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Mechanical run test of a compressor is a vibration demonstration test. Mechanical testing provides information related to the critical speed location and some indication of that modes behavior. The modal information is limited to only those modes located below the maximum test speed achieved (trip speed in most cases.) Typically, this is only the 1st critical speed.
What isn’t tested however can be significant. For example, subcritical motor applications (incorrectly termed stiff shaft) operate below the 1st critical. This critical speed can have high amplification factors and can be damaging if the separation margin is lost. Performing only a mechanical test will tell the user whether the mode is on or below the operating speed. The amount of separation remains untested and can only be inferred from the unverified analytical predictions.
This article contains excerpts from the paper, “Shop rotordynamic testing: options, objectives, benefits and practices” by John A. Kocur, Jr. of ExxonMobil Research & Engineering and C Hunter Cloud of BRG Machinery Consulting presented at the 2016 Asia Turbomachinery & Pump Symposia.
Mechanical run test of the rotating equipment should be viewed as a minimal test to determine the rotordynamic acceptability and should be considered for equipment that is designed-for-purpose in contrast to equipment selected from a catalog. For most types of equipment, API standards include specific procedures to follow during a mechanical test.
Important test factors related to the rotordynamic behavior of the equipment are: • Test speeds/duration • Lube oil parameters (temperature, flow rate and viscosity) • Rotor/support configuration
Operation of the compressor during the mechanical test should include a warm up portion where the rotor speed is incremented in 10 minute intervals carefully avoiding exclusions zones of critical speeds and blade natural frequencies. Following the warm up, operation at trip speed is specified for 15 minutes followed by a non-interrupted 4 hour run at maximum continuous speed. A coastdown from trip follows the 4 hour run. The warm up portion is included to ensure that the case and rotor are given time to thermally expand gradually to avoid creating unintended interferences between the two leading to rub damage. The warm up portion also permits examination of the rotor behavior at increasing speed intervals. Thus, faults can be detected at less energetic stages potentially avoiding rotor/stator damage and project delays.
Trip speed is included to ensure that vibration levels (and overall operation) are acceptable at this speed. The 4 hour run portion is used to set the thermal conditions of the system. Bearing temperatures and vibration levels are important factors to watch during the test. Stable levels of each parameter need to be reached during the test. If any parameter shows signs of continual movement (increase or decrease), the test should be extended until stable levels are achieved. If not, the test should be rejected. Following the 4 hour run, a coastdown from trip speed is performed. The coastdown is used to determine the overall and synchronous behavior of the rotor/support system. This data will also be used as the baseline for verification testing, if performed.
Operation at trip speed during the mechanical run test is important for several reasons. First, running to trip speed increases centrifugal forces on the rotor which may relax interference fits and permit the rotor’s static shape to change. This may alter the balance state of the rotor. Second, reaching trip speed is necessary for trip testing in the field. It is convenient to reach these speeds during testing to identify any potential problems. Finally, trip speed operation can help identify any critical speeds occurring just above maximum continuous speed that would otherwise not be seen on the test stand.
Additional shutdown/startup transient operation can be added at the beginning of the 4 hour test. When compared against the coastdown following the 4 hour test, thermal transient behavior of the rotor can be examined and any changes to the balance state of the rotor can be identified. This may prove useful in diagnosing Morton’s Effect, clearance closure of the radial bearings or fit relaxation due to operation at trip speed.
Lube oil parameters should mimic that used for the intended application. Bearing flow, lube oil inlet temperature and viscosity should be within the specified operating ranges set by the vendor for field operation. After stabilization during the 4 hour run, lube oil inlet temperature can be varied from the minimum to maximum specified range to examine the effect on vibration levels and bearing operation.
Operation at the range limits should be held until steady state conditions are achieved. Rotor/Support Configuration The rotor configuration should be as close to the operating condition as possible. The major concern centers around the overhung weight associated with the coupling. Often mechanical testing is done at partial rated power levels. Smaller shop drivers need not have, nor in some cases could accommodate, the larger couplings of the job. It is essential to closely mimic the overhung weight of the job ½ coupling. This may require that a simulator be added to the drive assembly to match the overhung moment. The opposite may be true for vendor’s smaller casings where the test coupling’s overhung moment is larger than the job coupling.
Rotordynamic predictions should be used to determine the impact of using the heavier overhung weights on the 2nd and 3rd critical speeds. Note: For the cases where rotordynamic predictions are used to study the impact of configuration changes, it is strongly recommended that verification testing be performed. The increased reliance on the rotordynamic model to predict behavior that may not be fully tested necessitates that some verification of the rotordynamic prediction be undertaken.