use the vibration chart

7/27/2019 Use the Vibration Chart

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February, 2009

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 thevibration 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 gs, which relates to gravity

In addition to the amplitude, it is imperative to determine thefrequency of the vibration signal in order

to identify the potential cause(s). Most electric power in the United States is produced by generatorsrunning 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 includea selectable amplitude meter and a tunable frequency meter. This IRD-320 analyzer is commonly foundin power plants and has various amplitude andfrequency ranges. Study Fig. 1 below.

Fig 1: Typical Vibration Analyzer Model IRD-320


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

Fig. 2: Vibration Amplitude Meter

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 aconvenient chart for use in converting vibration data. It is suggested that it be posted on the walls of theoffice 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


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For a gas or steam turbine generator operating at 3600 rpm, the primary vibration signal (commonlycalled 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 itis equivalent to .005 inches (peak-to-peak).

Note: Most field engineers commonly refer to this value as 5.0 mils of vibration. This isequivalent 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)

1.0g1.0 inches/sec/sec

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

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 setat 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 ofacceleration.

If you have additional questions, please contact David Lucier atPond and Lucier, LLCby email or

telephone: dlucier@pondlucier,.com or office phone: 518-371-1971.

use the vibration chart

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