Vibration Detectors Transducers and Cables

A variety of monitoring, trending, and analysis techniques are available that can and should be used as part of a total plant vibration monitoring program. Initially, such a program depends on the use of historical trends to detect incipient problems. As the program matures, however, other techniques such as frequency-domain signature analysis, time-domain analysis, and operating dynamics analysis are typically added.

An analysis is only as good as the data used, therefore, the equipment used to collect the data are critical and determine the success or failure of a predictive maintenance or reliability improvement program. The accuracy and proper use and mounting of equipment determines whether or not valid data are collected.

Specifically, three basic types of vibration transducers can be used for monitoring the mechanical condition of plant machinery: displacement probes, velocity transducers,

and accelerometers. Each has limitations and specific applications for which its use is appropriate.

Displacement Probes

Displacement, or eddy-current, probes are designed to measure the actual movement, or displacement, of a machine's shaft relative to the probe. Data are normally recorded as peak-to-peak in mils, or thousandths of an inch. This value represents the maximum deflection or displacement from the true centerline of a machine's shaft. Such a device must be rigidly mounted to a stationary structure to obtain accurate, repeatable data. Figure 8.1 shows an illustration of a displacement probe and signal conditioning system.

Permanently mounted displacement probes provide the most accurate data on machines having a rotor weight that is low relative to the casing and support structure. Turbines, large compressors, and other types of plant equipment should have displacement transducers permanently mounted at key measurement locations.

The useful frequency range for displacement probes is from 10 to 1000 Hz, or 600 to 60,000 rpm. Frequency components above or below this range are distorted and, therefore, unreliable for determining machine condition.

Figure 8.2 Schematic diagram of velocity pickup: (1) pickup case, (2) wire out, (3) damper, (4) mass, (5) spring, (6) magnet.

Figure 8.2 Schematic diagram of velocity pickup: (1) pickup case, (2) wire out, (3) damper, (4) mass, (5) spring, (6) magnet.

The major limitation with displacement or proximity probes is cost. The typical cost for installing a single probe, including a power supply, signal conditioning, etc., averages $1000. If each machine to be evaluated requires 10 measurements, the cost per machine is about $10,000. Using displacement transducers for all plant machinery dramatically increases the initial cost of the program. Therefore, key locations are generally instrumented first and other measurement points are added later.

Velocity Transducers

Velocity transducers are electromechanical sensors designed to monitor casing, or relative, vibration. Unlike displacement probes, velocity transducers measure the rate of displacement rather than the distance of movement. Velocity is normally expressed in terms of inches per second (in./sec) peak, which is perhaps the best method of expressing the energy caused by machine vibration. Figure 8.2 is a schematic diagram of a velocity measurement device.

Like displacement probes, velocity transducers have an effective frequency range of about 10 to 1000 Hz. They should not be used to monitor frequencies above or below this range.

The major limitation of velocity transducers is their sensitivity to mechanical and thermal damage. Normal use can cause a loss of calibration and, therefore, a strict recalibration program is required to prevent data errors. At a minimum, velocity transducers should be recalibrated every 6 months. Even with periodic recalibration, however, velocity transducers are prone to provide distorted data due to loss of calibration.

Figure 8.3 Schematic diagram of accelerometer: (1) base, (2) piezoelectric crystals, (3) mass, (4) case.

Figure 8.3 Schematic diagram of accelerometer: (1) base, (2) piezoelectric crystals, (3) mass, (4) case.

Accelerometers

Acceleration is perhaps the best method of determining the force resulting from machine vibration. Accelerometers use piezoelectric crystals or films to convert mechanical energy into electrical signals and Figure 8.3 is a schematic of such a device. Data acquired with this type of transducer are relative acceleration expressed in terms of the gravitational constant, g, in inches/second/second.

The effective range of general-purpose accelerometers is from about 1 to 10,000 Hz. Ultrasonic accelerometers are available for frequencies up to 1 MHz. In general, vibration data above 1000 Hz (or 60,000 cpm) should be taken and analyzed in acceleration or g's.

A benefit of the use of accelerometers is that they do not require a calibration program to ensure accuracy. However, they are susceptible to thermal damage. If sufficient heat radiates into the piezoelectric crystal, it can be damaged or destroyed. However,

Figure 8.4 Types of coiled cables.

thermal damage is rare because data acquisition time is relatively short (i.e., less than 30 sec) using temporary mounting techniques.

Cables

Most portable vibration data collectors use a coiled cable to connect to the transducer (see Figure 8.4). The cable, much like a telephone cord, provides a relatively compact length when relaxed, but will extend to reach distant measurement points. For general use, this type of cable is acceptable, but it cannot be used for all applications.

The coiled cable is not acceptable for low-speed (i.e., less than 300 rpm) applications or where there is a strong electromagnetic field. Because of its natural tendency to return to its relaxed length, the coiled cable generates a low-level frequency that corresponds to the oscillation rate of the cable. In low-speed applications, this oscillation frequency can mask real vibration that is generated by the machine.

A strong electromagnetic field, such as that generated by large mill motors, accelerates cable oscillation. In these instances, the vibration generated by the cable will mask real machine vibration.

In applications where the coiled cable distorts or interferes with the accuracy of acquired data, a shielded coaxial cable should be used. Although these noncoiled cables can be more difficult to use in conjunction with a portable analyzer, they are absolutely essential for low-speed and electromagnetic field applications.

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