VIBRATION MEASUREMENT

What is vibration ?

Everyone has Basic knowledge of vibration, when it is to be measured accurately and used as an indicator of a machinery mechanical condition, it should be explicitly understood. Technically vibration is defined as the oscillation of an object about its position of rest. It can best be explained by simple vibrating system shown in figure given below

If the mass is set in the motion, it will move back and forth some upper and lower limits. This movement of mass through all its positions and back to the point where it is ready to repeat the motion is defined as one cycle of vibration. The time it takes to complete this cycle is the period of the vibration. The no. of these cycles in a given length of time ( e. g. minute ) is the frequency of vibration. Frequency is usually stated in cycles per min or cycles per seconds or “Hertz”.

Generally found cause of vibration

Vibration is always present as a result of one or more exciting forces, otherwise there will not be vibrations. In rotating equipment the exciting forces could arise due to followings:

  • Unbalance of rotating part
  • Looseness of parts
  • Defects in alignments
  • Bending of shafts or wrong assemblies
  • Defects in bearings, gear or belts 􀂾 Air turbulence etc

These are all defects in the rotating equipment and each effect give rise to a different type of vibration. The overall vibration level is a result of all vibrations due to all causes. These are generally found most probable type causes which cause vibration in rotating machinery. Such type of vibration becomes more alarming if the machine is rotating at a very high speed.

Advantage of monitoring vibration

The first fundamental thing is that, should a given machine be monitored at all ? There are several objectives for vibration monitoring.

→ The physical safety of plant personnel and to avoid the potential damage to the machine and the equipment surrounding the subject machine.

→ To avoid the expense of making mechanical repairs in terms of both material ( replacement parts ) and time ( labor ).

→ To avoid expense of downtime or outrage due to a machinery failure.

Plant safety:

The operation of any rotating machine involves high forces and energy levels. When this energy operates in a controlled manner, then the machine performs its intended function in the overall process. However when these forces operate out of control, to the extent of a machinery failure then the severe damage will be realised. The most safety consideration is that the release of energy from a machine destruction can endanger the life of the plant personnel. This is generally a serious consideration on all large and /or high speed rotating machinery, but is especially so for machines in a dangerous plant process.

Two types of monitoring is usually done.

→ Intermittent as done by Inspection department.

→ Continuous monitoring using Bentley Nevada monitoring system.

Maintenance Costs:

Machine protection system can significantly reduce over all maintenance costs. Since monitoring systems can reduce the machine damage due to a malfunction, then the requirements for potential replacement parts can also be minimised. The inventory of replacement parts for a properly monitored machine need not be as expensive as for an unmonitored machine. Reduction in maintenance costs can be realised through the savings of maintenance personnel’s time. Monitoring system can indicate the part of machine in trouble. So that when repairs are necessary, the problem can be isolated quickly. Continuous monitoring also relive plant personnel from the task of measuring machinery vibration parameters by hand ( portable instruments ) and can make them more available for other job functions.

Plant availability:

The ultimate objective of any monitoring system is to improve plant normal running time. Even the most sophisticated and expensive monitoring system can be paid for in most plants, by saving just a few hours of plant process downtime. By evaluating and trending the data provided by the monitoring system, a given machine train can be kept on line as long as readings are satisfactory instead of bringing the machine down on a periodic inspection basis whether it needs or not. Also, most monitor system provide two levels of alarm, most machine malfunctions can be detected as they first appear, allowing operations personnel and plant management the time to adjust the process conditions for an orderly shutdown rather than loss in line product as a result of unexpected shutdown. Some time it is even possible to minimise the effect of a growing malfunction by reducing load, speed etc. and thus keep a machine on-line for a longer period before shutdown, if production requirement so dictates.

Instruments for vibration measurements

The basic transducer or pick-ups for the vibration measurement are of the three type described below.

i) Velocity pick-up

ii) Accelerometer

iii) Non-contact Pick-up

Velocity Pick-up

The design of a typical velocity pick-up involves a spring-loaded suspended coil surrounded by a permanent magnet which is attached to the velocity pick-up case. When the pick-up case is attached to a vibrating machine , the magnet vibrates back and forth past the coil, which remains stationary. The magnetic flux being cut by the coil creates a voltage in the coil proportional to the velocity of vibration. A pick-up schematic is shown below as Figure.

The parts of the pickup are Pick-up case, Wire coil, Dampner, Mass, Spring, Magnet. The typical frequency of a velocity pick-up is 15 htz to 2000 htz / 600 to 60,000 rpm. Measurements outside this range usually require application of correction factor. Because of its high output and other design characteristics it is relatively insensitive to minor cable problems and is unaffected by changes in cable length or long cable lengths ( e. g. upto 750 ft ) . it gives reliable results when hand held for quick periodic checks and is capable of operating with only minimal attenuation. The velocity pickup is designed to measure vibration velocity but when used with an instrument having integration capability, it can also be used to measure vibration displacements. For a very low frequency measurements on the order of 150 rpm, special velocity pick-up may be used. Such a pick-up has a prod attached directly to the pick-up coil. In this case the pick-up case is held stationery and the prod is attached to the vibrating surface. This pick-up is also useful for measuring the vibration of lightweight structures whose vibration response would be significantly affected by the weight of a conventional pick-up.

Accelerometer

It consists of piezoelectric crystals which are sandwiched between the accelerometer case and a small mass. The piezoelectric crystal posses the property of that causes them to generate a charge which is proportional to the amount by which they are compressed. When the accelerometer is attached to a vibrating machine, the vibration cause the small mass to compress the piezoelectric crystal in each cycle. This compression causes a voltage output by the crystals proportional to the vibration acceleration. Figure  below shows a schematic diagram of an accelerometer.

Accelerometer have a frequency range which is typically 120 to 60,000 cpm ( 2 – 10 khtz ). Special units can go to both higher and lower frequency. In addition to their wide frequency range, their small size, light weight, wide amplitude range, resistance to shock and ability to withstand high temperature make accelerometer extremely versatile. Characteristically, however, accelerometers have low output with high impedance. This means that care is required with the signal cabling to avoid ground loops, electromagnetic interference, and cable / connector vibration. In addition to the measuring acceleration, accelerometers can be used for measuring velocity and displacement when used with instruments having integration and double integration capability respectively. Where double integration is used, there is some sacrifice in frequency range or capability to detect low frequency vibration because of instrument noise introduced in the integration process.

Non Contact Pick-up

This is the widely used pickup in the industry for the measurement of vibration and displacement. It senses vibration in a different manner from both the velocity pick-up and accelerometer. A coil of the fine wire placed at the end of the pick-up ( also called Probe )generates a magnetic field when a very high frequency ac ( 1 Mhtz ) is applied to it. When the pickup coil is placed closed to a conducting surface such as shaft of a rotating machine, the magnetic field sets up eddy current in the shaft which acts as an added electrical resistance in the coil circuit. If the shaft is vibrating, the pick-up output voltage varies in a manner directly proportional to the displacement amplitude of the shaft vibration. Thus the non-contact type pick-up is capable of detecting both the gap distance between the pickup and the shaft, and vibration displacement amplitude. Unlike the velocity pickup and accelerometer which measure “absolute” motion ( i.e motion with respect to internal space ) the non contact type pickup measures the “ relative” motion between the structure supporting the pickup and the surface in close proximity to the pickup coil (e.g. motion between bearing and shaft ). Figure shows the schematic diagram of this type of pickup.

Non contact type pickups are designed primarily for measuring the relative vibration between a shaft and its bearings. While they have a frequency range on the order of 0-60,000 cpm ( 0- 1 khtz) shaft vibration displacements become so small at higher frequencies, even for rough running machines, that the frequency range is actually limited to about 0-60000 cpm by the amplitude resolution of the non contact pickup. This is on the order of 0.05 to 0.1 mil.

Gap Measurement:

The proximeter is always powered by –18 volts DC from an external source such as a power supply or monitoring device containing an –18 volts DC power supply. The proximeter converts this 18 volts into an RF signal that is applied to the probe through the 95 ohm coaxial extension cable, as shown in Figure . The probe coil radiates this RF signal into the surrounding area as magnetic field. If there is no conductive material within a specified distance to intercept the magnetic field there is no power loss in the RF signal, the RF signal at the proximeter output terminal is maximum approx. –14 volts. When a conductive material approaches the probe tip, eddy current are generated on the surface of the material, resulting in a power loss in the RF signals. As a power loss is developed in the RF signal the output signal voltage at proximeter OUTPUT terminal is reduced proportionately. As the observed conductive surface comes to the probe tip, more power is absorbed by the eddy currents on the surface of the material. When the probe is very close to conductive material surface nearly all the power radiated by the probe is absorbed by the material. This is reflected as a maximum power loss of the RF signal ,resulting in a min DC output signal at the proximeter output terminal. The proximeter measures the magnitude of the RF signal, and provides a negative DC output voltage O/P signal proportional to the peak of the RF signal.

Vibration Measurement:

If the observed surface is rotating and rapidly changing the gap distance, the RF signal amplitude is not a constant amplitude, but varies in direct proportion to the P-P movement of the observed surface as shown in figure. This Peak –Peak movement of the observed surface cause the RF signal to be amplitude modulated. The proximeter detects the modulated RF signal as an AC signal varying around a constant average dc voltage ( initial probe gap voltage setting) as shown in the figure below.

If the shaft vibration is 5 mils Peak-Peak, around an initial gap of 50 mils , the average dc voltage of approximately -8 volts remain constant, but the AC voltage is one volt P-P , -7.5 t0 –8.5 volts in direct proportion to the shaft vibration in figure . This is the process of radial vibration measurements, whether it is single plane or two plane ( X-Y).

A typical calibration curve for an eddy current displacement measuring system is shown in figure below. The curve may be divided into three regions, beginning with the probe contact , the conductive surface and a zero dc o/p from the oscillator demodulator.

In most systems the probe may be withdrawn a short distance before the output voltage will begin to change. At some point, as the probe is withdrawn, the output voltage will suddenly increase then transition to the second or linear region , where any change in distance ( gap) produces a corresponding proportional change in DC o/p from the oscillator demodulator.

Within the linear range, which may extend from 20-80 mils gap, current standard require either a 100 mV / mil or 200 mV / mil proportionality between gap and voltage. Thus a 10 mil change in gap should produce a voltage change of 1 volts at 100 mV / mil probe or 2 volts at 200 mV / mil probe.

To digress slightly, the probe, extension cable and oscillator demodulator makeup a tuned resonant circuit. In order to establish and maintain a constant ratio between gap and voltage, the probe, oscillator and demodulator and extension cable must be matched and calibrated. Most manufacturers will specify the type of probe, generally probe tip diameter, and the total electrical length of the extension and integral cables which must be used with each oscillator demodulator.

As the probe is withdrawn further, the system loses its linear relationship between output and gap as the output from the oscillator demodulator approaches its supply voltage. Thus whenever accuracy is desired, the probe must be set so that it is operating within its linear range.

The slope of the curve, the linear range, and the dc output corresponding to a given gap will vary with changes in a targets conductivity and permeability. If a probe and oscillator demodulator calibrated for 4140 steel are used without re-calibration on a material such as stainless steel or inconel, the curve shifts to the left, producing a higher voltage o/p for a given gap. In addition, the slope of the curve will change corresponding to a change in sensitivity. Due to this shift and potential inaccuracies, a non contact probe system calibrated for one material should not be used with another without re-calibration.

Temperature may also affect the range limits of a non contact probe and the DC output at a given gap; however, the shift is generally small across the temperature range experienced within a bearing housing. Elevated pressures may also affect the sensitivity of a non contact probe. If the probe is installed in an area of high or fluctuating pressure, its response should be tested in the actual environment to determine what changes in sensitivity or output will occur.

With every thing else equal, the maximum linear range with a non contact displacement measuring system will increase with increasing probe tip diameter and, as implied from figure , will likewise increase with increasing supply voltage. At a sensitivity of 200 mV / mil , linear range of typical non contact measuring system observing 4140 steel will vary from approx. 60 mils with a 0.190 inch ( 5 mm) tip dia and 18 volts dc supply to 85 mils with 8 mm tip diameter and –24 volts dc supply. The proximity transducer measurement system has a frequency response of 0 to 10 khtz ( 60,0000 rpm ), zero represents non-rotating condition or static condition. The upper frequency ( -3db ) of 1 Khtz (600,000 rpm) places no limitation on the systems ability to respond to radial vibration rates that are multiples of machine running speed.

KEYPHASOR

A Keyphasor is a special application of a probe and proximeter. The probe views a shaft marker (either a notch or a projection) to give a rotational reference of shaft speed. Figure below shows a relationship of probe pickup to oscilloscope display for both notch and projection.

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