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Subject:      Use of the Vibration Chart
Applies to:   Gas and Steam Turbines and Generators

Rotating machinery has many things in common. Turbines and generators are rotating machines and the vibration amplitude can be measured in three different ways:

  • Displacement (inches or mils) measured in peak to peak
  • Velocity (inches per second) measured in peak
  • Acceleration (inches per second per second) measured in g’s, which relates to gravity

In addition to the amplitude, it is imperative to determine the frequency of the vibration signal in order to identify the potential cause(s). Most electric power in the United States is produced by generators running at 3600 rpm (2-pole generators) or 1800 rpm (4-pole generators). In both cases, since generators must run in synchronism, the outgoing power is delivered at 60 cycles per second (abbreviated as cps or Hertz).

Two meters on a typical hand-held vibration analyzer are shown in Figures 1 and 2 below. They include a selectable amplitude meter and a tunable frequency meter. This IRD-320 analyzer is commonly found in power plants and has various amplitude and frequency ranges. Study Fig. 1 below.

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Fig 1: Typical Vibration Analyzer Model IRD-320

Close-up views of the amplitude meter (Fig. 2) and the frequency meter (Fig. 3) for the IRD-320 portable vibration device are shown below.

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Fig. 2: Vibration Amplitude Meter


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Fig. 3: Tunable Vibration Meter by IRD (reading in cycles per minute, cpm)

The chart in Figure 4 and instructions in Figure 5 were published by General Electric. They allow the user to relate vibration frequencies to amplitudes, regardless of how they are measured. This can be a convenient chart for use in converting vibration data. It is suggested that it be posted on the walls of the office in the power plant for easy access and usage.

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Fig. 4: Vibration Chart


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Fig. 5: Explanation of how to use Vibration Chart


For a gas or steam turbine generator operating at 3600 rpm, the primary vibration signal (commonly called 1/rev) would occur at 3600 cpm (analogous to 60 cycles per second, cps). Looking on the chart at the intersection of the 60 cps and 1.0 inches per second, you can read up to the right-side scale and see it is equivalent to .005 inches (peak-to-peak).

Note: Most field engineers commonly refer to this value as 5.0 mils of vibration. This is equivalent to five one-thousandth of an inch, peak to peak (sinusoidal).

Also, from the same intersecting point, it is possible to read down to the right side to the acceleration scale. There you will find the equivalent value in gravity units: 1.0 g of acceleration.

Signal Name

Value
Frequency 60 cps or 3600 cpm
Displacement

(mils or inches, peak-peak)

5.0 mils, p-p

.005 inches, p-p

Velocity,

(inches/sec, ips-peak)

1.0 inches/second, peak
Acceleration

(g, ips/sec)

1.0  g

1.0 inches/sec/sec


A typical, casing-mounted, velocity-type vibration pickup is mounted vertically on a gas turbine compressor casing. See Figure 6 below.

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Fig. 6: Typical Vibration Detector atop a turbine casing (seismic type, velocity output signal in ips-peak)


Most gas and steam turbines manufactured by General Electric have vibration trip (shutdown) limits set at 1.0 inches per second (ips). Thus, if you were reading vibration levels and tuned the frequency meter for 3600 cpm (same as 60 cps), this level would be equivalent to 5.0 mils of vibration or 1.0 g of acceleration.

If you have additional questions, please Contact Pal.