Maintenance Optimization [part 2]

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5. Maintenance Strategy - CBM

CBM / PdM Technologies

The start of Condition-Based Maintenance (CBM) - also called Predictive Maintenance or PdM - may have been when a mechanic first put an ear to the handle of a screwdriver, touched the other end to a machine, and pronounced that it sounded like a bearing was going bad.

We have come a long way since then with a variety of technologies for analyzing what's happening inside the asset. However, the need for a knowledgeable, experienced person to use the technology has not changed. Today, as in the beginning, successful predictive maintenance is a combination of man and technology.

Recent advances in technology have made CBM a reality - the ready availability of inexpensive computing power to gather, store, and analyze the data that makes CBM possible. By some counts, there are more than 30 technologies being used for condition-based maintenance. Others might argue that many of these are simply variations of each other. This section discusses some of the most-used CBM/PdM technologies.

Any condition-based maintenance program can be characterized by a combination of three phases:

• Surveillance - monitoring machinery condition to detect incipient problems

• Diagnosis / Prognosis - isolating the cause of the problem and developing a corrective action plan based on its condition and remaining life.

• Remedy - performing corrective action

Consistent, accurate data gathering is essential to all three phases.

Analysis of data is where the knowledge and experience of maintenance personnel becomes most important. It normally requires extensive training not only in the analysis techniques, but also in the use of the particular hardware and software employed.

Condition Monitoring and Data Collection

Condition monitoring uses primarily non-intrusive testing techniques, visual inspections, and performance data to assess machinery condition. It replaces arbitrarily timed maintenance tasks with maintenance scheduled only when warranted by asset condition. Continuing analysis of the asset condition allows planning and scheduling of maintenance or repairs in advance of catastrophic or functional failure.

Data Collection

Asset condition data is collected basically in two ways:

1. Spot readings - route based with portable instruments

2. Permanently installed data acquisition equipment for continuous online data collection

Generally, taking spot readings provides sufficient information for making informed decisions regarding maintenance of assets. The degradation of facility assets is usually not so rapid as to require the "up to the second" reporting that a permanent data acquisition system produces. The maintenance technician or a CMMS can usually keep a log of these spot readings and develop trends from these logs.

Permanent condition monitoring equipment is more expensive to install, and the databases created do cost money to analyze and maintain.

Typically, permanent data collection systems are installed only on critical and expensive assets and systems used in production processes. If they go down, it can cost the facility "money by the minute" when it’s not operating.

A variety of technologies are available to assess the condition of systems and equipment, and to determine the most effective time for scheduled maintenance. Some of the key technologies covering the basic theory of how the technology operates, the purpose of applying the technology, and acceptable applications are discussed in this section.

The data collected is used in one of the following ways to determine the condition of the asset and to identify the precursors of a failure:

• Trend Analysis. This method reviews data to see if an asset is on an obvious and immediate "downward slide" toward failure. It includes recognizing the changes in data as compared to earlier data or baseline data on similar assets.

• Pattern Recognition. This method reviews data to recognize any causal relationships between certain events and asset failure. For example, we might notice that after asset X is used in a certain production run, component Y fails due to stresses unique to that run. The method identifies deviations from established patterns.

• Correlation Analysis. This approach compares data from multiple sources, related technologies, or different analysts.

• Tests against Limits and Ranges. These tests set alarm limits and check if they are exceeded.

• Statistical Process Analysis. This analysis uses statistical techniques to identify deviations from the norm.

If published failure data on a certain asset or component exists, then we can compare failure data collected onsite with the published data to verify or disprove the data.

Several CBM technologies are available to assess the condition of an asset or system. In some instances, several technologies are used together to provide a more accurate picture of the asset condition. For example, to obtain the total picture of a cooling water system, a CBM effort may need to collect the following data: Flow Rates. Water flow is measured using precision, non-intrusive flow detectors.

Temperature. Differential temperature is measured to determine heat transfer coefficients and to indicate possible tube fouling of heat exchangers.

Pressure. Differential pressures across the pump and piping are measured to determine pump performance and to determine the condition of the tubes.

Electrical. Online and Offline testing is used to assess the condition of the motor.

Vibration. Vibration monitoring is used to assess the condition of rotating equipment, specifically compressors, pumps and motors. Additionally, structural problems can be identified through resonance and model testing.

Ultrasonic Testing. Pipe wall thickness is measured to determine erosion and corrosion degradation and also leaking pipes.

Airborne Ultrasonic Testing. Airborne ultrasonic indicates air leaking from control system piping and pumps.

Lubricant Analysis. Oil condition and wear particle analysis are used to identify problems with the lubricant, and to correlate those problems with vibration when wear particle concentrations exceed pre-established limits.

Infrared Thermography. Thermography scans motor control system and electrical distribution junction boxes for high-temperature conditions.

High temperature is indicative of loose connections, shorts, or failing conductor insulation. Piping insulation is checked for porosities. High temperatures are indicative of failed/failing areas in the pipe insulation.

Vibration Analysis

Vibration monitoring might be considered the "grandfather" of condition / predictive maintenance, and it provides the foundation for most facilities' CBM programs.

Vibration usually indicates trouble in the machine. Machine and structures vibrate in response to one or more pulsating forces that may be due to imbalance, misalignment, etc. The magnitude of vibration is dependent on the force and properties of the system, both of which may depend on speed.

There are four fundamental characteristics of vibration: frequency, period, amplitude, and phase. Frequency is the number of cycles per unit time and is expressed in the number of cycles per minute (CPM) or cycles per second (Hz). The period is the time required to complete one cycle of vibration, the reciprocal of frequency. The amplitude is the maximum value of vibration at a given location of the machine. Phase is the time relationship between vibrations of the same frequency and is measured in degrees.

The three key measures used to evaluate the magnitude of vibrations are:

• Displacement

• Velocity

• Acceleration

The units and descriptions of these measures are shown in FIG. 9.

Displacement measurement is dominant at low frequency and is caused by stresses in flexible members of the machine. It’s typically expressed in mils peak-to-peak. Displacement is a good measure for low frequency vibration, usually less than 20 Hz. Velocity is the time-rate change of displacement. It’s dependent upon both displacement and frequency. It’s related to fatigue characteristics of the machine. Greater displacement and frequency of vibration relates directly to a greater severity of machine vibration at the measured location. Velocity is generally used to evaluate machine condition in the frequency range of 10-1,000 Hz. The acceleration is the dominant measure at higher frequencies that exceed 1,000 Hz. Acceleration is proportional to the force on machine components such as gears and couplings. Both velocity and acceleration are expressed in PEAK.

Velocity and acceleration are calculated by the following formulas: Velocity (V) = 2 p f d Acceleration = 2 p f V = (2 p f)^2 x d where f = frequency in cycles per second d = peak displacement

Monitoring the vibration of facility machinery can provide direct correlation between the mechanical condition and recorded vibration data of each machine in the plant. This data can identify specific degrading machine components or the failure mode of plant assets before serious damage occurs.

FIG. 9 Vibration Measures

Vibration monitoring and trending works on the premise that every machine has a naturally correct vibration signature. This signature can be measured when the machine is in good working order, and subsequent measurements can be compared with what is considered the norm. As the component wears or ages, the vibration spectra change. Analyzing the changes identifies components that require further monitoring, repair, or replacement.

With a few exceptions, mechanical troubles in a rotating machine cause vibration. Common problems that produce vibration are:

• Imbalance of rotating parts

• Misalignment of couplings and bearings

• Bent shafts

• Worn, eccentric, or damaged parts

• Bad drive belts and chains

• Damaged / bad bearings

• Looseness

• Rubbing

• Aerodynamic and other forces

Under conditions of dynamic stress, displacement alone may be a better indication of severity, especially when the asset components exhibit the property of brittleness - the tendency to break or snap when stressed beyond a given limit. Consider a slowly rotating machine that operates at 60 RPM, and that exhibits vibration of 20 mils peak-to-peak displacement caused by rotor imbalance. In terms of vibration velocity, 20 mils at 60 CPM (1 Hz) is only 0.0628 in/sec [V = 2(3.14)(1)(0.02/2) = 0.0628]. This level would be considered good for general machinery and little cause for immediate concern. However, keep in mind that the bearing of this machine is being deflected 20 mils. Under these conditions, fatigue may occur due to stress (resulting from the displacement) rather than due to fatigue (caused by the velocity of displacement). Generally, the most useful presentation of vibration data is a graph showing vibration velocity (expressed in inches/second) on the vertical axis and frequency on the horizontal axis. By analyzing this data, a trained vibration technician can ascertain what kinds of problems exist. A trained technician can learn to read vibration signatures and to interpret what the different peaks in the different frequency ranges indicate. For example, when analyzing a 3580 RPM pump motor, a peak at 3580 RPM generally indicates some kind of mass imbalance. A peak at 7160 RPM (two times the rotational frequency) generally indicates a bent shaft.

All rotating machinery will exhibit a certain degree of vibration. The question then becomes "How much is too much?" There are no realistic figures for selecting a vibration limit, which, if exceeded, will result in immediate machinery failure. The events surrounding the development of a mechanical failure are too complex to set reliable limits. However, there are some general guidelines that have been developed over the years that can serve as general indication of the condition of a piece of machinery.

Some of the vibration equipment manufacturing and supplier companies can provide these guidelines and lessons learned.

FIG. 10 lists the forcing frequencies associated with machines as a guideline for possible fault sources.

FIG. 10 Forcing Frequencies Associated with Machines

Types of vibration analysis

• Broadband trending provides a broadband or overall value that represents the total vibration of the machine at the specific measurement point where the data was acquired. It does not provide information on the individual frequency components or machine dynamics that created the measured value. Collected data is com pared either to a baseline reading taken when the machine was new (or sometimes data from a new, duplicate machine) or to vibration severity charts to determine the relative condition of the machine.

• Narrowband trending monitors the total energy for a specific bandwidth of vibration frequencies and is thus more specific.

Narrowband analysis utilizes frequencies that represent specific machine components or failure modes. A narrowband vibration analysis can provide several weeks or months of warning of impending failure. Different vibration frequencies predict different potential failures.

• Signature analysis provides visual representation of each frequency component generated by a machine. With appropriate training and experience, plant personnel can use vibration signatures to determine the specific maintenance required on the machine being studied.

When setting up a vibration monitoring program that uses handheld vibration instrumentation, it’s necessary to ensure that the measurements are taken consistently. A slight variation in the location where a measurement is taken on machinery can significantly alter its accuracy. This becomes an issue especially when several technicians take measurements at different times on the same machinery.

If applied by a trained professional, vibration monitoring can yield information regarding: wear, imbalance, misalignment, mechanical looseness, bearing damage, belt flaws, bent shaft, sheave and pulley flaws, gear damage, flow turbulence, cavitations, structural resonance, and material fatigue.

Detection Interval/Amount of Data Collected

The frequency of data collection depends on machine type and failure category. Typically, spot readings of facility assets with handheld vibration monitoring equipment once per month, per quarter, or per 500 operating hours usually pro vides sufficient warning of impending problems. Facility rotating equipment, e.g., fans and pumps, usually don’t deteriorate fast enough to war rant continual real time data collection. However, critical and expensive assets may warrant having real-time, continuous data collection system.

Spectrum Analysis and Waveform Analysis

Spectrum analysis is the most commonly-employed analysis method for machinery diagnostics. In this type of analysis, the vibration technician focuses on analyzing specific "slices" of the vibration data taken over a certain range of CPM.

Spectrum analysis can be used to identify the majority of all rotating equipment failures (due to mechanical degradation) before failure. Waveform analysis, or time domain analysis, is another extremely valuable analytical tool. Although not used as regularly as spectrum analysis, the waveform often helps the analyst more correctly diagnose the problem.

Shock Pulse Analysis

This type of analysis is used to detect impacts caused by contact between the surfaces of the ball or roller and the race way during rotation of anti-friction bearings. The magnitude of these pulses depends on the surface condition and the angular velocity of the bearing (RPM and diameter). Spike energy is similar in theory to shock pulse.

Alignment

Misalignment of shafted equipment won’t only cause equipment malfunctions or breakdowns; it may also be an indicator of other problems. Checking and adjusting alignment used to be a very slow procedure. The advent of laser alignment systems has reduced labor time by more than half and increased accuracy significantly.

Laser alignment is a natural compliment to vibration analysis.

Properly aligning shafts eliminates one of the major causes of vibration in rotating machines and also drastically extends bearing life. For the mini mal amount of work involved, the payback is great.

Vibration Equipment

For permanent data collection, vibration analysis systems include microprocessor-based data collectors, vibration transducers, equipment-mounted sound discs, and a host personal computer with software for analyzing trends, establishing alert and alarm points, and assisting in diagnostics. Portable handheld data collectors consist of a data collection device about the size of a palm-top computer and a magnetized sensing device.

The effectiveness of vibration monitoring depends on sensor mounting, signal resolution, machine complexity, data collection techniques, and the ability of the analyst. This last factor, the ability of the analyst, is probably the most important aspect of establishing an effective vibration monitoring program. The analyst must be someone who possesses a thorough understanding of vibration theory and the extensive field experience necessary to make the correct diagnosis of the acquired vibration data.

Infrared Thermography

As one of the most versatile condition-based maintenance technologies available, infrared thermography is used to study everything from individual components of assets to plant systems, roofs, and even entire buildings.

Infrared inspections can be qualitative or quantitative. Qualitative inspection concerns relative differences, hot and cold spots, and deviations from normal or expected temperatures. Quantitative inspection concerns accurate measurement of the temperature of the target. One must be careful not to put too much emphasis on the quantitative side of infrared because temperature-based sensors are better for accurate temperature measurements.

Infrared instruments include an optical system to collect radiant energy from the object and focus it, a detector to convert the focused energy pattern to an electrical signal, and an electronic system to amplify the detector output signal and process it into a form that can be displayed.

Most instruments include the ability to produce an image that can be displayed and recorded. These thermographs, as the images are called, can be interpreted directly by the eye or analyzed by computer to produce additional detailed information. Mid-wave range instruments detect infrared in the 2-5 micron range; long-wave range instruments detect the 8-14 micron range. High-end systems can isolate readings for separate points, calculate average readings for a defined area, produce temperature traces along a line, and make isothermal images showing thermal contours.

It’s essential that infrared studies be conducted by technicians who are trained in the operation of the equipment and interpretation of the imagery. Variables that can destroy the accuracy and repeatability of thermal data , for example, must be compensated for each time data is acquired.

Infrared Thermography (IRT) cameras are non-contact, line-of-sight, thermal measurement and imaging systems. Because IRT is a non-contact technique, it’s especially attractive for identifying hot and cold spots in energized electrical equipment, large surface areas such as boilers and building roofs, and other areas where "stand-off" temperature measurement is necessary. Instruments that perform this function detect electro magnetic energy in the short wave (3-5 microns) and long wave (8-15 microns) bands of the electromagnetic spectrum.

Because of the varied inspections (electrical, mechanical, and structural) encountered, the short wave instrument is the best choice for facility inspections. However, the short wave instrument is more sensitive than long wave to solar reflections. Sunlight reflected from shiny surfaces may make those surfaces appear to be "hotter" than the adjacent surfaces when they really are not. IRT instruments-cameras are portable, usually sensitive to within 0.20oC over a range of temperatures from -100 to +3000oC, and accurate within +/-3 percent. In addition, the instrument can store images for later analysis.

IRT inspections attempt to accurately measure the temperature of the item of interest. To perform an inspection requires knowledge and under standing of the relationship of temperature and radiant power, reflection, emittance, and environmental factors, as well as the limitations of the detection instrument. This knowledge must be applied in a methodical manner to control the imaging system properly and to obtain accurate temperature measurements.

The qualitative inspections are significantly less time-consuming because the thermographer is not concerned with highly-accurate temperature measurement. In qualitative inspections, the thermographer obtains accurate temperature differences (?T) between like components. For example, a typical motor control center will supply three-phase power, through a circuit breaker and controller, to a motor. Ideally, current flow through the three-phase circuit should be uniform so the components within the circuit should have similar temperatures. Any uneven heating, perhaps due to dirty or loose connections, would quickly be identified with the IRT imaging system.

IRT can be used very effectively to identify degrading conditions in facilities' electrical systems such as transformers, motor control centers, switchgear, substations, switch yards, or power lines. In mechanical systems, IRT can identify blocked flow conditions in heat exchangers, condensers, transformer cooling radiators, and pipes. IRT can also be used to verify fluid level in large containers such as fuel storage tanks. IRT can identify insulation system degradation in building walls and roofs, as well as refractory in boilers and furnaces. Temperature monitoring, infrared thermography in particular, is a reliable technique for finding the moisture-induced temperature effects that characterize roof leaks, and for determining the thermal efficiency of heat exchangers, boilers, building envelopes, etc.

Deep-probe temperature analysis can detect buried pipe energy loss and leakage by examining the temperature of the surrounding soil. This technique can be used to quantify ground energy losses of pipes. IRT can also be used as a damage control tool to locate mishaps such as fires and leaks.

Thermography is limited to line of sight. Errors can be introduced due to color of material, material geometry, and by environmental factors such as solar heating and wind effects. Emissivity is a key concern which can introduce 5-20% error in measurements. Shiny or highly polished surfaces can be very tricky to measure; even dull smooth metal surfaces may not be equally emissive in all directions. Be especially careful where surfaces are highly curved. If the emissivity is very low, some polished surfaces have an emissivity of 0.2 or less, and then an accurate reading is unlikely to be possible. Many low cost Infrared cameras have a fixed emissivity (usually around 0.95), which is a reasonable value in normal circumstances.

Because IRT images are complex and difficult to measure and analyze, training is required to obtain and interpret accurate and repeatable thermal data and to interpret the data. With adequate training and certification, electrical/mechanical technicians and engineers can perform this technique.

Training is available through infrared imaging system manufacturers and vendors. Also, the American Society of Non-destructive Testing (ASNT) has established guidelines for nondestructive testing (NDT) thermographer certification. These guidelines, intended for use in nondestructive testing, may be used as guidelines for thermography in CBM if appropriately applied.

Ultrasonic Testing

Ultrasonic testing is extremely useful in the diagnosis of mechanical and electrical problems. Testing instruments are usually portable hand held devices. Their electronic circuitry converts a narrow band of ultra sound (between 20 and 100 kHz) into the audible range so that a user can recognize the qualitative sounds of operating equipment through head phones. Intensity of signal strength is also displayed on the instrument.

Ultrasonic instruments-scanners are most often used to detect gas, liquid, or vacuum leaks.

Ultrasonic detectors are somewhat limited in their use. For example, they may help identify the presence of suspicious vibrations within a machine, but they are generally not sufficient for isolating the sources or causes of those vibrations.

On the plus side, ultrasonic monitoring is easy, it requires minimal training, and the instruments are inexpensive. Airborne ultrasonic devices are highly sensitive listening "guns" (similar in size to the radar speed guns used by police at speed traps). They provide a convenient, non-intrusive means of assessing asset condition. Airborne ultrasonic monitoring is especially easy and useful in testing remote electrical equipment, as well as shielded electrical equipment, e.g., connections inside switchgear and panels. In the case of high voltage insulator failures, airborne ultrasonic devices can often detect faults earlier than infrared thermography can.

Except for severe cases where a current path to ground was established, infrared thermography would not detect high-voltage insulation failures because the corona or tracking typically produces little or no heat.

Airborne ultrasonic devices can also detect the noise caused by loose connections as they vibrate inside of panels.

Airborne ultrasonic devices operate in the frequency range from 20 to 100 kHz and translate the high frequency signal to a signal within the audible human range. This allows the operator to hear changes in noise levels associated with air leaks, corona discharges, and other high frequency events. For example, a maintenance technician could use ultrasonic equipment to "hear" a bearing ring and surrounding housing resonating at the resonant frequency. Once detected, a maintenance technician could then proceed to find the cause of the problem; insufficient lubrication or minor bearing material defects would be the likely cause of this malfunction.

Some of the most common applications of ultrasound detection are:

• Leak detection in pressure and vacuum systems (e.g., boiler, heat exchanger, condensers, chillers, vacuum furnaces, specialty gas systems)

• Bearing inspection

• Steam trap inspection

• Pump cavitations

• Detection of corona in electrical switchgear

• Valve analysis

• Integrity of seals and gaskets in tanks and pipe systems

All operating equipment and most leakage problems produce a broad range of sound. The high frequency ultrasonic components of these sounds are extremely short wave in nature, and a short wave signal tends to be fairly directional. It’s therefore easy to isolate these signals from background noises and detect their exact location. In addition, as subtle changes begin to occur in mechanical equipment, the nature of ultrasound allows these potential warning signals to be detected early - before actual failure.

Leak Detection in Mechanical Systems

Ultrasound is a very versa tile technique that detects the sound of a leak. When a fluid (liquid or gas) leaks, it moves from the high pressure side through the leak site to the low pressure side, where it expands rapidly and produces a turbulent flow.

This turbulence has strong ultrasonic components. The intensity of the ultrasonic signal falls off rapidly from the source, allowing the exact spot of a leak to be located.

Generally gas leak detection is easy to apply. An area should be scanned while listening for a distinct rushing sound. With continued sensitivity adjustments, the leak area is scanned until the loudest point is heard.

Some instruments include a rubber focusing probe that narrows the area of reception so that a small emission can be pinpointed. The rubber focusing probe is also an excellent tool for confirming the location of a leak. This is done by pressing it against the surface of the suspected area to determine if the sound of the leak remains consistent. If it decreases in volume, the leak is elsewhere.

Vacuum leaks may be located in the same manner; the only difference being that the turbulence will occur within the vacuum chamber. For this reason, the intensity of the sound will be less than that of a pressurized leak. Though it’s most effective with low-mid to gross leaks, the ease of ultrasound detection makes it useful for most vacuum leak problems.

Liquid leaks are usually determined through valves and steam traps, although some successes have been reported in locating water leaks from pressurized pipes buried underground. A product can be checked for leak age if it produces some turbulence as it leaks.

Valves are usually checked for leakage with the contact probe on the downstream side. This is accomplished by first touching the upstream side and adjusting the sensitivity to read about 50% of scale. The downstream side is then touched and the sound intensity is compared. If the signal is lower than upstream, the valve is considered closed; if it’s louder than upstream and is accompanied by a typical rushing sound, it’s considered to be leaking.

Steam traps are also inspected easily with ultrasonic translators.

During the steam trap operation and while observing the meter, trap conditions can be interpreted. The speed and simplicity of this type of test can allow every trap in a plant to be routinely inspected.

Leaking tubes in heat exchangers and condensers as well as boiler casing leaks are detectable with ultrasonic translators. In most power plants, the problem of condenser in-leakage is a major concern.

Condenser fittings are often routinely inspected using the leak detection method previously described. If a leak is suspected in a condenser tube bundle, it’s possible to locate the leak by putting a condenser at partial load and opening up a water box of a suspected tube bundle. After the tube sheet is cleared of debris, the tube sheet is scanned.

Bearing Inspections

Ultrasonic inspection and monitoring of bearings is a reliable method for detecting incipient bearing failure. The ultra sonic warning appears prior to a rise in temperature or an increase in driving torque. Ultrasonic inspection of a bearing is useful in recognizing the beginning of fatigue failure, brinelling of bearing surfaces, or flooding of (or lack) of lubricant. In ball bearings, as the metal in the raceway, roller, or bearing balls begins to fatigue, a subtle deformation begins to occur.

This deforming of the metal will produce an increase in the emission of ultrasonic sound waves.

It’s observed that as the lubricant film reduces, the sound level will increase. A rise of about 8 dB over baseline accompanied by a uniform rushing sound will indicate lack of lubrication. When lubricating, add just enough to return the reading to baseline. Some lubricants will need time to run in order to cover the bearing surfaces uniformly.

One of the most frequent causes for bearing failure is over-lubrication. The excess stress of lubricant often breaks bearing seals or causes a buildup of heat, which can create stress and deformity. To avoid over lubrication, don’t lubricate if the baseline reading and baseline sound quality is maintained. When lubricating, use just enough lubricant to bring the ultrasonic reading to baseline. Recently new grease guns have become available in the market with built-in ultrasonic systems that can provide the appropriate amount of grease. These are very practical and easy to use devices.

Detection in Electrical Systems

Three types of high voltage electrical problems detectable with ultra sound are:

• Arcing. An arc occurs when electricity flows through space.

Lightning is a good example.

• Corona. When voltage on an electrical conductor, such as an antenna or high voltage transmission line, exceeds threshold value, the air around it begins to ionize to form a blue or purple glow.

• Tracking. Often referred to as "baby arcing," electricity follows the path of damaged insulation, using surrounding dirt, debris, and moisture as the conductive medium.

Theoretically, ultrasonic detection can be used in low, medium, and high voltage systems; however, applications have only normally been used with medium and high voltage systems. When electricity escapes in high voltage lines or when it jumps across a gap in an electrical connection, it disturbs the air molecules around it and generates ultrasound.

Often this sound will be perceived as a crackling or frying sound. In other situations, it will be heard as a buzzing sound. In substations and distribution systems, components such as insulators, transformers, cable, switchgear, bus bars, relays, contractors, junction boxes, and bushings may be tested.

Ultrasonic testing is often used for evaluation at voltages exceeding 2,000 volts, especially in enclosed switchgear. This is especially useful in identifying corona problems. In enclosed switchgear, the frequency of detection of corona greatly exceeds the frequency of serious faults identified by infrared. It’s recommended that both tests be used with enclosed switchgear. When testing electric equipment, be sure to follow safety procedures.

The method for detecting electric arc and corona leakage is similar to that discussed in mechanical leak detection. Instead of listening for a rushing sound, a user will listen for a crackling or buzzing sound. Determining whether a problem exists is relatively simple. By comparing sound quality and sound levels among similar equipment, the problem will become easy to identify, even though the sound itself will differ somewhat as it resonates through various types and sizes of equipment.

On lower voltage systems, a quick scan of bus bars will often pick up a loose connection. Checking junction boxes can reveal arcing. As with leak detection, the closer one gets to the leak site, the louder the signal. If power lines are to be inspected and the signal does not appear to be intense enough to be detectable from the ground, a parabolic reflector which is an ultrasonic waveform concentrator, will increase the detection distance of the system and provide pinpoint detection.

Lubricant (Oil) and Wear Particle Analysis

The objective of oil analysis is to determine:

• An asset's mechanical wear condition

• Lubricant condition

• If the lubricant has become contaminated

A wide variety of tests can provide information regarding one or more of these areas. The three areas are not unrelated. Changes in lubricant condition and contamination, if not corrected, will lead to machine wear.

Lubricant Condition

Bad lubricating oil is either discarded or reconditioned through filtering or by replacing additives. Analyzing the oil to determine the lubricant condition is, therefore, driven by costs. Small machines with small oil reservoirs have the oil changed on an operating time basis. An automobile is the most common example of time-based lubricating oil maintenance. In this example, the costs to replace the auto mobile oil change (which includes the replacement oil, labor to change the oil, and disposal costs) are lower than the cost to analyze the oil, e.g., the cost of sample materials, labor to collect the sample, and the analysis. In the case of automobile oil, time-based replacement is cheaper than analysis due to competition and the economies of scale that have been created to meet the consumer need for replacing automobile oil.

In an industrial set-up, lubricating oil can become contaminated due to the machine's operating environment, improper filling procedures, or through the mixing of different lubricants in the same machine. If a machine is "topped off" with oil frequently, we should periodically send the oil out for analysis to check the machine for any serious problems.

The full benefit of oil analysis can be achieved only by taking frequent samples and trending the data for each asset in the program. The length of the sampling intervals varies with different types of equipment and operating conditions. Based on the results of the analyses, lubricants can be changed or upgraded to meet the specific operating requirements.

It cannot be overemphasized that the sampling technique is critical to meaningful oil analysis. Sampling locations must be carefully selected to provide a representative sample and sampling conditions should be uniform so that accurate comparisons can be made.

Standard Analytical Test Types

Lubricating oil and hydraulic fluid analysis should proceed from simple, subjective techniques such as visual and odor examination through more sophisticated techniques. The more sophisticated tests should be performed when conditions indicate the need for additional information and based on asset criticality.

Visual and Odor

Simple inspections can be performed weekly by the equipment operator to look at and smell the lubricating oil. A visual inspection looks for changes in color, haziness or cloudiness, and particles. This test is very subjective, but can be an indicator of recent water or dirt contamination and advancing oxidation. A small sample of fresh lubricating oil in a sealed, clear bottle can be kept on hand for visual comparison. A burned smell may indicate oxidation of the oil. Other odors could indicate contamination. Odor is more subjective than the visual inspection because people's sensitivity to smell varies, and there is no effective way to compare the odor between samples. The operator must also be careful not to introduce dirt into the system when taking a sample.

Viscosity is one of the most important properties of lubricating oil; it’s often referred to as the structural strength of liquid. The analysis consists of comparing a sample of oil from an asset to a sample of unused oil to determine if thinning or thickening of the oil has occurred during use.

Viscosity is critical to oil film control and is a key indicator of machine oil condition.

Viscosity is a measure of oil's resistance to flow at a specified temperature. A change (increase or decrease) in viscosity over time indicates changes in the lubricant condition, or it may indicate lubricant contamination. Viscosity can be tested using portable equipment, or it can be tested more accurately in a laboratory using the ASTM D445 standard. Viscosity is measured in centistoke (cSt) at 40°C, and minimum and maximum values are identified by the ISO grade.

Water (Moisture) Test

Water is generally referred to as a chemical contaminant when suspended in oils. Its effects in bearings, gearing, and hydraulic components can be very destructive. Like particles, control must be established to minimize water accumulation to the oil and machines.

Water in lubricating oil and hydraulic fluid contributes to the corrosion and formation of acids. Small amounts of water (less than 0.1 per cent) can be dissolved in oil and can be detected using the crackle test or infrared spectroscopy (minimum detectable is approximately 500 ppm), the ASTM D95 distillation method (minimum 100 ppm). If greater than 0.1 percent water is suspended or emulsified in the oil, the oil will appear cloudy or hazy. Free water in oil collects in the bottom of oil reservoirs and can be found by draining them from the bottom of the reservoir.

Using a titration process with a Karl Fischer method, low levels of water can be detected and quantified. The volumetric titration test uses ASTM D1744, and Coulometric titration uses ASTM D4928. This test is useful when accepting new oil.

Wear Particle Count

High particle counts indicate that machinery may be wearing abnormally or that failures could be caused by blocked orifices. Particle count tests are especially important in hydraulic systems.

The wear particle test emphasizes the detection and analysis of current machine anomalies -- the symptoms of failure. The oil serves as the messenger of information on the health of the machine. When a machine component is experiencing some level of failure such as rubbing, it will shed particles in the oil. The presence of abnormal level of wear particles, their size, shape, color, orientation, and elements define the cause, source, and severity of the condition.

Total Acid Number (TAN)

Total acid number (TAN) is a measure of the amount of acid or acid-like materials in oil. It’s an indicator of the lubricating oil condition and is monitored relative to the TAN of new oil.

In some systems, the TAN will also be used to indicate acid contamination. TAN is measured in milligrams of potassium hydroxide (KOH) per gram of oil (mg KOH/g). KOH is used in a titration process and the end point is indicated by color change (ASTM D974) or electrical conductivity change (ASTM D664). Total Base Number (TBN) Total base number (TBN) indicates oil's ability to neutralize acidity. Low TBN is often an indicator that the wrong oil is being used for the application, intervals between oil changes are too long, oil has been overheated, or a high-sulfur fuel is being used.

Spectrometric Metals Analysis

Also known as emission spectroscopy, this test examines the light (spectrum) emitted from the sample during testing, and identifies approximately 21 metals. Metals are categorized as wear, contaminate, or additive metals. The procedure identifies both soluble metal and metal particles.

Infrared Spectroscopy

This test is also known as infrared analysis, infrared absorption spectroscopy or spectrophotometry, and Fourier Transform Infrared (FTIR) spectroscopy. The technique examines the infrared wavelength that is absorbed by the oil sample. The test is used to identify nonmetallic contamination and lubricant conditions (e.g., oxidation, anti-oxidant, other additive depletion). Connecting a computer expert system with known oil spectrums can produce highly accurate diagnosis of small changes in the oil condition.

Analytical Ferrography

Analytical ferrography is often initiated based on changes in Direct Reading (DR) indicating increases in metal or particle counts. DR Ferrograph quantitatively measures the concentration of ferrous wear particles in lubricating or hydraulic oil.

Analytical ferrography is qualitative and requires visual examination and identification of wear particles. Properties and features of the wear debris are inventoried and categorized. This includes size, shape, texture, color, light effect, density, surface oxide, etc.

This analysis is sometimes performed on a regular basis on expensive or critical machines. The test process is labor intensive and involves the preparation of a sample and examination under magnification. Results vary with the analyst's capability, but the procedure can provide detailed information regarding wear: e.g., wear type (rubbing, sliding, cutting), color, particle types (oxide, corrosive, crystalline), and other nonferrous particles. This detailed information can be critical in finding the root cause of wear problems.

Foaming

Some oils may have anti-foam agents added to improve the lubrication capability in specific applications such as gear boxes or mixers. ASTM test D892 can be used to test the oil's foam characteristics. The test blows air through a sample of the oil and measures the foam volume.

Rust Prevention

Some systems are susceptible to water contamination due to equipment location or the system operating environment. In those cases, the lubricating oil or hydraulic fluid may be fortified with an inhibitor to prevent rust. The effectiveness of rust prevention can be tested using ASTM D665 (or ASTM D3603). Rotating Bomb Oxidation Test (RBOT)

Also known as the Rotary Bomb Oxidation Test, ASTM D 2272 is used to estimate oxidation stability and the remaining useful life of oil. The test simulates aging, identifying when rapid oxidation takes place, and indicating that anti-oxidants have been depleted. The test is not a one-time test; it must be performed over time, starting with a baseline test of the new oil. Subsequent tests are necessary to develop the trend line. Because of the high cost and the multiple tests required, this test is usually only performed on large volume reservoirs or expensive oil.

Sampling and Frequency

Oil samples must be collected safely and in a manner that won’t introduce dirt and other contaminates into the machine or system, or into the sample. It may be necessary to install permanent sample valves in some lubricating systems. The oil sample should be representative of the oil being circulated in the machine. The sample should, therefore, be collected from a mid-point in reservoirs and upstream of the filter in circulating systems. Clean sample collection bottles and tubing must be used to collect the sample. Oil sample pumps for extracting oil from reservoirs are used to avoid contamination. Samples must be collected from the same point in the system to ensure consistency in the test analysis. Therefore, the maintenance procedure must provide detailed direction on where and how to collect samples. It’s a good practice to have operators that use the equipment collect the samples.

Typically, lubricating oil analysis should be performed on a quarterly, yearly, or 500 / 2000 hours of operations basis for most of the assets. The analysis schedule should be adjusted based on asset usage, criticality, and cost. Analyze more frequently for machines that are indicating emerging problems and less frequently for machines that operate under the same conditions and are not run on a continuous basis. A new baseline analysis is recommended following machine repair or oil change out.

Grease is usually not analyzed on a regular basis. Although most of the testing that is done on oil can also be done on grease, getting a representative sample is usually difficult. The machine may have to be disassembled in order to get a good sample that is a homogeneous mixture of the grease, contaminants, and wear.

Oil Contamination Program

A concern common to all machines with lubricating oil systems is keeping dirt and moisture out of the system.

Common components of dirt, such as silica, are abrasive and naturally promote wear of contact surfaces. In hydraulic systems, particles can block and abrade the close tolerances of moving parts. Water in oil pro motes oxidation and reacts with additives to degrade the performance of the lubrication system. Ideally, there would be no dirt or moisture in the lubricant; this, of course, is not possible. The lubricant analysis program must therefore monitor and control contaminants.

Oil analysis is a reliable predictive maintenance tool and is very effective in detecting contaminants in oil that are a result of ingressed dirt or internal wear debris generated by the effect of machine degradation and wear. An increase in contaminant levels accelerates the wear-out process of all components in industrial machine applications. Oil cleanliness is measured by ISO standard, ISO 4406. For each numerical increase in ISO contaminant level, the amount of contaminants in the oil almost doubles.

If the standard is a 16/14/11, then the increase in contaminants in the oil for a 22/21/17 is about 64 times dirtier than the standard.

Contaminants in oils can be prevented. Good filtration on the return side of hydraulic power units will help in taking out dirt and other ingressed particles. Usually 3-micron filtration with a 200 beta ratio is the standard set for most machinery.

Large systems with filters will have steady-state levels of contaminants. Increases in contaminates indicate breakdown in the system's integrity (leaks in seals, doors, heat exchangers, etc.) or degradation of the filter. Use of "Air Breathers" in gearboxes and hydraulic system tanks is a best practice to control contaminants. Unfiltered systems can exhibit steady increases during operation. Operators can perform a weekly visual and odor check of lubricating systems and provide a first alert of contamination. Some bearing lubricating systems may have such a small amount of oil that a weekly check may not be cost effective.

A basic oil contamination control program can be implemented in three steps:

1. Establish the target fluid cleanliness levels for each machine fluid system.

2. Select and install filtration equipment (or upgrade current filter rating) and contaminant exclusion techniques to achieve target cleanliness levels.

3. Monitor fluid cleanliness at regular intervals to achieve target cleanliness levels.

It’s a good practice to establish a quality control program for incoming oil. Set up a minimum oil cleanliness standard (ISO) for all oils, new or old, before they are used in the machines.

Electrical Condition Monitoring

Electrical condition monitoring encompasses several technologies and techniques used to provide a comprehensive system evaluation.

Electrical equipment represents a major portion of a facility's capital investment. From the power distribution system to electric motors, efficient operation of the electrical systems is crucial to maintaining operational capability of a facility.

Monitoring key electrical parameters provides the information to detect and correct electrical faults such as high resistance connections, phase imbalance, and insulation breakdown. Because faults in electrical systems are seldom visible, these faults are costly due to increased electrical usage and increased safety concerns. They involve life cycle cost issues due to premature replacement of expensive assets. According to the Electric Power Research Institute (EPRI), voltage imbalances of as little as 5% in motor power circuits result in a 50% reduction in motor life expectancy and efficiency in a three-phase AC motors. A 2.5% increase in motor temperatures can be generated by the same 5% voltage imbalance accelerating insulation degradation.

Monitoring intervals of several weeks to several months for various technologies will provide sufficient condition information to warn of degrading equipment condition. Specific expectations of the length of warning provided should be factored into developing monitoring intervals for specific technologies.

Until fairly recently, predictive maintenance technologies for motors were limited to vibration testing, high-voltage surge testing for winding faults, meg-Ohm and high-potential tests for insulation resistance-to ground, and voltage and current tests for testing phase balance. Many of these tests still have their place in plant maintenance, but several of them are dangerous or harmful when tests are conducted with motors in place.

New technologies allow for portable, safe, and trendable tests that can be used for more accurate troubleshooting or identifying problem areas.

Each of these technologies has its strengths and weaknesses. But as part of a CBM program, they can accurately detect potential faults and avoid costly downtime.

Electrical equipment evaluation can be divided into two categories: online monitoring/testing and offline testing. Online monitoring/testing is the measuring of any aspect of electrical equipment while it’s in service and operating. Offline testing is usually done by inducing voltage or cur rent into equipment and taking electrical measurements, which requires that the equipment be de-energized and completely isolated from its nor mal circuit. Both are very valuable in evaluating an electrical system; in most cases, they detect different types of faults or potential problems, meaning much of online monitoring/testing cannot replace offline testing and vice versa.

Online monitoring is continuous whereas online testing takes measurements at periodic intervals. Some examples of online monitoring include dissolved gas analysis for transformers, temperature monitoring for motors and transformers, power quality, and partial discharge. An example of online testing is current signature analysis, which can detect broken rotor bars and air gap eccentricity in squirrel cage motors. Current signature analysis is also capable of detecting mechanical imbalances, gear mesh problems, broken fan blades, bearing issues, and any other type of problem that results in torque pulsations that result in changes to the current draw. Currently, some of the mechanical faults such as bearing damage are more clearly detected through vibration analysis. All online testing could be converted to continuous monitoring, but the amount of data acquired and cost of online testing equipment makes portable devices and periodic testing more feasible in some cases.

Much of offline testing often involves evaluating the equipment's insulation. Other tests, such as low ohm resistance measuring, will inspect other aspects of the equipment such as high resistance connections. There are a wide array of tests available that use AC signals, DC signals, and varying frequency signals. The most common maintenance insulation tests are DC insulation resistance tests, step voltage tests, high potential tests, dissipation/power factor tests, and the transformer turns ratio test.

Note that insulation tests are sensitive to environmental factors such as temperature, humidity, and contamination or cleanliness of the insulation.

Insulation testing is standard industry practice and crucial in determining the condition of the electrical insulation.

Several of the technologies outlined below are also effective when used for acceptance testing and certification for new systems.

Motor Current Readings

Clamp-on ammeter attachments provide the capability of taking actual current draw information while the equipment is operating. On three-phase equipment, comparison of current draws can reveal phase imbalance conditions.

Motor Current Signature Analysis (MCSA)

MCSA is performed by taking current data and analyzing it using Fourier transform analysis. The primary purpose of the test is rotor bar fault detection, but it’s also useful for detecting rotor faults and power quality problems as well as other motor and load defects in later stages of failure. Motors must be energized and loaded during tests.

MCSA is a proven method of detecting the presence of broken or cracked rotor bars or high resistance connections in end rings. Motor current spectrums in both time and frequency domains are collected with a clamp-on ammeter and Fast Fourier Transform (FFT) analyzer. Rotor bar problems will appear as side-bands around the power supply line frequency. MCSA evaluates the amplitude of the side bands that occur about the line frequency.

AC High Potential Testing (HiPot)

HiPot testing applies a voltage equal to twice the operating voltage plus 1000 volts to motor windings to test the insulation system. This is typically a "go/no-go" test. Industry practice calls for HiPot tests on new and rewound motors. This test stresses the insulation systems and can induce premature failures in marginal motors. Due to this possibility, AC HiPot is not recommended as a routinely repeated condition monitoring technique, but as an acceptance test.

Surge Comparison Testing

This testing uses high-voltage pulses to detect winding faults. Only experienced operators should conduct these tests because of the potentially harmful effects of high voltage impressed on used windings and cables. There are also challenges with testing assembled motors due to rotor effects on the motor circuit. The motor being tested must be de-energized with controls disconnected.

Surge

Testing uses equipment based on two capacitors and an oscilloscope to determine the condition of motor windings. This is a comparative test evaluating the difference in readings of identical voltage pulses applied to two windings simultaneously. This test is primarily an acceptance go/no-go test. Data are provided as a comparison of waveforms between two phases indicating the relative condition of the two phases with regard to short circuits. The readings for a particular motor can be trended, but the repeated stress of the insulation system is not recommended.

Conductor Complex Impedance

The total resistance of a conductor is the sum of its resistance, capacitive impedance, and inductive impedance. Accurate measurement of the conductor impedance allows minor degradations in a motor to be detected and addressed prior to motor failure. The condition of the insulation system can be determined by measuring the capacitance between each phase and ground. The presence of moisture (or other conducting substance) will form a capacitor with the conductor being one plate, the insulation being the dielectric, and the contaminate forming the second plate. Maintaining proper phase balance is imperative to efficient operation and realizing the full lifetime of electrical equipment.

Megohmmeter Testing

A handheld meter is used to measure the insulation resistance phase-to-phase or phase-to-ground of an electric circuit. Readings must be temperature-corrected to trend the information, as the winding temperatures affect the test results.

Polarization

Index is another term which gives an idea of the cleanliness of the motor or generator windings. It’s a ratio of the Insulation Resistance Measured for 10 minutes to the insulation resistance value measured after 1 minute. Because it’s a ratio; it does not have any units.

Polarization Ratio = Insulation Resistance after 10 minutes / Insulation Resistance after 1 minute The recommended minimum value of the polarization index for AC and DC motors and generators is 2.0. Machines having windings with a lower index are less likely to be suited for operation. The procedure for determining the polarization index is covered in detail by IEEE Standard No. 43.

Time Domain Reflectometry

In this test, a voltage spike is sent through a conductor. Each discontinuity in the conductor path generates a reflected pulse. The reflected pulse and time difference between initial pulse and reception of the reflected pulse indicate the location of the discontinuity.

Radio Frequency (RF) Monitoring

RF monitoring is used to detect arcs caused by broken windings in generators. It consists of establishing RF background levels and the amplitude trend over a narrow frequency band.

Power Factor and Harmonic Distortion

Maintaining optimum power factor maximizes the efficient use of electrical power. The power factor is the ratio of real power to reactive power usage. Dual channel data-loggers are used to determine the phase relationship between voltage and current, then to calculate the power factor. If this process detects a low power factor, subsequent engineering analysis will be required to devise a means of improving power system power factor.

Application of Various. Technologies to Electrical Assets Specific electrical assets that can be monitored by CBM technologies are:

• Electrical Distribution Cabling. Megohmmeter, Time Domain Reflectometry, HiPot, Infrared Thermography(IRT), and Airborne Ultrasonic.

• Electrical Distribution Switchgear and Controllers. Timing, Visual Inspection, IRT, and Airborne Ultrasonic

• Electrical Distribution Transformer. Oil Analysis, Turns Ratio, Power Factor, and Harmonic Distortion

• Electrical Motors. Current Draw, Motor Current Spectrum Analysis, Motor Circuit Analysis, Megohmmeter, HiPot, Surge Test, Conductor Complex Impedance, Starting Current, and Coast Down Time

• Generators. Megohmmeter, RF, and Coast-Down Time Technologies Limitations

The technologies discussed earlier can be divided into two categories:

• Energized. These technologies can safely provide information on energized systems and require the system be energized and operational. They include IRT, Ultrasonics, Motor Current Readings, Starting Current, Motor Current Spectrum Analysis, RF, Power Factor, and Harmonic Distortion.

• De-Energized. These technologies require the circuit to be de energized for safe usage. They include Surge Testing, HiPot Testing, Time Domain Reflectometry (TDR), Megohmmeter, Motor Circuit Analysis, Transformer Oil Analysis, Turns Ratio, and Conductor Complex Impedance.

Each technology will require specific initial conditions to be set prior to conducting the test. For instance, prior to an IRT survey, typical equipment powered through the switchboard should be running to bring the distribution equipment to normal operating temperatures. Higher load accentuates problem areas. Conducting the survey at low load conditions may allow a problem to remain undetected.

HiPot and surge testing should be performed with caution. The high voltage applied during these tests may induce premature failure of the units being tested. For that reason, these tests normally are performed only for acceptance testing, not for condition monitoring.

Other Miscellaneous Non-Destructive Testing

Non-Destructive Testing (NDT) evaluates material properties and quality of expensive components or assemblies without damaging the product or its function. Typically, NDT has been associated with the welding of large high-stress components such as pressure vessels and structural supports. Process plants such as refineries or chemical plants use NDT techniques to ensure integrity of pressure boundaries for systems processing volatile substances. The following are various NDT techniques.

Radiography

Radiography is performed to detect sub-surface defects. Radiography or X-ray is one of the most powerful NDT techniques available in industry. Depending on the strength of the radiation source, radiography can provide a clear representation (radiograph) of discontinuities or inclusions in material several inches thick. X-ray or gamma ray sensitive film is placed on one surface of the material to be examined. The radiation source is positioned on the opposite side of the piece. The source may be either a natural gamma emitter or a powered X ray emitter. The source is accurately aligned to ensure the proper exposure angle through the material. When all preparations and safety precautions are complete, the radiation source is energized or unshielded.

Radiography, though a versatile tool, is limited by the potential health risks. Use of radiography usually requires the piece be moved to a special shielded area, or that personnel be evacuated from the vicinity to avoid exposure to the powerful radiation source required to penetrate several inches of dense material.

Ultrasonic Testing (Imaging)

Ultrasonic imaging provides detection of deep sub-surface defects. Ultrasonic inspection of welds and base material is often an alternative or complementary NDT technique to radiography. Though more dependent on the skill of the operator, ultrasonic inspection does not produce the harmful radiation experienced with radiography. Ultrasonic inspection is based on the difference in the wave reflecting properties of defects and the surrounding material. An ultrasonic signal is applied through a transducer into the material being inspected.

The speed and intensity with which the signal is transmitted or reflected to a transducer provides a graphic representation of defects or discontinuities within the material.

Due to the time and effort involved in surface preparation and testing, ultrasonic inspections are often conducted on representative samples of materials subjected to high stress levels, high corrosion areas, and large welds.

Magnetic Particle Testing (MPT)

Magnetic Particle Testing uses magnetic particle detection of shallow sub-surface defects. It’s a very useful technique for localized inspections of weld areas and specific areas of high stress or fatigue loading. MPT provides the ability to locate shallow sub-surface defects. Two electrodes are placed several inches apart on the surface of the material to be inspected. An electric current is passed between the electrodes producing magnetic lines. While the current is applied, iron ink or powder is sprinkled in the area of interest. The iron aligns with the lines of flux. Any defect in the area of interest will cause distortions in the lines of magnetic flux, which will be visible through the alignment of the powder. Surface preparation is important because the powder is sprinkled directly onto the metal surface and major surface defects will interfere with sub-surface defect indications. Also, good electrode contact and placement is important to ensure consistent strength in the lines of magnetic flux.

A major advantage of MPT is its portability and speed of testing. The handheld electrodes allow the orientation of the test to be changed in seconds. This allows for inspection of defects in multiple axes of orientation.

Multiple sites can be inspected quickly without interrupting work in the vicinity. The equipment is portable and is preferred for onsite or in-place applications. The results of MPT inspections are recordable with high quality photographs.

Hydrostatic Testing

Hydrostatic Testing is another NDT method for detecting defects that completely penetrate pressure boundaries.

Hydrostatic Testing is typically conducted prior to the delivery or operation of completed systems or subsystems that act as pressure boundaries.

During the hydrostatic test, the system to be tested is filled with water or the operating fluid. The system is then sealed and the pressure is increased to approximately 1.5 times operating pressure.

This pressure is held for a defined period. During the test, inspections are conducted to find visible leaks as well as monitor pressure drop and make-up water additions. If the pressure drop is out of specification, any leaks must be located and repaired. The principle of hydrostatic testing can also be used with compressed gases. This type of test is typically called an air drop test and is often used to test the integrity of high pres sure air or gas systems.

Eddy Current Testing

Eddy current testing is used to detect surface and shallow subsurface defects. Also known as electromagnetic induction testing, eddy current testing provides a portable and consistent method for detecting surface and shallow subsurface defects. This technique provides the capability of inspecting metal components quickly for defects or homogeneity. By applying rapidly varying AC signals through coils near the surface of the test material, eddy currents are induced into conducting materials. Any discontinuity that affects the material's electrical conductivity or magnetic permeability will influence the results of this test.

Component geometry must also be taken into account when analyzing results from this test.

Why Have a CBM Program

Condition Based Maintenance can:

• Warn of most problems in time to minimize unexpected failure, the risk and consequences of collateral damage, and adverse impact on safety, operations, and the environment. It will reduce the number of preemptive corrective actions.

• Increase equipment utilization and life; minimize disruption to mission and schedule. It will decrease asset and process down time, resulting in increased availability.

• Reduce maintenance costs - both parts and labor.

• Reduce a significant amount of calendar / run-based preventive maintenance.

• Minimize cost and hazard to an asset that result from unnecessary overhauls, disassemblies, and PM inspections.

• Increase the likelihood that components operate to optimum life time. In some cases, replacement prior to end-of-life is more efficient for meeting operational requirements and optimum cost.

• Reduce requirements for emergency spare parts.

• Increase awareness of asset condition.

• Provide vital information for continuous improvement, work, and logistic planning.

• Improve worker safety.

• Increase energy savings.

However, CBM cannot:

• Eliminate defects and problems, or stop assets from deteriorating.

• Eliminate all preventive maintenance.

• Reliably and effectively warn of fatigue failures.

• Reduce personnel or produce a major decrease in lifetime maintenance costs without a commitment to eliminating defects and chronic problems.

CBM is not a "silver bullet." Some potential failures, such as fatigue, or uniform wear on a blower fan are not easily detected with condition measurements. In other cases, sensors may not be able to survive in the environment; measurements to assess condition may be overly difficult and may require major asset modifications.

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