Instruments For Vibration Analysis

One fact about end-suction and between-bearing pumps is that external visual evidence of mechanical problems is very limited. Only three gauges for mechanical trouble exist: temperature, vibration, and sound.

FIGURE 20 Complex vibration signal resolves into sine wave spectrum

It is normal for a machine to vibrate at some level; such vibrations are caused by manufacturing defects, design limits of the pump, casting irregularities, less than optimum application, and a maintenance/installation problem. When the velocity vibration level starts to increase 0.1 in/s (2.5 mm/s) zero to peak (0-P) above the "as new installed level," the vibration should be analyzed to determine the possible sources of the mechanical and/or hydraulic problem. Several mechanical and/or hydraulic problems may be producing, for instance, the 1X running speed frequency vibration. The key in using vibration to define the mechanical and/or hydraulic problems is to determine the frequency at which the vibration occurs. Vibration amplitude is also an important factor because it indicates the severity of the vibration. Field vibration data are normally a complex vibration waveform. By using a tunable analyzer, the complex vibration signal, as shown in Figure 20, can be filtered or tuned into its basic frequency components; that is, all complex signals are summations of the harmonics and subharmonics 1X, 0.5X, 6X, 30X, and so on. By comparing these filtered components of the complexed vibration signal with an analysis chart and some common-sense experience, probable causes of the vibration can be listed.

The first step toward resolving the vibration problem is to convert the mechanical movement to an equivalent electrical signal so it can be filtered and measured.

Often used analyzer systems are

1. A turbine ac-battery-powered analyzer with strobe light, providing amplitude, frequency, and phase. A plotter accessory can also be attached for copies of the data.

2. A small battery-powered, internally driven, tunable analyzer with a built-in plotter using an accelerometer or velocity sensor.

3. A spectrum analyzer, ac powered, that receives the signal directly from a vibration transducer or the recorded signal from a battery-powered four-channel FM/AM cassette tape recorder.

For startup or where the problem is tougher, one can add

1. Eight-channel FM tape recorder

2. Four-channel oscilloscope with blanking and time display

3. Tracking filter displaying revolutions per minute, amplitude, and phase, capable of tracking runup/rundown data

FIGURE 21 Limitations on machinery vibration analysis systems and transducers (mils X 0.0254 = mm; in/s ; 25.4 = mm/s) (Reference 10)

Provisions should be made for the use of all types of sensors, as there are advantages in each. As more complex problems continue to appear, tunable analyzers with a sensor are not just a requirement but a necessity in any maintenance reliability program. The choice of a displacement sensor (eddy current probe), velocity or seismic sensor, or an accelerometer depends on the frequency range to be analyzed and the type of pumping equipment. There is no one vibration sensor for all jobs.

Of the three types of vibration measurements, acceleration and displacement are dependent on frequency and velocity is independent of frequency. Most engineers and technicians select a measurement that is independent of frequency for a datum to judge the general health of new and used pumps. With the exception of low-speed pumps and motors, 1750 rpm or less, unfiltered velocity and filtered velocity are used for most basic data. Figure 21 shows the frequency relationships (log) versus output (log) of three different measurement sensors with reference to a constant velocity of 0.3 in/s (7.6 mm/s). The figure gives an overview of present sensor limits and shows that each sensor is like a window through which portions of the frequency spectrum may be observed. The figure also shows that the accelerometer is the choice sensor at high frequency because it measures the square of the frequency. The advantage of displacement at low frequencies is due to its high output; the disadvantage of displacement at high frequencies is that the output signal will disappear into the background noise of most measuring systems.

One should not confuse the measurement parameters (displacement, velocity, and acceleration) with the sensors (eddy current probes for displacement, velocity sensors, and accelerometers). The basic relationship of these measurement parameters with commonly used units are shown on a simple sine wave in Figure 22.

Although the velocity sensor is not necessarily the best all-around type of sensor, it does have the advantage of high self-generating output (up to 1000 ft [300 m] of cable), can be mounted in any position, and is influenced only slightly (less than 5%) by transverse sensitivity (side forces). The disadvantages are that the output signal below 600 cpm is significantly nonlinear but can be corrected, the accuracy is limited at ±8% to 1000 Hz, and

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