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Performance and mechanical running tests of centrifugal compressors – III
HOW THE TESTS ARE DONE
A radially split type compressor is set up for shop tests. Various spectral plots are generated during mechanical running test of the compressor. The compressor is installed on baseplate after test.
A graphical representation of the vectorial subtraction of vibration levels between mechanical run test and rotor unbalance response test, at each speed, is used to verify if the compressor rotor's response to known unbalance at all speeds was within the predicted levels at minimum/maximum radial bearing clearances. This Amplitude versus Speed Chart includes vibration level lines for:
1. Unbalance weight Influence (unbalance response - Mechanical Run Test vectorial subtraction)
2. Mechanical Run Test - Steady State conditions
3. Unbalance Response
4. Predicted - maximum radial bearing clearance and Predicted - minimum radial bearing clearance.
HOW TESTING AFFECTS PLANT REVENUE
More often, compressor’s flow and discharge pressure are directly related to production level and in turn, to the revenues of process plants. In many cases, compressors also influence performance guarantees of new process facilities. While shop testing is expensive, it is justified because any performance deficiencies noticed at site after start-up can cost a lot more money and time to rectify.
CODE PROCEDURES
Most users specify ASME PTC-10 power test Code based performance test for their purchased compressor(s).
The Code procedures are used as the basis to predict compressor performance. It is usually either not possible or is impractical to test compressors with specified or design gas. Therefore, suitable test gases are used for performance testing. These gases are either single gases or gas mixtures. The three most significant criteria used in the selection of test gas are Volume ratio, Mach number, and Reynolds number.
Mechanical running tests serve the following objectives:
1. Confirm that transient and steady-state rotor vibrations are within specified limits and check that potential troublesome or discrete frequencies are not present in the vibration plots.
2. Demonstrate mechanical integrity of the compressor.
3. Verify rotor-bearing system stability and response to unbalance.
4. Check damped critical speed(s) and compare with calculated values shown in lateral critical speed analysis.
5. Assess quality of compressor assembly; in specific, cleanliness of lubrication passages, the internal clearances, unit alignment, and freedom from leakage across casing joints and shaft end labyrinths.
6. Coupling hub fit-up and contact with shaft.
7. System response to variations in lube oil supply pressure and temperature.
8. Provide baseline readings for bearing metal temperatures, bearing oil return temperature, bearing oil flow rates, shaft seal primary vent flow (assuming that dry gas seals are installed), balance piston flow, axial position of rotor, and unit noise level.
