Ifyou're hearing a background beat in your facility but there's no band in sight, you've likely got trouble with your machinery. If you ignore it, the problem may get even bigger, so it's important that you take steps to beat this 'beating' problem.
If two vibrations of a similar frequency are superimposed in operating machines, beating occurs. This phenomenon is clearly audible in acoustic measurements. The beating tone, which is made up of two individual tones of slightly different frequencies, changes its volume at the beat frequency, rhythmically becoming louder and softer.
Due to the high amplitudes that this can cause, beating generated by two neighbouring machines can lead to enormous vibration damage and even machine failure. Because the cause of beating cannot always be eliminated, it is important to keep its damaging effects as small as possible.
The principle of beating and the beat frequency are illustrated in Fig. 6 and can be demonstrated in a simulation. Beating can also be detected in the display of FFT vibration analyzers. The recorded time signals of the vibration velocity and other measured quantities must be of sufficient length. If this type of analyzer is not available, you should become sceptical if amplitudes fluctuate wildly.
This was the case at a pumping station. The operator detected conditions with strongly fluctuating machine and building vibrations. The source was thought to be the pump and an inadequate building foundation.
In the control room, the sense was that the vibration intensity was linked to how the pump was operated and that the vibrations only occurred during high pump output. But why?
As a result, systematic vibration analyses were contracted with the Prüftechnik Machinery Service. Its basic measurements revealed a number of unfavourable conditions that led to pronounced building vibrations.
The pumping station concept
In this pumping station, the pressure increase is achieved in three consecutive centrifugal pumps that run at different speeds. Pump 1 is the slowest, Pump 2 runs somewhat faster, and Pump 3 is the fastest. On the measurement day, Pumps 2 and 3 had a relatively high vibration level when running at a high delivery rate. It dominated the first order in the vibration velocity.
Measurements of the building revealed additional beating in the vibration velocity. Beating changed as the pump speed increased. The operating mode of the pumps also had an influence on the beating. It could be shown that the beating was influenced by the pressure ratio of the pumps to each other.
When only Pumps 1 and 3 were operated together, the beating was reduced (see Fig. 4). However, this was not the solution the operator was looking for. On the contrary, increasing the delivery rate further was the order of the day. Therefore, avenues were sought to actively reduce vibration.
Imbalance
The vibration analyses showed that relatively strong rotational vibrations arose as the motor speed increased (1st order). The balance condition was checked. In the process, it was noticed that keys in the couplings protruded or were missing altogether. One of the first measures was to correct this situation. But this was not the only issue.
Critical speed
The motor type in use was not free of natural frequencies in the operating speed range for which it was designed. At about 3600 rpm, the motor even passed through a bending critical speed that led to a marked increase in amplitude peaks on the motor. Phase jumps were also found in the coast-down curves.
Precision balancing
A precision balancing procedure was performed. This reduced the vibration amplitudes and beating was no longer noticeable in the building. Ultimately, beating in the area of 6 mm/s remained when operating at critical speed, which is why this speed could not be cleared for continuous operation.
Extensive reconstruction or the use of a motor without natural frequencies was a recommendation that was made. It was also recommended that the new pump motors meet the requirements of EN ISO 13709.
Dr. Edwin Becker is the head of the diagnostic department at Prüftechnik in Ismaning, Germany. Florian Buder is the sales and marketing manager at Prüftechnik Canada, Montreal, QC. For more information, visit www.pruftechnik.ca.
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