Overview of Condition Monitoring Techniques
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1 - Noise Analysis Techniques Overview
Noise is a well proven technique for detecting and diagnosing advanced bearing surface damage, rubbing between moving components, loose banging components, leaks in pressurised systems, various electrical problems and can be used to assist in any vibration monitoring or diagnosing work. Noise monitoring is also important in Occupational, Health and Safely programs to ensure the workplace is a safe and health environment.
Five Senses
The human ear is a very powerful noise analyser and can sometimes detect small rubbing noises, tones and other sound patterns which may not be detected by the most complex signal analysing systems. An observant person who regularly goes into a specific area of plant will often pick up small changes in the noises which may be the signs of equipment deteriation. There are many specific problems in which the ear can be an aid to monitoring and diagnosis such as pump cavitation and rolling element bearing deterioration. Using sound to diagnose, for example, a bearing involves listening to the suspect bearing and comparing to a bearing or a number of bearings on similar machines. Even though noise monitoring with the human ear can often be effective it should not replace measurement instrumentation such as vibration meters for critical equipment situations. This is because noise may not tell the whole picture and will not reliably detect a number of equipment failure modes. Use of the human ear often becomes important to assist in difficult vibration monitoring applications such as for very slow speed bearings.
There are a number of instruments available to assist in using the human ear as a condition monitoring tool. An example of this is the SKF Electronic Stethoscope TMST2, see figure 3.4 below, which amplifies the noise coming from equipment into a set of headphones. This makes indistinct noises more audible and cuts out background noise which might interfere.
Ultrasonic Leakage Noise Detectors
Leakage of gas or liquids from pressurised or evacuated systems creates high frequency noise in the ultra sonic range. Electrical arching and corrona, especially high voltage equipment, also produce noise at the same frequencies as do bearings with race surface damage. There are a number of instruments from different suppliers that are designed to detect ultrasonic noise. An example is the Ultraprobe 2000 show in figure 3.5 which has a number of standard features and attachments as listed below:-
- Microphone probe for picking up airborne noise from, for example to trace an external leak in a network of air pipes, finding a flange leak on a vacuum vessel or finding arcing noise in an electrical panel.
- Solid probe adaptor to use for finding internal valve leakage when leakage can't be externally observed, monitoring the performance of steam traps or monitoring the surface condition of rolling element bearing.
- Air tight vessels can be tested for integrity by placing an ultrasonic noise generator in the vessel and testing for any noise external to the vessel.
- A set of headphones are supplied which convert the ultrasonic noise to an audible range to help the instrument user to discriminate between different noises and their sources.
- The instrument has a sensitivity adjustment and a display gauge so that quantitative measurements can be recorded.
One of the biggest uses to this type of equipment tends to be in monitoring valve and steam trap internal leakage. Valves of almost any sort can be monitored. With very low pressures systems there is little leakage noise generated but the system has been used successfully on low pressure water systems. The typical application is having a group of valves coming of a common header and the need to find which valve is leaking without having to split flanges. The system works on hydraulic systems except if there is a pressure relief or other type of valve nearby which is producing a large amount of noise, making leakage testing impossible. These systems have been used to monitor leakage in piston compressor one way valves. They are a well recognised way of monitoring steam trap leakage which can be a significant source of energy wastage.
The use of this equipment for finding and diagnosing electrical discharge noise in electrical equipment is well recognised. Equipment such as wiring, switchgear and transformers can be monitored with this technique to complement the use of thermography.
Other Noise Measurement systems
There may be some equipment problems where the only symptoms available for measurement are transmitted as airborne noise. Diagnostic speciallist companise often are able to carry out analysis on noise using microphone transducers and their vibration analysis equipment. It is also possible to connent a microphone to a data collector instead of a vibration transducer.
Speciallist diagnostic comanies often have instrumentation to carry out noise testing if any Occupational, Health and Safely problems with noise occur. A number of companies such as Bruel & Kjaer and Rion make environmental noise meters.
2 - Lubricant Condition Analysis Overview
There are a number of techniques described below for lubricant condition analysis as well as techniques for analysis of equipment condition described under the section on wear debris analysis. For equipment to operate reliably with a long and useful life, moving metal components which come in regular contact must be effectively lubricated. This includes components such as slides, bearings, pistons and gears. Lubrication tries to minimise the wear and deterioration by getting in between the components and reducing or eliminating metal to metal contact. Problems with lubrication can come from not enough lubricant, incorrect specification, oxidisation or contamination. Lubricant condition analysis is checking the condition of the lubricant rather than the condition of the equipment.
Simple On-Site Tests
The simplest on-site lubricant monitoring is the oil level check. This is very important to equipment where loss of oil, or ingress of liquid such as water to the system is a risk. Loss of lubricant from greased bearing system is not usually a risk unless the temperature is very high. Many oil systems, such as for steam turbines, can have water ingress as a problem. Water being more dense than oil it will sinks to the bottom of the tank. Opening a drain valve at the bottom of the tank and taking a sample in a glass container will show evidence of the water in the oil. Visual inspection of a sample of oil may show signs of overheating by darkening of the oil colour. If changes in oil viscosity is highlighted as a significant problem then simple viscosity testing kits are available to do on-site tests. These work by directly comparing used oil with new samples in the speed the oils travel down a special slide.
A simple instrument is also available to test the level of oxidisation deterioration of oil by testing the acidity level and giving a simple read out of condition. The meter is called the ICC Oilcheck.
Lubricant Laboratory Tests
Laboratory lubricant testing is used to compare the condition of an oil to it's manufactured specification. The parameters tested in oil are viscosity, acidity, %water and %solids. These tests show up significant levels of oil degridation, contamination or dilution. If necessary the presence of special additives can be tested for by spectrographic oil analysis. Grease can be tested using a penetration test and a %water test. For oil used in electrical equipment such as transformers other tests which measure Power Factor and Inter Facial Tension (IFT) to monitor the insulation characteristic of the oil. If information is required about the actual lubrication abilities of a new or used lubricant, detailed friction and load testing machines are available in some oil laboratories.
3 - Wear Debris Analysis Overview
As an oil lubricated machine wears the oil becomes contaminated with fragments of the worn particles. Several method are available which try to determine the severity, source, type of wear or location of wear by analysis of the wear debris particles in the oil. Wear debris analysis attempts to measure both the condition of equipment and or the contamination condition of the lubricant. A number of these techniques require taking oil samples for analysis in laboritories. Samples have to be taken in a way which ensures that the wear particles produced within the equipment are present in the sample. The other major problem when taking oil samples is contamination as the amount of debris in the oil is very small and so evern small levels of contamination can effect results.
Magnetic Plugs
Monitoring magnetic debris using removable magnetic plugs in circulating oil systems is a monitoring system that is widely used in aircraft jet engines. The magnetic plugs are placed in the return line from critical bearings of gear systems. The plugs can be removed while the machinery is running and checked visually for unusual build-up or the debris can be removed from the plug and measured in a special instrument. Vibration or wear debris monitoring of complex gear and bearing systems can be very time consuming and needs considerable expertise. Magnetic Plugs provide a quick on-site method which does not require high levels of expertise and the magnetic plug socket can also act as an oil sampling point for further laboratory wear debris analysis.
Filter Inspections
Most recirculating oil systems and hydraulic systems have oil filter systems which collect the solid contaminants in the oil. The rate at which the pressure drop across the filter increases or the rate at which the filters have to be changed or cleaned can be an important indicator of system problems. To track down the source of excess contaminant the debris from a removed filters can be visually examined and a sample of the debris could be tested for material type in a laboratory.
Spectrographic Oil Analysis
Spectrographic Oil Analysis uses an extremely high temperature to heat a small amount of oil and the light given off in this process is analysed. The amount of different elements in the oil, such as iron or copper, can be measured. It is a laboratory technique ans requires strict oil sampling procedures as the technique is very sensitive to contamination. This technique can be performed by most large oil laboratories. A number of oil companies such as Mobil, Caltex and Century can provide this service. This technique has been used widely around the world on mobile equipment internal combustion engines, transmisions and gearboxes. Often it is possible to predict the source of a problem from the types and quantities of elements found in the oil. For example, bearings are a source of cadmium, piston rings are a source of molybdenum, and coolant leaks are a source of sodium. Traces of silver could come from silver-soldered pipe joints, and so on. The main limitation with the technique is that only very small particles are detected and that larger more dangerous particles are ignored.
Particle Counting
Particle Counting is a method of quantifying the amount of contaminant in an oil sample. It works by passing an oil sample through a small glass tube and the number of times a light beam is interrupted by a particle is counted. It is a laboratory technique and is only suitable for relatively clean oil such as from hydraulic systems. As with spectrographic analysis the technique is very sensitive to contamination. Because the reliability of hydraulic systems is usually directly related to the cleanliness this is a recommended monitoring method for hydraulic systems. The output from the tests is a NAS or ISO cleanliness rating which look at both the size and number of particles in the oil. Instrumentation is now available which measures cleanliness rating on-site with a data collector type portable instrument. This instrument uses a filter blockage measurement technique but gives an equivelent output with standard cleanliness ratings. Figure XX show an example of this equipment.
Filtergrams
Filtergram analysis requires an oil sample to be forced through a filter paper which collects any particles in the oil. The fliter paper is attached onto a glass slide, dried and then examined under a microscope. Filtergram analysis can show what sort of wear is occurring in the machine (eg. metal fatigue or rubbing) through analysing the range of particle shape, size and colour on the filter paper. Any small laboratory can be set up to create filtergrams at very little cost but experience is required to get full potential from the analysis of the filtergram under the microscope. Filter analysis can be performed by a number of groups such as Monash University and Mobiltech. Filtergrams and ferrography give the potential for very early detection of equipment problems. They both look at the full range of particle sizes and so provides more information than spectrographic oil analysis but is slower and more expensive.
Ferrography
In this method a set quantitiy of oil runs over a glass slide and a strong magnet underneath affects the way particles are deposited on the slide. Ferrous particles in the sample are deposited at different positions because of their size and some non-ferrous particles settle out as well. The analysis of the slide is very similar to the analysis of the filtergram slide. Monash University and XXPeter Ball has a ferrography expert available.
On-Line oil analysis
There are a number of on line oil analysis systems available using a number of different technologies. One avilaiable system is an automated version of a magnetic plug which every 20 seconds magnetises and collects debris, measures the amount of debris and then releases the debris to start the collection cycle again. The system is called Sensys from a company called Gas Tops in Canada.
5 - Temperature Measurement Overview
Temperature is a very important indicator of the condition of equipment, especially for heat machinery such as furnaces and refrigeration systems. Temperature generated by electrical resistance or mechanical friction are anther important areas for condition monitoring of equipment. There is a very wide range of temperature monitoring techniques ranging from the simple and inexpensive to the very sophisticated and costly.
Five Senses
Temperature monitoring with the human hand is one of the oldest and most widely used methods for monitoring equipment. Also for very high temperatures, such as with fire, any exposed skin and the human eyes can also be used as a non-contact temperature sensor. The human senses can't accurately measure temperature but in many machine monitoring application this is not necessary. For example if the heat insulation deteriorates in a furnace wall or if a bearing fails causing mechanical rubbing, human senses are more than adequate to detect the large changes in temperature that occur. This type of temperature monitoring can be combined with any other operational or maintenance activity associated with the equipment and is of special importance to combine it with inspection and monitoring activities as it involves little extra time or cost. Where five senses type temperature monitoring is not applicable is where accurate monitoring or trending is required, where small changes need to be detected or were access to the equipment is not available or not safe. An example where safety is a very important factor is for live electrical equipment.
Thermometers
Where accurate monitoring and trending of temperature parameters is necessary there is a wide range of equipment available such as glass bulb thermometers, magnetic contact thermometers, hand held contact thermometers, hand held non-contact thermometers and a wide range of temperature measurement instruments for permanent installation. Many portable condition monitoring data collector a have non-contact thermometer as a standard attachment, so equipment surface temperatures can efficiently be collected, trended and alarmed using this system. The introduction of hand held non-contact thermometers has greatly increased the speed and ease of collecting equipment surface temperatures. There are also instruments available that can measure from a reasonable distance away from the user where access is a problem. Applications for this sort of instrument are for monitoring high or low temperature equipment such as furnaces, boilers or refrigeration systems where energy efficiency is critical. Also for electrical equipment where electrical joints, electrical insulation and resistance of components such as fuses, resistors and thyristors can be monitored. Temperature generated by friction in mechanical components such as bearing, gearboxes and slides can be quickly monitored. Also with a non-contact measurement instrument the temperature of rotating shafts or other moving components can be easily by monitored. For many production processes temperature is a critical parameter which has to be controlled or monitored. For these applications portable instruments are not usually satisfactory and permanent instrumentation is required. In these situation equipment operators usually monitor the measurements and inform maintenance of parameter alarms or trends so that repairs or further diagnostics can be carried out. The two most widely used sensors for on-line measurement of temperature are Thermocouples and Resistance Temperature Detectors (RTD). Thermocouples operate by a pair of differing metal wires generating a voltage relative to temperatue and RTD’s operate by the resistance of special materials changing with temperature. As with all permanently installed sensors calibration becomes a key consideration.
Temperature Sensitive Crayons, Paints and Stickers
Thermographic Indication Paint is one of the cheapest and simplest methods of thermographic inspection. Thermal paint changes colour permanently when subjected to a temperature above a specific level. Versions are also available where the colour change is not permanent. Thermal paint is available in many temperature ranges to suit varied application from 25 deg C to 1100 deg C. For Condition Monitoring of electrical components such bus-bars, fuses, contactors and cable junctions the most popular temperature ranges have been 46 deg C and 110 deg C, although 25 deg C has been used successfully. The temperature sensitive paint is of most use in monitoring systems that can't be measured during operation for safety reasons but the paint can be checked on any shut-down to ensure the service temperature has not reached it's critical limit.
Thermography
Thermography or infra-red imaging equipment produces a heat image of a viewed object. This image uses colours to highlight areas of equal temperature usually viewed on a video screen and gives a highly accurate temperature distribution. Temperatures up to 1600oC and temperature differences down to 0.1oC can be measured. This technique has been extensively used to monitoring thermal insulation or refractories especially for furnaces to check for heat loss, insulation wear and defects. The system has application in electrical equipment condition monitoring as well as in many other areas. Thermography has the advantage over simple temperature monitoring instruments in that a large amount of equipment can be monitored very quickly by taking advantage of the human ability to recognise shape and colour patterns. Using this method problems are often found in unexpected areas which would never have been looked at with the more time consuming techniques. In most cases the information from thermography is very simple to interpret with just a basic knowledge of the equipment being monitored.
6 - Visual Inspection
The eye is the most powerful of the human senses. Most condition monitoring instruments and systems convert measurement information into a visual form for analysis and diagnosis. The eye combined with the human mind has the ability to make sense of complex shapes, patterns, shades and colours. This section covers the direct use of human vision and instruments used to extend human's optical capacity.
Five Senses
Simple visual inspections are the most powerful condition monitoring system available. Many equipment condition problems would be visually evident to any normal person, for example liquid leaks from pipework or significant physical damage. Other condition problems are evident only to experienced personnel who understand the plant process and the equipment components which make it up. To an experienced person a small pile of material in a certain location, small movement of a component, signs of wear in a component or a significant reduction of flow in a certain discharge pipe may all indicate a possible future equipment failure and thus the need for maintenance. The key to setting up efficient visual inspections of plant is knowing the importance of equipment, understanding how equipment is likely to fail and determining the visual clues that many indicate deterioration has reached a critical stage.
Simple qualitative visual inspections are best combined with more quantitative inspections by use of portable instruments, for example a portable vibration data collector, and by permanently installed instruments, for example temperature and pressure gauges.
Stroboscope
A stroboscope is an instrument that gives a repetitive pulse of light. When the pulse frequency approximately matches the rotational speed it appears to visually freeze or slow down rotating or reciprocating components. A stroboscope can be used to measure rotational speed, help show looseness of moving components and inspect for worn or damaged components such as V belts or cone ring couplings rubbers. The main advantage of stroboscopes as an inspection tool is that equipment does not need to be out of service to check these moving components. Battery operated units are the most versatile, such as the unit shown in Figure 3.7.
Users of stroboscopes have to be doubly cautious about the safety aspects of working near moving machinery and to resist the temptation to touch items that appear stationary but are not.
Rigid Endoscopes and Fibrescopes
Some equipment components can only be visually inspected directly by eye after expensive disassembly, for example jet engine combustion chambers, and there are other components that could never be inspected directly by eye, for example the inside of small diameter pipework. Rigid Endoscopes and Fibrescopes provide a way to see into inaccessible equipment locations as well as do simple mirror systems in less difficult applications. The Rigid Endoscope is a small diameter rod with an eye piece at one end and viewing optics and light source at the other. They are available in a range of lengths. Viewing angles range from 90 degrees to the rod to straight ahead. they are ideal to view through small holes to detect signs of internal wear or damage. For example, inspection through a spark plug hole into the combustion chamber and cylinder of a petrol engine. Many aircraft and jet engines are specifically designed to allow Rigid Endoscope inspection of inaccessible areas and components such as a jet engine combustion chamber.
Fibrescopes carries out the same role as a Rigid Endoscope but is more versatile in the places it can view. Rather than use a solid rod it is constructed of a 3 meter long flexible bundle of glass fibres protected by a flexible stainless steel sheaf. This allows the fibrescope to view around multiple bends which is the norm for pipework. The viewing head of the Fibrescope is usually equipped with a two directional steering device, which with operator experience, makes it a very power full inspection tool.
When trying to view down small diammeter holes or through small gaps the biggest problem can be to apply illumination to the inspection point. A simple device which can assist consists of a small torch attached to a fibre optic cable the end of which illuninates the inspection point. Because of the small diameter of the fibre optic cable it does not restrict viewing access through the gap or hole.
Television Systems
Television camera systems are used in some plants to increase the efficiency and effectiveness of plant process and equipment monitoring. One operator through a number of TV screens could visually monitor a wide area of plant on an almost continual basis. This technology also assists in inspection of areas which are too dangerous or unpleasant to be inspected by people, for example where poisonous gases or very high temperatures are present. Another television technology offers and alternative to the Fibrescope by using a miniature TV camera at the tip of a long flexible cable. The advantage of this system is that the possible length of the cable is not limited. An example of an application is inspection inside boiler wall tubes which could easily be over 20 meters long.
7 - Dimension Measurement
In a large amount of industrial equipment deterioration and failure can often be associated with wear, corrosion or some other physical change in dimension. There is a large range of tools and instruments to assist in quantifying these changes.
Rulers, Tapes and Diameter (PI) Tapes
There a many applications where dimension changes such as through wear, misalignment and component position changes can be measured with adequate accuracy by simple rulers or tapes. There is also a wide range of simple tradesmen tools to assist with measurements such as callipers, squares and dividers. Special measurement tapes are available to measure pipe or rod outside diameters by rapping the tape around the circumference and reading diameter directly from the tape, these are often called Pi Tapes.
Often equipment inspection is carried out relying only on visual qualitative measurements of these dimension or position variables. Often with very little extra effort and with a dramatic increase in consistency and reliability quantitative measurements can be made with just simple measurement tools. Another very effective approach is to make qualitative methods more accurate, for example drilling small holes at a set depth into the back a wear plates so that when a small rod can be poked through a hole the wear plate needs replacement.
Verniers and Micrometers
Where measurement with rulers and tapes are not accurate enough, verniers and micrometers can be used to measure to much higher levels of dimensional accuracy. This type of equipment is now available with digital readouts which increases their speed and ease of use. Inside diameters or gaps between 3mm to 13mm can be easily measured with the assistance of adjustable "Holes Gauges" and larger inside diameters can be measured with adjustable "T Gauges" or with inside micrometers. For small clearances or gaps the simple feeler gauge is the most widely used tool and is used to check the clearance on items such as larger diameter spherical rolling element bearings. Another method of measuring small clearances when feeler gauges can’t be used is assembling an item with a small lead wire in the clearance gap, tightening bolts and then disassembling and measuring the lead wire thickness. Rather than using lead wire a Flexi-gauge can be used which is a thin plastic cord which is calibrated so that the width of the crushed cord can be simply related to the thickness. This system is used extensively for measuring the clearance in journal bearings. When very small clearances or interference fits needs to be checked the same method can be used by inserting standard thickness spacers into the housing and then after tightening, comparing the lead wire thickness to the spacer thickness. This method is used for checking bearing to housing clearances.
Dial Indicators and Linear Transducers
Dial indicators are a measurement device consisting of a small spring loaded rod in which axial displacement is indicated on a dial. Dial indicators are used to check shaft out of round, bent shafts, misalignment, axial float in shafts, small linear movement of components, housing strain (often called soft foot) and many other applications. Bearing clearances can often be checked with the shaft lift method using a dial indicator. The dial indicator is positioned on the top of a shaft as close to the bearing as possible. Then the shaft is lifted with the help of a lever or jack for heavy shafts and the vertical bearing clearance will be measured. Axial clearances are measured in a similar way. Electronic dial indicators are available for applications where extremely high accuracy is required.
Where a linear position or movement is required to be converted to an electrical signal for testing, monitoring or control of a machine, linear position transducers can be used. These are available in a wide range types and configurations. Angular position transducers are also widely used on machines. Hysteresis measured by this type of transducers is often an indication of wear or deterioration in system or component being measured.
Ultrasonic Thickness Gauge
There are may applications where the thickness of an item can't be measured with normal tools because measurement access is not available. An example is thickness measurement of a wear plate or wall thickness of a pipe main. It is possible to measure thickness for most metals and a number of other materials with an Ultrasonic Thickness Gauge. This type of instrument has a probe which emits an ultrasonic sound waves and when it has a good contact with a metal surface the ultra-sound is reflected off the opposite metal surface and the time measured. Using the speed of sound for the material allows the calculation of component thickness. To get a good transfer of the ultrasonic sound into the material from and back to the probe a coupling medium such as water, grease or oil has to be used.
Instruments are available which have a simple digital read out calibrated for steel materials and is ideal for monitoring the thickness of pipework, wear plates, pressure vessels or structural steel which may be subject to corrosion or wear. Where the item being measured has a more complicated shape instruments are available which displays the reflections off the different surfaces of the component which makes it more difficult to interpret but more flexible. Instrument are also available which measure the thickness of paint or other surface coating using the same technology.
Laser and Survey Equipment
Laser technology is widely used in many areas of position, shape and displacement measurement. Some applications are shaft alignment, equipment set-up, distance measurement for survey work and measuring wear of furnace or ladle refractories while still hot. Special instrmentation is also available for non-contact vibration measurement using lasers.
8 - Non-Destructive Testing
Non-Destructive Testing (NDT) is a generalised term referring to a large group of measuring techniques which can be useful in condition monitoring or quality control.
Dye Check
Dye Penetrant testing is a method for finding cracks and discontinuities open to the surface in solid non porous bodies. The method employs a penetrating liquid which is applied over the surface and enters the discontinuity or crack. The excess penetrant is then cleaned from the surface and sprayed with a special paint. The penetrant which exudes or is drawn back out of the crack is observed as a colour change indicating the presence and location of the defect.
Magnetic Crack Detection
This NDT technique allows cracks, tears, laps and similar defects which come to or are very close to the surface to be detected. It uses relatively simple test equipment and the interpretation of results is generally straight-forward. It is applicable only to ferromagnetic materials. Non ferrous products, for example aluminium and copper, need to be tested by other techniques.
The magnetic particle test makes use of the fact that an external magnetic field is developed around a discontinuity. A magnetic field is applied to the object which is then sprayed by liquid containing iron particles. The fine iron particles will be attracted to this magnetic flux leakage around the defect and can be visually detected by patterns in these particles.
Ultrasonic Crack Detection
Ultrasonic inspection is a test to determine the presence of internal flaws. It involves the transmission of high frequency sound waves which are reflected by internal defects. There are two fundamental methods of ultrasonic testing:
- Sound energy is transmitted through the object under examination and the amount reflected is measured - referred to as pulse echo method. This method is also used extensively for thickness testing.
- Sound energy is transmitted through the object under examination and the amount received at the far end is measured - referred to as through transmission method.
Eddy Current Crack Detection
Eddy Current testing uses an electrical coil containing alternating currents and when it is brought into proximity of a conductive material such as a steel plate the coil excites curculating or eddy currents in the test material. The work in generating these eddy currents can be measured at the coil. When the coil is moved across the surface of the material changes in material elctrical conductivity, changes in material quantity and defects in the material will all produce eddy current changes. See figure XX. Different size and shape coils are required for different applications. One application is for testing of defects in heat exchanger tubes by feeding a coil down the inside of the tube. The main advantage of eddy current testing is that it is a non-contact technique and large areas can be tested quickly.
Radiography
Radiography is very similar to medical X-ray techniques. It uses a radiation source to penetrate through an object and record an image onto photographic type film positioned on the opposite side of the object. This system is used extensively in the testing of critical welds and where other simpler methods of defect measurement are not practical.
9 - Force and Strain Measurement
A large number of failures are caused or accelerated by the forces applied to the equipment. It is often sensible to measure the force which causes the damage rather than to detect the symptoms of damage.
Strain Gauges
A strain gauge is small sensor made of a thin flexible material with special wires embedded in it. It is designed so that if it is glued to an object and then strain is placed on the object the wires will be stretched causing them to change resistance. Special instruments are available to measure the resistance thus the strain on the object. Strain gauges are often used to measure stress in equipment such as pressure vessels, bridges or aircraft. Quantifying the actual static and dynamic load on a component may enable the expected fatigue life to be calculated. Another application is measuring torque in rotating shaft by using any of a number of non-contact methods to transfer the strain signal from the rotating shaft.
Load Cells
Load Cells are usually strain gauges or other types of force transducers designed for special force measurement applications. The major application for load cells is measureing the weight of objects. This could be anything from a few grams of material to a 1000 Tonne material hopper. As with strain gauges load cells are used in a wide range of equipment testing, monitoring and control application.
10 - Performance Measurement
Performance Monitoring is the regular checking of performance indicators for a machine against standard values. Performance indicators of a process or a piece of equipment measures its ability to fulfil it's required function. It may also indirectly indicates the condition of components. Some examples of simple performance measures are the time for a large valve to close or the liquid discharge rate of a pump. The standard values are often called the new performance. A fall-off in performance is an indication that something may be wrong. On most new plant there is some level performance or acceptance testing during commissioning. Repeating a simplified version of these tests is the method most often use on performance testing. In some modern plants around the world performance monitoring has been fully automated for example the BSC Port Talbot Steelworks Hot Strip Mill has a once a week 15 minute performance check for its whole Mill process. Performance Indicators often take the form of a multi-dimension relationships between parameters usually specifies by lines or areas on a graph, for example a pump head to flow graph.
Temperature
An example of a temperature performance monitoring application is monitoring the steam discharge temperature of a boiler. A drop of this temperature could indicate a control system problem, a measuement calibration problem or any number of other process or component problems.
Pressure
An example of a pressure performance monitoring application is a piston air or gas compressor where the cylinder pressure to stroke relationship of each cylinder can be measured and compared to the as new relationship.
Flow
An example of a flow performance monitoring application is a water pump where the flow and pressure increase through the pump are measured and compared to the pumps commissioning head flow graph.
Time
An example of a time performance monitoring application is a automatic valve where the closing or opening time is critical for a process. The valve operating time could be automatically measured on each operation and an alarm triggered if a predetermined value or a trend in indicated.
11 - Electrical Equipment Monitoring Overview
There are many ways to measure the condition of electrical equipment and two of the most important which are not mentioned in this section are temperature monitoring and dimensional measurement. The section on temperature measurement is relevant for electrical equipment especially for electrical panel equipment. For example fuse clips, bus conections, relays, breakers, contacts, control conections, thermal overloads, bolted electrical connections, transformers and conductors with excess current can all be monitored by temperature. Yearly monitoring with thermography or use of temperature sensitive paints should be a standard maintenance strategy for critical electrical panel equipment to detect possible hot joints, uneven load or resistance problems.
The sections on dimentions measurement is very relevant for monitoring of electrical sliding surfaces. This includes wear monitoring applications such as brushes, DC motor commutators, AC motor slip rings, generator exciters and a large range of wearing components in switch gear and contactors. In most cases these wear measurement must be carried out off-line while the equipment in down for maintenance for safety reasons.
Electrical Resistance Testing
The Multi-meters are the most widely used resistance measurement equipment and are used mostly for testing circuits and small electrical components. They are usually not suitable for measurement of resistance or insulation condition of larger electrical components. A method to measure circuit resistance for larger equipment is attaching a DC or AC current generator to the circuit and measuring the voltage across and current flow to calculate the resistance.
Insulation condition for electrical components is a measured of resistance to earth. Insulation breakdown is a major form of electrical failure especially in high-voltage electrical equipment (as in some large electric motors). When an insulator breaks down, electric current flows through the breakdown path, resulting in heating. Insulation deterioration usually occurs slowly until significant heat generation occurs which acceleration of deterioration and can even cause an explosion. Dust, moisture and oil contamination are some of the major causes of insulation deterioration on equipment such as motors, generators, contacts and switchgear.
The MegOhmmeter (Megger) is the most widely used instrument for isulation condition measurement of larger electrical equipment such as motor and switchgear. The circuit for testing needs to be isolated and the instrument attached. The instrument creates a predetermined voltage level between the circuit and earth and the deterioration of this voltage is checked over time. This is called Time Resistance Readings and the measurements have to be temperature corrected if equipment has not cooled to around 40oC. As insulating materials get contaminated or deteriates their resistance, which is ideally infinite, decreases. Equipment must be off-line to carry out this test and is usually carried out when equipment is out of service for other reasons. Other insulation condition parameters such as Dielectric Absorption Ratio and Polarisation Index are widly used for more critical high voilage equipment as they give a longer problem warning time.
It is possible in some situations to get an indication of electrical insulation condition while equipment is in operation. One method is earth leakage testing which measures the current which flows between the circuit and earth. There are various levels of sophistication for this type of equipment and is usually used on large motors and generators and integrated into the control equipment as an on-line measurement for alarming and tripping.
Dissolved Gas Analysis
Large high voltage electrical equipment such as transformers often use oil as a part of the insulation and cooling systems. Taking sampling of oil and doing spectral analysis of the gases in this oil is a standard method of understanding the internal condition of the equipment. If the insulation condition of an oil filled transformer deteriorates various gasses and material from the internal insulation are liberated into the oil which can be measured in these laboratory tests. Other parameters are usually monitored at the same time such as acidity, Interfacial Tension (IFT), Metals in oil (ICP) and moisture to assist in diagnostics. Technical information is available to interpret the results and analyse the equipment condition. Since most industrial equipment require electrical power to operate the reliability of electrical supply transformers are often critical to the reliability of the plant.
Motor Current Analysis
Induction motors tend to fail from either stator winding or bearing problems. Bearings are usually the major problem and the favoured monitoring method is using vibration analysis. Vibration analysis can often detect developing rotor or stator problems. Analysis of motor current is another method of detecting motor problems and for rotor bar failures, rotor eccentricity and phase current differences. Rotor bar and rotor eccentricity problems required spectrum analysis of the motor current using an instrument such as the IRD Fast Track. There is a software package available called Entek Motor Monitor which can analysis the spectrum and automatically analyse the collected current spectrum and recommend a maintenance action or further monitoring.
A specialised technique is available for the SPPD central condition monitoring group for monitoring the condition of conductors in DC motor armatures.
Offline Motor Circuit Analysis
There are now intruments (PMDA, All Test) available to do offline integrated testing of motor circuits. These have been highly successful in detecting winding problems in both installed motors and motor after overhaul of stored as spares. They also have been sucessful in detecting higher resistance joints in motor cableing.
Control Systems Analysis
Control systems are often a critical part of equipment and processes and variables such as static accuracy, response times, calculated frequency characteristics and hysteresis can measure the performance and condition of these systems. The monitoring of closed loop control system performance is possible with the use of a Real Time Transfer Function Analyser (RTTFA). This instrument measures the response of a control system by injecting a small random noise signal into the input of a control system and then measuring the signal feedback part of the system. Output of the instrument is a Bode plot that can then be used to determine the gain and phase characteristics of the control system.
Much new equipment is provided with sophisticated electronic control and monitoring systems and usually have many instruments and sensors. The information recorded from these sensors is often valuable for understanding the equipment condition.
