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Liquid Air-Separation Compressor Testing Serious compressor problems often appear during startup and shutdown. During this time, dynamic stresses from acceleration and deceleration inertia typically run higher on certain components than when the compressors operate at constant speed, and the stresses may aggravate a defect. Also, some anomalies may be more observable. This is not to say that problems do not arise during steady-state operating conditions; they do, but they may be less obvious during the onset of a failure. Centrifugal compressors that separate liquid air belong to the dynamic compressor family and operate at extremely high speeds. They contain a large bull gear that drives high-speed pinions, which develop the pressure. A prime mover, such as an electric motor, connects its drive shaft to the compressor unit through a coupling to spin the bull gear. During the start up or shut down, events take place very quickly and it requires a system with real-time recording capability to detect and identify problems that may arise. For example, Nelson Baxter, a consultant with AZIMA Corp., Woburn, Mass., was recently requested to measure the startup operation of a four-stage, 5.0 k-hp liquid-air separation compressor. The purpose of the test was to evaluate the unit’s condition and detect any root problems that previously might have caused an excessive vibration transient in the first stage that resulted in a subsequent trip event. Baxter initially recorded some sub-synchronous instability, but for five succeeding days, no unusual vibration transients appeared. The data recorded during the test came from IOtech’s 650u Dynamic Signal Analyzer (DSA). The client’s original data acquisition system is acceptable for recording overall data, but it has no time or spectra data collection capabilities. In contrast, the 650u DSA collects data from each probe continuously, and then streams it to a computer’s hard drive for post-processing analysis. The DSA was set to collect 5120 samples/sec during the startup phase. It recorded each revolution of all four stages from startup through the first seven minutes of operation. Following the startup it was then reconfigured to obtain waveform and spectra data every minute or during a level increase thereafter. Although the system continued to operate without an automatic shutdown from excessive vibration transients, Baxter closely analyzed the recorded data. During the first ten seconds of startup, he found that all four stages of the compressor experienced periods of extremely high vibration. The bull gear generated tangential forces that substantially shifted the centerlines of the pinion shafts. He also found an unexpected amount of high-amplitude, dynamic sinusoidal motion during two separate periods; the first immediately upon switch closure, and the second near the synchronous point.These events explain why the compressor shows such high levels of motion during startup. Data analysis indicates that, most likely, the two peaks in the dynamic amplitude come from the bull gear’s torsional response. The torque pulse generates the first peak during initial motor startup. The second peak appears to come from the synchronization pulse rate, around 80% of full speed, which indicates a suspected natural, torsional frequency. This is not particularly surprising, since synchronous motors are commonly known to generate torsional vibration during startup. After the motor reaches synchronous speed, the amplitudes level off and then remain relatively constant. After the unit stabilizes, stage 3 generates the highest vibration level. Time-based wave shapes were taken from stage 3 when operating at a relatively low speed. They show 1.5-mil amplitude notches every revolution. Notches such as these that appear at low speeds usually indicate a scratched shaft, the most probable reason that the stage 3 proximity probe shows abnormally high values. Conclusion The steady-state data were sampled each minute with the provision that any increase in vibration would also trigger the data-capture feature. The trigger did initiate a few data sets, but none appeared to be high enough to trigger a trip. Unfortunately, the source of the original trip remains unknown. A random event might have caused the sub-synchronous vibration to spike, which is always present at some level on the first stage, or the original instrumentation had an intrinsic transient problem.
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[ IOtech PC-Based Data Acquisition (DAQ) and Instrumentation - PDF Format IOtech PC-based products (USB, PCI, Ethernet, GPIB) and signal conditioning options capture waveforms and measure most physical parameters including temperature, vibration, strain, velocity, acceleration, position, as well as common voltage, current, power, and data logging. IOtech's solutions are used in environments such as |