476 - Compression Machinery for Oil and Gas

456 SECTION III Applications

blank-off flanges. Typical gases used in the test include air, nitrogen, carbon
dioxide, and some noble gases like argon and helium.
During a mechanical run test, the compressor is brought up to maximum
continuous speed until operating conditions are settled. API 617 [4] requires
that the compressor shall be run continuously for at least 4h during the test.
Vibrations and lube oil parameters are closely measured, observed, and
recorded at various data points along different operating speeds.
A performance test is conducted to verify the aerodynamic performance of
the compressor. It is usually performed in a closed loop, using nitrogen, CO 2 ,
methane, or other gases. ASME PTC 10 [1] or ISO 5389 [5] define the require-
ments for this type of tests. Depending on the specific requirements, a test can
also be performed in an open cycle, using ambient air.
Performance testing conducted in accordance with ASME PTC-10 [1] usu-
ally requires a closed loop. A type 1 test is the most straightforward way of test-
ing. The test gas is similar to specified gas used for the compressor design. In a
type 2 test, inert gases like nitrogen, carbon dioxide, and nitrogen-helium mix-
tures, and different types of Freon can be used. The test conditions have to be
defined such that aerodynamic similarity between test conditions and specified
field conditions is achieved. Specifically, the flow coefficient at the design point
is maintained within 4%, the ratio between volumetric flow at the inlet and the
discharge of the compressor within 5%, while maintaining Mach and Reyn-
olds numbers within specified limits. Such parameters are defined by ASME
PTC 10 [1] as follows:
_ m rotor
φ ¼
3
D
ρ 2πN
i
12
v i
r v ¼
v d
U 2
Ma m ¼
a i
U 2 b
Re m ¼
ν
Compressors are operated at the required conditions for a period of time long
enough to demonstrate that conditions have settled. Usually five points are mea-
sured to complete performance curves (if required). Each point along the curve
needs to be checked for equivalency. Specified operating conditions are calcu-
lated (flow rates, discharge conditions, polytropic head, power, etc.) based on
computed test performance parameters after the application of machine Reyn-
olds number correction factors. To determine the compressor stability limit, the
flow is reduced at constant speed until detecting the instability, either by observ-
ing pressure oscillations or hearing the “surge noise” as well as monitoring the