characteristics of parts of a machine, without in my way affecting that part physically or functionally. These testing facilities are developing very rapidly all over the world, and are progressively being made available indigenously as well. They are frequently being used and relied upon, for application and use in predictive maintenance practices and systems. The operating parameters which can define the health or condition of an operating machinery need to be monitored thoroughly.
In order to monitor the health of a machine, there are several parameters; these parameters are now discussed.
(i) Process variables. These include pressure, temperature, flow of process fluids, lubricating oil, seal oil, and cooling water circulation.
(ii) Mechanical running condition variables. These variables are: a) Temperature of vulnerable machine components
b) Mechanical vibrations c) Machine RPM
d) The relative motion of moving elements with respect to stationary elements of a machine e) Crack detection.
(iii) Rotating machinery malfunctions. These include imbalance, misalignments, foundation problems, fracture and warpage of casing due to a variety of loads such as thermal loads, and bearing failures.
NON-DESTRUCTIVE TESTING METHODOLOGIES
In order to monitor the above parameters, a variety of non-destructive testing (NDT) methodologies have been developed; some of these are now described in brief.
Boroscope
This is a tool that can be inserted into inaccessible places through small openings, and with its aid one can see a highly magnified image of the part under examination, through an eyepiece, a prism, or a reflector which has lighting arrangements.
Flexlscope
This is an instrument that is used to see contoured surfaces and U-bends, which are otherwise not within easy reach or view.
Liquid Dye Penetrant
This dye can seep into minute surface openings by capillary action to detect surface cracks, porosity and lamination.
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Magnetic Particle Detection
This technique is used for locating surface and sub-surface discontinuities in ferromagnetic materials. When a test piece is magnetised and finely divided, the ferromagnetic particles sprinkled over the surface forms an outline of the discontinuity formed by the magnetically held collection of particles, indicating the size, location, shape and extent of the problem of discontinuity.
Eddy Current Testing
This method is employed to measure electrical conductivity, magnetic permeability, grain size, heat treatment condition and hardness. The eddy current detects seams, laps, cracks, void and inclusions and sorts out dissimilar metal compositions.
Ultrasonic Devices
These devices use beams of high frequency waves to detect surface/sub-surface flaws. The waves travel through the material with attendant loss of energy and are reflected at the inner faces. The reflected beam is analysed to define the presence/location of flaws, cracks, laminations, shrinkage, cavities, pores, inclusions, etc.
Radiography
Because of the variation in the density and thickness of the radiographed item, it absorbs different amounts of radiation energy, with the unabsorbed radiation passing through the part The radiation can be recorded on film, or on to photo sensitive paper and viewed through a radiographic viewer to locate defects.
Hardness Testers
A variety of hardness testers, such as the Rockwell hardness tester, the rebound hardness tester and other allied tools, are available today. They are all portable and can indicate resistance to penetration and wear.
Creep Tester
Creep is a phenomenon that results in permanent deformation and damage when an item has been operating at high temperatures and stresses. The creep tester measures the changes that have come about in the dimensions. This has to be done at periodic intervals.
Spark Testing
In this test, a visual examination of the spark pattern that emerges when an alloy is held against a grinding wheel is used as an indicator for classification of ferrous alloys according to their chemical composition.
Leak Testing
Here, the assembly is filled up with air up to a specific pressure, and then a testing is done with a soap solution, halogen, helium or freon, to locate the leak.
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Fig. 7,1 Vibralion analysis.
Thermal Testing
The measurement of temperature can indicate abnormal or overhot conditions. The measurements can be done by making use of contact thermometers, heat sensitive paint, heat sensitive stickers, infra-red thermometers or optical pyrometers.
Acoustic Emission Testing
Acoustic emission is defined as a high frequency stress wave generated by the rapid release of strain energy that occurs within a material during crack development or plastic deformation. This method is capable of detecting the minutest of increasing flaws. There is no other NDT method which can match its capability.
Holography
Holography is a three-dimensional image of a diffusely reflecting object having an arbitrary shape. Both the amplitude and the phase of any type of wave motion emanating from the object are recorded by encoding this information in a suitable medium. This reading is a holograph. It can be obtained by using visible light waves and is known as optical holography or ultrasonic waves, called acoustical holography. It is used for detecting/locating debonds within honeycomb core sandwich structures, unbonded regions within pneumatic tyres, cracks in hydraulic fittings, stress of all kinds, corrosion, cracking in metals, fatigue in turbine blades, etc. Acoustic holography has been used commercially for inspecting various types and sizes of welds.
In Situ Metallography
The particular part to be examined is polished and etched in its location and the surface structure is transferred to a thin film using suitable chemicals. Then it is examined under a microscope to review the metallurgical changes that have taken place such as graphitisation, carburisation, intergranular cracking, and grain-growth.
Strain Monitoring
Strain gauges arc used to monitor the condition of parts subject to strains due to variations in their operating conditions. This is used extensively in aircraft testing during design stages and after overhauls during test flights.
Vibration Monitoring
Vibralion in rotating machines is caused due to factors such as imbalance, misalignment, looseness, cavitation, turbulence, wear, bearing damage, coupling damage, and rubbing. As the defects grow in magnitude, the vibration levels increase. By trend monitoring, the faults can be identified.In vibration monitoring, portable vibration analysers, digital vector filters, spectrum analysers and oscilloscopes, and the data generated can pin- point the cause of vibration (see Fig. 7.1).
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THE a-z OF DIAGNOSTIC INSTRUMENTS
More and more advanced portable diagnostic instruments are being developed each day. There are a host of them; some of these are now described, giving their particular functional details.
Diagnostic Instruments
1. Pocket-sized thermistor thermometer. This is shaped like a pocket watch with battery and probes, and gives a temperature reading within a few minutes.
2. Ultrasonic hardness tester. This instrument is used to read surface hardness in Rockwell C, of bearing, races, shafts, etc. A light weight probe, which is held against the surface, makes a reading. The answer is delivered in 2-3 seconds.
3. Ultrasonic corona detector. This device is employed to hear the 'carona* in the voids in cables or in the splice insulation slung across insulators before the carona can damage the insulation. 4. Laser beam source and detector readout. This permits the alignment of shafts, fixtures, or
structures to be made even when the items to be aligned are hundreds of feet apart This is done to a precision of 0.001 inch.
5. Pistol-grip static meter. This instrument measures the electrostatic charge on any surface at which it is aimed from a foot away.
6. Portable sonic resonance tester. It measures the thickness and soundness of concrete or wood, and can check the uniformity of fire bricks or metal. 7. Eddy current tester. This has a pointed probe which spots tiny
discontinuities on or below the metal surface without touching the object being scanned.
8. Pencil-probe leak detector. A neon light within the transparent probe Hashes whenever the point of the probe gets near a freon leak.
9. Tension checker (pencil sized) for V-belts.
Thermopile heat flow sensor. This gadget can be connected to any vacuum-tube voltmeter, and then it can be calibrated to read the extent of heat loss due to insulation (B.T.U. per square ft per hour) or to check the efficiency of the different areas on a heat transfer surface.
10. Stethoscopes. Overcoming external noise and disturbances, these can pick up mechanical problems like bearing problems which develop within. There are electronic stethoscopes as well as those commonly used by doctors everywhere.
11. Smoke bombs. These are used to ascertain the wind direction.
12. Thermistor thermometer. Unlike the pocket version, this is a probe attached to a flexible lead on portable instruments. It reads the temperature directly and accurately.
13. Vacuum-tube voltmeter. This is characterised by its ability to read voltage across points in an electronic system without drawing current from the circuit.
14. Fibre optic inspection probe. This enables one to make an examination of the internal
mechanism of a closed or inaccessible gear case or housing. Being avery small probe, it is a very useful tool for the inspection of gear teeth, or for locating broken/lost parts within a rotating