DoctorKnow Application Paper

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1 DoctorKnow Application Paper Title: Fundamentals of Ultrasonics Source/Author: Bob Cecil EGroup: Product: Ultrasonics Program: Technology: Ultrasonics Classification: Basic Section 1 Theory Sound Theory Definition of Sound Sound is the physical quality of waves traveling through air that stimulates the sense of hearing. The source of a sound has some vibrating property. The waves created by the vibration travel through a medium and the listener detects them as sound. Three stages of matter act as a medium - solids, liquids and gases. Each form of matter transfers vibration energy with different levels of impedance. Solid forms of matter have particles that are relatively fixed with a definite shape and volume. Liquid forms of matter have particles that are free and vibrate about moving points. Container determines the shape of this form of matter. Gas form of matter has independent particles that have no definite shape or volume. A vibrating body causes particles around it to vibrate. The energy produced causes a disturbance in the medium in which the vibration occurs. This energy is transferred from one particle to the next until it is received as sound. Sound waves experience different levels of impedance in the different forms of matter. Wave Motion

2 Energy propagates through a medium by a series of small displacements of particles. When a medium is compressed in one direction, it typically expands in another direction. Elasticity is the phenomena that cause a medium to retain its shape. Air, liquids and solids possess various degrees of elasticity. The transmission of vibration through a medium is related to the elastic property of the specific medium. There is no general displacement of the medium, as a whole, in which the sound travels. Only the energy of the wave motion spreads; no matter actually moves far. For example, imagine a series of metal balls, each suspended by a string and touching each other. If one end ball is moved and allowed to hit the next ball, very little movement will be observed and yet the ball on the opposite end will fly out. This allows the transfer of energy to be observed. Each ball carried a certain amount of energy to the adjoining ball until it was observed in the end ball. Sound waves can spread longitudinally or transversely. An example of a transverse wave is the motion of a rope that is fastened at one end and shaken up and down. A sound wave is a longitudinal wave in which air molecules move back and forth in the same direction as the wave motion. Each molecule passes energy on to neighboring molecules, but after the sound wave has passed, each molecule remains about where it was before. If a violin string vibrates, or moves back and forth, its movement in one direction pushes the molecules of the air before it, crowding them together. When it moves back again past its original position and on to the other side, it leaves behind it a nearly empty space. Alternately the body thus causes in a given space a crowding together of the air molecules (a condensation) and a thinning out of the molecules (a rarefaction). The condensation and rarefaction make up a sound wave. Sound Components The above diagram illustrates a cycle. The ball moves from the line to a peak, across the line to a peak in the opposite direction and finally resting at the line opposite from where it started. The movement on each side of the line represents a displacement. Two displacements, on in one direction and the other in the opposite direction forms a cycle. The time that it takes for this ball to make this complete cycle is called a period. The number of complete cycles during a given period is called frequency. For example, if it takes the ball one second to cycle 60 times we say the

3 frequency is 60 cycles per second (cps). The term hertz has replaced cycles per second as the term to use for the measure of frequency for sounds. Any simple sound can be described by three perceived characteristics: pitch, intensity, and quality. These result from three physical characteristics: frequency, amplitude and harmonic constitution, respectively. The frequency of a sound wave is the number of waves passing a point in one second. For a sound, the higher its frequency is, the higher its pitch is. The amplitude of a sound wave is the amount of motion of air molecules within the wave. The higher the amplitude, the greater the intensity of the sound. The greater the amplitude of the wave, the harder the molecules strike the eardrum, and the louder the sound that is perceived. Low frequency waves will vibrate solid surfaces and make walls and large objects appear transparent. High frequency waves, on the other hand, are short and weak and therefore can not penetrate solid objects. Velocity of Sound λ = V/F where, λ = wavelength V = Velocity F = Frequency

4 Wavelength is the distance between two successive crests of waves. The product of the wavelength and the frequency is the speed of the wave. From the diagram above, we see that low frequency sounds have greater wavelengths than high frequency sounds. The speed of sound is directly proportional to temperature. If the temperature increases, the speed of sound increases. Changes in pressure at controlled density, however, have virtually no effect. Wave speed remains constant in a given medium. Sound generally moves faster in liquids and solids than in gases. This is due to the extra time it takes for collisions to occur between molecules of a gas. Gases are less dense (have a lower density) than solids and liquids. The velocity of sound in many gases depends on the density of the gases. If the molecules are heavy, they move less readily, and sound progresses more slowly. The velocity of sound in most gases depends on the specific heat. They also have a lower modulus of elasticity. Velocity also varies directly as the square root of the elasticity. Angle of Incidence, Refraction, Reflection and Interference Angle of Incidence is the angle at which sound interfaces with the second medium. Only when an ultrasonic wave is incident at right angles on an interface between two materials do transmission and reflection occur at the interface without any change in beam direction. Any other angle of incidence will create mode conversion, which is a change in the nature of the wave, and refraction must be considered. Refraction is a change in direction of wave propagation. Like light, sound is subject to refraction, which bends sound waves from their original path. Mode conversion is the transformation of a wave into other modes of vibration because of an impedance mismatch at the interface of a medium.

5 When ultrasound is emitted from a source, some of the wave energy striking an interface may be transmitted through the interface and some will be reflected. The amount of energy that is reflected is dependent upon the specific acoustic impedance of the two media. Some of the energy is attenuated, some reflected, some refracted, and some wave mode conversion has taken place. This essentially reduces the amount of sound but does not change the wavelength or velocity of the wave in a given medium. If the angle of incidence is other than normal, the reflected longitudinal wave angle will also change by an equal amount. The angle of reflection of a longitudinal wave at an interface is always equal to the angle of incidence. Angle of Reflection = Angle of Incidence Sound is also subject to diffraction and interference. If sound from a single source reaches a listener by two different paths - one direct and the other reflected - the two sounds can reinforce one another. If they are out of phase, the waves can interfere negatively, so that the resultant sound is actually less intense than the direct sound without reflection. Interference paths are different for sounds of different frequencies, so that interference produces distortion in complex sounds. Two sounds of different frequencies can combine to produce a third sound with a frequency equal to the sum or difference of the two original frequencies. Decibel The decibel, db, is the unit of sound intensity. The faintest audible sound is arbitrarily assigned a value of 0 db, and the loudest sounds that the human ear can tolerate are about 120 db. The difference in decibels between any two sounds is equal to 10 log10(p1/p2), where P1 and P2 are the two power levels. Physics of Ultrasonics Sounds are generally audible to the human ear when the frequency lies between 20 and 20,000 vibrations per second (Hz). Sound waves with frequencies below the audible range are subsonic, and those with frequencies higher than the audible range are ultrasonic. Ultrasonics deals with high-frequency sound waves, usually above 20,000 hertz (20 khz), which is above the audible range. It is different than supersonics, which deals with phenomena arising when the speed of a solid object exceeds the speed of sound. Modern ultrasonic generators produce frequencies up to more than several gigahertz (1 GHz = 1 billion Hz) by transforming alternating electric currents into mechanical oscillations.

6 Because of their high frequency nature, ultrasonic waves are directional. An ultrasonic sound wave will move more in a straight line than spread out as it propagates from the source. This makes utilizing ultrasonics for detection of leaks, corona, and mechanical problems ideal. Ultrasonic waves are shortwave, meaning they typically have a wavelength between 0.3 and 1.6 cm long, while sonic sounds have a wavelength between 1.9 cm to 17 meters. Properties of Ultrasonics Directional propagate from source in straight line Shortwave - Attenuate quickly as they propagate away from source Section 2 Equipment Ultrasonic Detection Equipment Using ultrasonics in a plant requires the user to know the equipment scanned and the ultrasonic device used. This section covers the basic components necessary to perform ultrasonic inspections. Ultrasonic Detector The base component necessary for ultrasonic inspections is an ultrasonic detector. The detector should provide: Heterodyning Basic heterodyne capability to allow the ultrasonic signal to be heard A quantified db reading for comparative analysis Volume control Multiple signal processing parameters Ability to isolate faults at specific frequencies

7 Figure 1 Figure 1 provides a general diagram of the heterodyning process. An ultrasonic signal is detected through the 40 khz sensor and is amplified through a gain control. The signal then flows into a mixer where four frequencies are produced. The first frequency is the original 40 khz signal(f1). The second signal results as from the original frequency of the local oscillator (F2). The third and fourth frequencies are results of the sum (F1 + F2) and difference (F1 - F2) of the two signals. The signal resulting from the difference of F1 and F2 is passes through the low pass filter and on to the headphones. For example, when using the 40 khz sensor, F1 equals 40 khz. In this example, when the original frequency of the local oscillator, F2, equals 38 khz. The last two frequencies are F1 + F2 = 78 khz and F1 - F2 = 2 khz. Of the four frequencies, 2 khz is the only frequency the human ear can detect. This frequency passes on to the headphones as the other signals are filtered through the low pass filter. Quantified db level Figure 2 When using ultrasonics, db levels for similar components or faults can be compared to ascertain severity. The db level is provided through an output on the ultrasonic detector. This level may have to be obtained by using the detector s sensitivity and volume setting and finding the db value on a chart. Some digital ultrasonic instruments provide a calibrated db value allowing in-field analysis and comparison. Figure 2 shows a separate processing path for a digital ultrasonic instrument that provides a calibrated db value for the ultrasonic signal. Volume Control Using an ultrasonic detector with precision volume control is crucial for accurate detection and diagnosis of ultrasonic anomalies. Traditional analog ultrasonic devices volume control and sensitivity are dependent. This means that when the volume is increased, so is the sensitivity, thus the need for a chart to quantify the db value. Some digital ultrasonic

8 guns have independent volume and sensitivity settings. This allows the volume to be increased or decreased without affecting the db level of the signal. This is diagramed in Figure 2 with the creation of the digital processing path in addition to the analog path for heterodyning. Signal Processing Parameters Having the ability to collect different parameters allows in-depth analysis of an ultrasonic signal. An ultrasonic detection device should allow the signal to be broken down, providing the real time value, the average level, and the highest peak detected of the ultrasonic signal. This allows analysis of the ultrasonic signal s nature for accurate analysis of the fault s true source. Ability to isolate faults at specific frequencies Listening to specific frequencies for particular faults enhances the value of an ultrasonic detection device. It is not only important to discover anomalies, it is crucial to isolate the cause of the problem. For example, a bearing with a high db value can result from a lack of lubrication or a fault in the bearing s components. Identifying the cause allows for swift correction. Sensors Two basic sensors are required for a complete ultrasonic program. Airborne Airborne Contact Probe The purpose of this sensor in airborne ultrasound is to receive exterior ultrasounds. The process of receiving airborne or structure borne ultrasound involves the use of piezoelectric transducers. Typical applications for this sensor are leak detection and electrical inspections. Contact Since ultrasound does not travel through more than one medium effectively, it is necessary to use a contact sensor. Stimulated by ultrasounds, this probe acts as a wave-guide when touched against surfaces. The probe is connected to the transducer where the acoustic energy is converted into electrical energy. Typical applications for this probe include mechanical testing, bearing inspection, valves and steam trap testing.

9 Accessories Focus Cone The focus cone is designed to allow the inspector to pinpoint the exact location of a leak. As the inspector approaches the leak, he is able to pinpoint the leak through a back and forth motion and when safe to do so can confirm the leak by placing the rubber focusing probe over the leak site. Tone Generator Section 3 Leak Detection Leak Detection Compressed air systems are an integral component to industry. It is estimated that 10% of total electrical power consumed by industry is used to generate compressed air. A substantial amount of resources must be used to produce the needed compressed air. Using ultrasonics to detect leaks in compressed air system is an extremely effective method of managing loss. Table 1 reflects the percentage breakdown of the total cost involved in compressed air. The largest percentage is involved in the energy used to generate compressed air. Leaks in an air system cause this percentage to increase. It is possible to save 10% to 20% of the total cost of producing compressed air by conducting air leak surveys. Table 1 - Total Cost of Compressed Air

10 In conducting leak surveys, it is possible to improve the performance of compressed air systems. An excellent system operates with a 5% demand as leaks and most consider 10% leaks is good. However, it is not unusual for systems to run with 30% leaks. Compressed air leaks holds the greatest potential of savings in a plant as compared to the capital outlay required conducting air leak surveys. Establishing the Leakage Rate To determine the performance of a compressed air system, the leakage rate must be established. The best method is to install a flow meter and pressure transducer in the compressor-feeding main and after the receiver. The outputs should be fed to a Chart Recorder and readings recorded over a representative period of time. This enables you to read your outgoing pressure and your receiving pressure. Any loss due to leaks will be reflected in the receiver pressure. Another method of determining the leakage rate requires you to pump the system up to operating pressure during nonproductive hours using a compressor of known capacity. As the system pressure drops due to leakage, the compressor will load at its minimum running pressure. Taking the averaged loaded and unloaded time will enable an estimation of leakage rate to be estimated. After the leakage rate is established, targets should be set for your system. Determining where your system needs to operate for maximum efficiency and savings is the next step. Careful monitoring of results will provide a measure of the system changes and operation. As a result, loss can be decreased and efficiency can be increased, putting money back in your program. Avoiding Loss in Compressed Air Systems Careful consideration should be given to all uses of compressed air, as it is often misused in industry. One of the biggest examples of misuse of compressed air is when it is generated during off-peak hours, when it is not in use. Another means of saving on cost of compressed air is to use air at lower pressures to perform task. A third common mistake in compressed air systems is basing the supplied pressure on one machine in the system. One unit may require a higher pressure so all machines are supplied with the same pressure when they actually require less pressure. Misuse of Compressed Air Loss due to Distribution Air Knives Air Gates Blow Guns Air Lances Air Agitation Air-Jets for Ejection Process Using air at too much psi Powder transfer Compressed air is distributed to machines via a piping system. This is the second area that must be tested for leaks. Air pressure may be lost through leaks in the joints of the piping system. Energy can be wasted by having to generate more pressure than is necessary to overcome incorrect pipe, valves and other air line component sizing. When any component in the piping system is not the correct size, it may cause pressure to drop and high flow velocities. All systems should be designed to minimize drops in pressure. Distribution systems require regular maintenance. Perform air-leak inspections to locate leaks and problem areas.

11 Why check for leaks? Economies Safety Environment Quality Performance Detecting a Leak Everything leaks. Determining when and what type of leaks to look for depends on many variables. Economies, safety, performance, impact on related objects or products (e.g. quality), and the economies of and the ability to repair the leak are examples of these variables. Leakage occurs when a material can move from one medium to another. A particle flows where there is little or no resistance. The force produced by some form of energy allows the material to flow through the path of least resistance. Several factors that make a leak detectable using ultrasonics, such as: turbulence, orifice shape, viscosity of fluid, pressure differentials, distance from leak, competing ultrasounds, accessibility to leak, and atmospheric conditions. The effectiveness of using airborne ultrasonics to detect a leak depends on certain characteristics. The leak must be turbulent to emit ultrasound qualities. Some gases are less dense than others in which case it will flow faster through an orifice are. If pressure differentials between high pressure and low pressure are greater due to the atmospheric condition in one leak as opposed to another, the turbulence will be greater as well. A large orifice in one instance may produce more turbulence than a small orifice but in another situation, the reverse might be true. As a rule of thumb, a small orifice will generate more ultrasound than a large orifice. A jagged orifice usually produces greater turbulence than a smooth opening. A thick wall will tend to dampen some of the forces of pressure and reduce pressure differentials more so than a thin wall. Sound absorbent material such as rubber will produce less ultrasound than a conductive material such as stainless steel. Since there are so many variables, we cannot ever say with certainty that a leak of such and such a pressure at X diameter will produce a detectable ultrasound. We have to state our threshold limitations with the understanding that they are not absolutes. There are two classes of viscous flow, laminar and turbulent. In laminar flow, the particle path is almost linear, undisturbed and free of eddy currents. Turbulent flow is very erratic as a result of innumerable eddies. When performing leak detection using airborne ultrasound, the important characteristic is that the fluid flow be turbulent since a laminar flow leak does not generate sound. If the leak is laminar, it is necessary to scan low frequencies to detect flow. Pressure differential is a significant issue when performing most leak tests. Pressure differential is created when pressure across a leak is changed, and the flow changes in proportion to the differences of the square of the absolute pressure. When performing airborne ultrasound leak inspection, it is important to consider the viscous flow that a given pressure differential acting across the leak.

12 It is important to remember that a smooth orifice will not produce as much turbulence as a jagged orifice. An orifice that has many edges can affect fluid flow and produce more turbulence The Reed Effect. A nontraditional leak, permeation leaks, may be detected using ultrasonic detection. Permeation leaks are membrane type leaks and seem to penetrate through the walls of the container. Permeation is the passing of fluid into or through a solid barrier. The solid substance has characteristics that allow molecules of the leaking substance to escape through the solid. This process involves diffusion, absorption, dissociation, migration and desorption. Permeation leaks, (or membrane leaks) are almost impossible to plug up. Leaks through orifices can permit the flow of any gas when a pressure differential is created. The size of the opening is a factor influencing how detectable a leak is. Orifice leaks are detectable by airborne ultrasound at considerable distances. The detectability of leaks is dependent upon the orifice diameter, pressure of the leak and distance. Attention should be given to leaks as they may have an irregularly shaped orifice. An irregularly shaped orifice has the potential for stress risers. These stress risers can be the starting points for a tearing or cracking process. As the pressure in the vessel failure are conditions of service, vessel shape, size, wall thickness and material, and fabrication methods. Viscosity of fluid The viscosity of a fluid is a measure of the fluid s internal friction, or its resistance to flow. When talking about viscosity of air, the dynamic viscosity of the fluid is the subject. Dynamic viscosity is essentially the measure of activity of air molecules with increases in temperature. With increase in temperature, there is an increase in molecule movement, with this increase comes increased collision of molecules. A higher dynamic viscosity is created resulting in a greater resistance to flow. The factors that influence gaseous flow through leaks are: Distance from the Leak Molecular weight of the gas Viscosity of gas Pressure difference causing the flow Absolute pressure of the system Length and cross section of the leak path. Another factor influencing the detectability of a leak is the distance from the leak. In airborne ultrasound, attenuation of sound relates to the decrease in sound intensity from the source of ultrasound. The intensity of the ultrasound signal decreases as the distance from the source sending the ultrasound increases. Intensity refers to the relative strength of a sound signal at a certain point. Accessibility to the Leak It is important that the leak is accessible. Providing it is safe, the closer the inspector can get to the leak the better the inspector will be able to detect and evaluate the leak. If a leak is buried behind several structures, the leak will have a tendency to reflect off the various structures. The ultrasound from the leak bounces from one object to another and consequently the inspector can become confused as to the leak s source is located. In some cases, ultrasound may be hitting material that absorbs the sound waves. The further that the leak has to travel, the more likely the leak is to attenuate and weaken. Get closer to the leak source, remove interfering objects, and use aids to gain access to the leak, such as wave-guide, parabolic reflector and extension cables. Atmospheric Conditions

13 Atmospheric conditions will not normally change your airborne ultrasound to any great degree. There is a variation of atmospheric pressure with change of altitude. This is because there are fewer gas molecules present. In many leak detection methods, atmospheric pressure is a very important aspect of the detectability of a leak. Logistics of Leak Detection: Strategies Pressure/Vacuum Test Determine the type of potential leak (pressure/vacuum) Set up a background and foreground in your mind and position your scan angle/approach accordingly. Be sure to keep the background in the back. This means, angle away from the background sounds. Adjust the volume for each test zone. In some cases, you will adjust up and in others down. Typically, setting the volume to a lower setting provides the best results. Determine what is normal ultrasound for the test zone versus a difference that will indicate a problem or leak. As an example, if a test area has spraying machines, there will be a steady, loud hiss from the machines. You will be looking for an increase in the hissing over the background to determine a leak. Keep all of the above in mind: reduce the volume, scan close to the test area, angle away from the hiss. Ultrasonics can be used to locate leaks in pressurized system regardless of the type of gas used. This is especially beneficial in areas where vacuum processes exist. Ultrasonics allows areas and machines to be tested for leaks while on-line, making it an easy to use convenient technology. Two types of leaks can be tested for: Pressure and Vacuum. Pressure Leaks: The pressure gas will move through the leak site producing a recognizable turbulent flow. Since the turbulence is produced in the scan side, the ultrasound will be louder than in a vacuum test. For this reason, the pressure test is quite effective in locating mid to low-level leaks. The ultrasonic detection device can locate pressure leaks at great distances usually.

14 Vacuum Leaks: During a vacuum leak, the airflow moves from atmosphere to vacuum. As it does, it will produce a turbulent flow on the vacuum side. Since the turbulence is generated within the vacuum side, the leak signal will tend to be relatively low. For this reason, it is important to scan close to the leak site. Liquid Leak Amplifier Liquid Leak Amplifier is often described as an ultrasonic bubble test, this method is quite effective in locating low flow leaks (ranging from 1 x 10-3 std cc/sec to 1 x 10-6 std cc/sec). Using a liquid with a low surface tension, a small amount of fluid is poured over the test area. Within a few seconds small soda-like bubbles will form and collapse over the leak. The collapsing produces a strong ultrasound that is readily detected. Even in situations where the bubbles are not visible, the leak will still be sensed with ultrasonic detection. Virtual Leak A Virtual Leak is when the actual leak site is usually several inches from the exit path of the bubbles. Gravity and/or pressure force the bubbles into a downward or upward path of least resistance and exiting from between the outer skin of the container. This is particularly common with recessed areas that are not visible to the eye. Airborne ultrasound can pinpoint the leak site. Since ultrasound is directional, the airborne ultrasonic detector can sense the opening allowing the operator to hear and see the db deflection as the probe is moved over the leak site. Confirming Leak Locations Gross to Fine Method Perform the gross to fine method in order to pinpoint and isolate leaks. Start at Maximum and move closer to the area and listen for the loudest leak signal (Using the focus cone is necessary). Remember to scan around the suspected leak area. It is possible to pinpoint exactly where the leak is coming from if you scan completely around the area of interest. Positioning In order to pinpoint the actual ultrasound it is sometimes necessary to move from one area to another. This is a common technique when it is difficult to get close to the area emitting ultrasound. By moving from one area to another and taking ultrasound readings, the inspector is able to determine the actual area emitting the ultrasounds through a process of elimination. Sonic Deflection

15 Sonic Deflection can be a source of confusion and can lead to false readings. Sound reflects of solid surfaces and although there is some degree of attenuation, the sound energy can be misinterpreted as the primary leak source. It is important that you confirm the leak location. Scanning Techniques There are several techniques to use when confirming leak locations. To confirm the leak site: Scan in all directions, verify Angle of approach to ensure that you are approaching the sound source in direct line with the leak flow. Shielding and Barriers Shielding is the process of moving a solid material between the inspection site and the ultrasonic detection device. Any solid material can be used to shield ultrasound. This technique will not eliminate sound but will give you a background foreground effect. Take advantage of existing barriers. Examples of existing barriers are doors, partitions, out of service machinery, and walls. Competing ultrasounds can be minimized or eliminated in situations by using existing barriers. Body

16 Using your body as a shield provides a substantial mass that will absorb and reflect many sounds if strategically placed between the competing ultrasound and the inspection area. Always ensure that any shielding method you use can be performed safely. Angulation Angulation is an effective means of reducing the effect of competing ultrasounds. When confronted with a competing ultrasound change the angle of inspection so that the scanner is facing away from the competing sound and is pointed at the test area. This is diagramed above. Hand Techniques Clipboard Wet Hand Technique uses a wet hand or finger to confirm a leak. Most have tried to determine the direction of wind by wetting our finger and placing it in the air. Hundreds of nerves are found at the tip of the fingers. The sensitivity of these nerves is increased by the coolness caused by the air hitting a wet finger. This is essentially the same principle. Wet your fingers and place them in the direction, but NOT ON the suspected leak. Caution: Do not use this technique on electrical leakage or highpressure leaks. Sealing is a technique used with a focusing module.. This technique is used to identify and confirm leaks, by using the focus cone to cover and seal a leak. Also, do not drag the focusing probe across the surface, it will sound like a leak. Sometimes it is necessary to cover the end of the focusing module with a cloth or a gloved hand in order to completely seal the area emitting ultrasound. Gloved Hand is a technique used to minimize competing ultrasounds that could enter through the tip of the ultrasonic detector and is best accomplished when using the focusing module. Wrap you hand around the tip of the focusing module while covering the suspected leak area. Shielding the ultrasound can be as easy as placing a clipboard between competing ultrasounds and the subject for inspection. Ultrasound reflects off the surface of the material you use. In some cases, the material may absorb ultrasounds in addition to reflecting sounds. Things to Consider

17 Is the volume of the ultrasonic detector at the optimum level? Should I take the ultrasonic reading from another position Did I follow the loudest sound? Pinpoint the leak? Did I do everything possible to shield the airborne ultrasonic detector from competing ultrasounds? Did I confirm my readings? Section 4 Steam Traps Steam Trap Analysis Basic Concepts of Steam Steam is an invisible gas generated by adding heat energy to water in a boiler. To raise the temperature of the water to the boiling point energy must be added. Then additional energy - without any further increase in temperature changes the water to steam. Steam is an efficient and easily controlled heat transfer medium. It is most often used for transporting energy from a central location (boiler) to any number of locations in the plant where it is used to heat air, water, or process applications. As noted, additional Btu (British thermal units) are required to make boiling water change to steam. A Btu is the amount of heat energy required to raise the temperature of one pound of cold water by 1 o F. A Btu is also defined as the amount of heat energy given off by one pound of water in cooling, say, from 70 o F to 69 o F. These Btu are not lost but stored in the steam ready to be released to heat air, cook tomatoes, press pants or dry a roll of paper. Definitions Steam Mains or Mains: carry steam from the boiler to an area in which multiple steam-using units are installed. Steam branch lines: take steam from steam main to steam-heated unit. Trap discharge lines: move condensate and flash steam from the trap to a return line. Condensate return lines: receive condensate from many trap discharge lines and carry the condensate back to the boiler room. Temperature is the degree of hotness with no implication of the amount of heat energy available. Heat is a measure of energy available with no implication of temperature. To illustrate, the one Btu, which raises one pound of water from 39 o F to 40 o F, could come from the surrounding air, at a temperature of 70 o F or from a flame at a temperature of 1000 o F. Heat of vaporization or latent heat is the heat required to change boiling water into steam. The quantity is different for every pressure/temperature combination.

18 Steam at Work How the heat of steam is utilized Heat flows from a higher to a lower temperature level in a process known as heat transfer. Starting in the combustion chamber of the boiler, heat flows through the boiler tubes to the water heater. When the higher pressure in the boiler pushes steam out, it heats the pipes of the distribution system. Heat flows from the steam through the walls of the pipes into the cooler surrounding air. This heat transfer changes some of the steam back to water. Insulating distribution lines can minimize this wasteful and undesirable heat transfer. Transfer of heat from the steam is desirable when steam reaches the heat exchangers in the system. Heat flows to the air in an air heater, to the water in a water heater or to food in a cooking kettle. Nothing should interfere with this heat transfer. Condensate Drainage Why it is necessary? Condensate forms in the distribution system due to unavoidable radiation and is the by-product of heat transfer in a steam system. It also forms in heating and process equipment as a result of desirable heat transfer from the steam to the substance heated. Once the steam has condensed and valuable latent heat has dissipated, the hot condensate must be removed immediately. Although the available heat in a pound of condensate is negligible as compared to a pound of steam, condensate is still valuable hot water and should be returned to the boiler. The need to drain the distribution system. Condensate lying in the bottom of steam lines can cause one kind of water hammer. Steam traveling at up to 100 miles per hour makes waves as it passes over this condensate as seen above. Condensate allowed to collect in pipes is blown into waves by steam passing over it until it blocks steam flow. This is diagramed in point A above. If enough condensate forms, high-speed steam pushes it along, creating a dangerous slug (point B above) which grows larger and larger as it picks up liquid in front of it. Anything which changes the direction pipe fittings, regulating valves, tees, elbows, blind flanges can be destroyed. In addition to damage from this battering ram, high velocity water may erode fittings by chipping away at metal surfaces. The need to drain the heat transfer unit.

19 When steam comes in contact with condensate cooled below the temperature of steam, it can produce another kind of water hammer known as thermal shock. Steam occupies a much greater volume than condensate, and when it collapses suddenly, it can send shock waves throughout the system. This form of water hammer can damage equipment, and it signals that condensate is not being drained from the system. Obviously, condensate in the heat transfer unit takes up space and reduces the physical size and capacity of the equipment. Removing it quickly keeps the unit full of steam as seen above. As steam condenses, it forms a film of water on the inside of the heat exchanger. The above diagram shows potential barriers to heat transfer. Steam heat and temperature must penetrate these potential barriers to do their work. Non-condensable gases do not change into a liquid and flow away by gravity. Instead, they accumulate as a thin film on the surface of the heat exchanger along with dirt and scale. The need to remove air and CO2 Air is always present during equipment start-up and in the boiler feedwater. In addition to air, feedwater may contain dissolved carbonates, which release carbon dioxide gas. The steam velocity pushes the gases to the walls of the heat exchangers where they may block heat transfer. This compounds the condensate drainage problem because these gases must be removed along with the condensate. Effect of air on steam temperature

20 When air and other gases enter the steam system, they consume part of the volume that the steam would otherwise occupy. The temperature of the air/steam mixture falls below that of pure steam resulting in decreased efficiency of the steam system. Effect of air on Heat Transfer The normal flow of steam toward the heat exchanger surface carries air and other gases with it. Since they do not condense and drain by gravity, these non-condensable gases set up a barrier between the steam and the heat exchanger surface. The excellent insulating properties of air reduce heat transfer. In fact, under certain conditions as little as ½ of 1% by volume of air in steam can reduce heat transfer efficiency by 50%. When non-condensable gases (primarily air) continue to accumulate and are not removed, they may gradually fill the heat exchanger with gases and stop the flow of steam altogether. The unit is then air bound. Corrosion Two primary causes of scale and corrosion are carbon dioxide (CO2) and oxygen. CO2 enters the system as carbonates dissolved in feedwater and when mixed with cooled condensate creates carbonic acid. Extremely corrosive, carbonic acid can eat through piping and heat exchangers. Oxygen enters the system as gas dissolved in the cold feedwater. It aggravates the action of carbonic acid, speeding corrosion and pitting iron and steel surfaces. Back Pressure Back pressures excessive by normal standards may occur due to fouling of return lines, increase in condensate load or faulty trap operation. Depending on the operation of the particular trap, back pressure may or may not be a problem. If a back pressure is likely to exist in the return lines, be certain the trap selected will work against it.back pressure does lower the pressure differential and, hence, the capacity of the trap is decreased. In severe cases, the reduction in capacity could make it necessary to use traps one size larger to compensate for the decrease in operating pressure differential. What the steam trap must do The job of the steam trap is to get condensate, air and CO2 out of the system as quickly as they accumulate. In addition, for overall efficiency and economy, the trap must also provide:

21 Minimal steam loss. The above table shows how costly unattended steam leaks can be. Long life and dependable service. Rapid wear of parts quickly brings a trap to the point of undependability. An efficient trap saves money by minimizing trap testing, repair, cleaning, downtime and associated losses. Corrosion resistance. Working trap parts should be corrosion resistant in order to combat the damaging effects of acidic or oxygen-laden condensate. Air venting. Air can be present in steam at any time and especially on start-up. Air must be vented for efficient heat transfer and to prevent system binding. CO2 venting. Venting CO2 at steam temperature will prevent the formation of carbonic acid. Therefore, the steam trap must function at or near steam temperature since CO2 dissolves in condensate which has cooled below steam temperature. Operation against back pressure. Pressurized return lines can occur both by design and unintentionally. A steam trap should be able to operate against the actual back pressure in its return system. Freedom from dirt problems. Dirt is an ever present concern since traps are located at low points in the steam system. Condensate picks up dirt and scale in the piping, and solids may carry over from the boiler. Even particles passing through strainer screens are erosive and, therefore, the steam trap must be able to operate in the presence of dirt. A trap delivering anything less than all these desirable operating/design features will reduce the efficiency of the system and increase costs. When a trap delivers all these features the system can achieve: Steam Trap Analysis Fast heat-up of heat transfer equipment. Maximum equipment temperature for enhanced steam heat transfer. Maximum equipment capacity. Maximum fuel economy. Reduced labor per unit of output. Minimum maintenance and a long trouble-free service life. A steam trap, which is simply and automatic valve which opens for condensate, air and CO2 and closes for steam, does this job. For economic reasons, the steam trap should do its work for long periods with minimum attention. Periodic monitoring of steam traps can prevent problems, increase system efficiency, and save money. Ultrasonic detectors are successfully used in many steam trap maintenance programs. Steam can escape from a typical steam system from many areas. Malfunctioning control valves often overheat, undercook and exceed operating parameters and potentially shutting down production. A single failed steam trap can cost over $1,000 per year in energy loss. Considering the number of traps in a facility, costs and expenses increase exponentially. Two of the biggest complaints on using ultrasonic equipment for steam trap analysis is

22 the operator is not knowledgeable of the equipment and ultrasonic equipment will pick up competing ultrasonic noise. Steam Trap Operation Stage 1 At start-up, when a large quantity of condensate flows into the trap, the float (A) rises by buoyancy, opening the orifice (E) into the control chamber (F), building up the pressure there. The pressure causes the piston (D), and accordingly the main valve (B), to move up. The orifice of the valve seat (C) is then opened to discharge condensate. Stage 2 When condensate has been discharged, the float (A) falls, closing the pilot orifice (E). Pressure in the control chamber (F) decreases due to leakage through small holes. The valve (B) moves downward, closing the orifice of the valve seat (C). The orifices are completely water-sealed during operation, allowing no steam leakage.

23 Stage 3 After the large quantity of condensate formed at start-up has been discharged, the steam equipment reaches a thermally balanced state, forming condensate in accordance with the load. The trap then discharges condensate by a modulating float dynamic principle. The Float Dynamic Principle When a large quantity of condensate flows into the trap, the float (A) rises immediately, opening the orifice (E) wide. Condensate passes through the pilot orifice at a high velocity into the control chamber (F), where the pressure increases rapidly due to flashing condensate. The rapid expansion causes a force to be exerted on the piston, opening the large orifice instantly. As condensate discharges through the main orifice at a high velocity, condensate in the trap equipment and trap inlet pipe is also induced and discharged. Steam Trap Failure It is important to understand the kind of trap being inspected and how it is suppose to operate. The rhythm of the trap is used to identify a malfunction within the traps operation. Changes in the steam system should be recorded and analyzed for changes. A metered steam system allows the facility operator to monitor the amount of steam being used and wasted. The amount of steam being wasted can be converted into the amount of dollars being wasted in the steam system. Without this information, it would be difficult to make decisions on steam usage and measure your success.

24 Ultrasound Inspection The contact method clearly detects sound of passing steam in trap and db level increases when a trap exhausts. When the trap is stuck open, a blowing sound is constantly present and there is a tremendous amount of steam lost. When the trap is stuck shut, there is no sound, condensate flow is prevented and causing a loss in the system s efficiency. The diagram above shows two test points for a float and thermostatic trap. The first point is the valve or discharge orifice and the second point is the air vent of the trap. If the air vent is louder than the discharge orifice, the trap has a problem. The inlet, outlet and trap body provide valuable information on the trap s temperature profile. Testing steam traps for proper operation using ultrasonic testing is fast and very accurate. The operator must be knowledgeable about the equipment and combat competing ultrasounds. Understanding how a steam trap operates enables the understanding of how it may fail. There are several types of steam traps, all of which, fail in similar manners. Using various information obtained through ultrasonic detection allows the user to diagnose the condition of a steam trap. A trap that has failed open will exhibit a high degree of ultrasonic noise downstream, as steam escapes through the trap. Additionally, the trap will not cycle, allowing steam to constantly flow through the trap. The temperature of the feed and exhaust lines is also a good indicator of the condition of the steam trap. The temperatures should be considerably different, but when a steam trap has failed the temperatures of the two lines will be the same. A trap that has failed closed will usually not exhibit any ultrasonic noise downstream nor in the trap, because there is no flow going through the trap. Ultrasonic and Temperature It is beneficial to test steam traps with temperature analysis and airborne ultrasound. The temperature will of the steam line ahead and after the trap, but in many cases that is not enough. Note the pressure/temperature relationship of steam. The lower the steam pressure, the more difficult it is to test steam traps based on temperature, and therefore, a pressure reading at any point in the system. Ultrasonic testing will let you hear what is happening in the trap so you know what has to be done to repair it. When inspecting steam traps: Replacing faulty insulation or insulating where needed. Installing new steam supply lines to augment overload existing lines Re-evaluating of present lines to eliminate inefficient uses of steam Checking operation of steam supply and condensate return lines to locate problem areas due to defective valves, regulators, heat exchangers, traps and coils. Return lines may have shut by mistake, causing a deadhead situation.

25 Check if trap is working Research the trap types Compare similar traps Wait for trap to cycle

26

27 Troubleshooting Steam Traps The following summary will prove helpful in locating and correcting many steam trap troubles. Many of these are actually steam system problems rather than trap troubles. Whenever a trap fails to operate and the reason is not readily apparent, the discharge from the trap should be observed. If the trap has a test outlet installed, this will be a simple matter otherwise, it will be necessary to break the discharge connection. Cold Trap No Discharge If the trap fails to discharge condensate, then:

28 A. Pressure may be too high. Wrong pressure originally specified. Pressure raised without installing smaller orifice PRV out of order Pressure gauge in boiler reads low Orifice enlarged by normal wear. High vacuum in return line increases pressure differential beyond which trap may operate. B. No condensate or steam coming to trap. Stopped by plugged strainer ahead of trap. Broken valve in line to trap Pipe line or elbows plugged. C. Worn or defective mechanism. Repair or replace as required. D. Trap body filled with dirt. Install strainer or remove dirt at source. E. For IB, bucket filled with dirt. Prevent by: Installing strainer. Enlarging vent slightly Using bucket vent scrubbing wire. F. For F&T traps, if air vent is not functioning properly, trap will likely air bind. G. For thermostatic traps, the bellows element may rupture from hydraulic shock, causing the trap to fail closed. H. For disc traps, trap may be installed backward. Hot Trap No Discharge A. No condensate coming to trap. Steam Loss Trap installed above leaky by-pass valve. Broken or damaged syphon pipe in syphon drained cylinder. Vacuum in water heater coils may prevent drainage. Install a vacuum breaker between the heat exchanger and the trap. If the trap blows live steam, the trouble may be due to any of the following causes: A. Valve may fail to seat. Piece of scale lodged in orifice. Worn parts. B. IB trap may lose its prime.

29 If the trap is blowing live steam, close the inlet valve for a few minutes. Then gradually open. If the trap catches its prime, the chances are that the trap is all right. Prime loss is usually due to sudden or frequent drops in steam pressure. On such jobs, the installation of a check valve is called for. If possible locate trap well below drip point. C. For F&T and thermostatic traps, thermostatic elements may fail to close. Continuous Flow If an IB or disc trap discharges continuously, or an F&T or thermostatic trap discharges a full capacity, check the following: A. Trap too small. A larger trap, or additional traps should be installed in parallel. High pressure traps may have been used for a low pressure job. Install right size of internal mechanism. B. Abnormal water conditions. Boiler may foam or prime, throwing large quantities of water into steam lines. A separator should be installed or else the feed water conditions should be remedied. Sluggish Heating When trap operates satisfactorily, but unit fails to heat properly: A. One or more units may be short-circuiting. The remedy is to install a trap on each unit. B. Traps may be too small for job even though they may appear to be handling the condensate efficiently. Try next larger size trap. C. Trap may have insufficient air-handling capacity, or the air may not be reaching the trap. In either case, use auxiliary air vents. Mysterious Trouble If trap operates satisfactorily when discharging to atmosphere, but trouble is encountered when connected with return line, check the following: A. Back pressure may reduce capacity of trap. Imaginary Troubles Return line too small trap hot. Other traps may be blowing steam trap hot. Atmospheric vent in condensate receiver may be plugged trap hot or cold. Obstruction in return line trap hot or cold. Excess vacuum in return line trap cold. If it appears that steam escapes every time trap discharges, remember: Hot condensate forms flash steam when released to lower pressure, but it usually condenses quickly in the return line. Primary Source of Information: Armstrong Steam Traps Section 5 Mechanical

30 Mechanical Applications Moving metal parts wear each other down. As the parts move relative to each other, ultrasonic noise is produced. With experience, the causes of the noise can be quickly identified and resolved. It is important to understand the nature of the working force behind mechanical equipment in order to perform ultrasonic inspections. By understanding the working forces, motion, work, and energy involved in particular test equipment the inspector is better equipped to inspect, analyze, diagnose, and report with a high level of competence. Before mechanical objects can perform their work, force must be applied to the equipment. Force can be applied mechanically or manually to hold it in position or maintain equilibrium. Every force produces a stress in the part on which it is applied. When dealing with mechanical equipment, there are four major working forces: tension, compression, torque, and shear. Notice in the above illustration that all four of these forces are working together at the same time. Ultrasonics can be used to detect problems that may develop as a result of the different forces. Principles of Motion Principles of Motion Direction Linear Rotary Displacement Relative Harmonic Speed Velocity Acceleration How fast an object changes velocity The principles of motion are the changing of position or displacement. For instance, when a shaft turns, it has motion even though it does not go anywhere. Linear (straight line) and Rotary (circular) are considered absolute motion since there is a fixed reference point to determine position change. When both linear and rotary are combined, they create a motion called cam-shaped motion (also an absolute motion). Relative motion involves the motion between two objects. Harmonic motion is created in springs caused by springs and also in the simple back and forth motion found in a pendulum. Linear motion: The airborne ultrasonic inspector is often concerned with the rate of speed at which it takes an object to move. When performing airborne ultrasonic inspections the inspector may perform an analysis using baseline

31 trending, comparative trending or historical trending. By understanding the normal rate of motion for a particular motion an airborne ultrasound inspector can determine anomalies in the test subject. Airborne ultrasound gives the inspector an audible picture of whether the motion is consistently linear and also if the motion is at optimal velocity. Circular Motion (radial motion): can be either centripetal (inward) or centrifugal (outward). Centripetal force can be illustrated by a rock swinging on a string and is held in a circular path by the centripetal force applied by the string. Centrifugal force is what causes a car to overturn when it takes a corner too fast (flying off a tangent to its circular motion. Rotary motion: is a fixed or restricted motion about a given point. The rate of motion is often expressed in rpm or rps. Relative motion: is the difference between the motions of two objects. Harmonic motion: is a back-and-forth motion that can be illustrated using a pendulum or spring. For example: the motion occurs whenever the net force acting on an object is proportional to the displacement of the object from its equilibrium position. In a spring, net force results from two separate forces acting on the spring in the equilibrium stage. The spring is pulled or pushed away from equilibrium. The net force is larger as the spring extends further away or contracted tightly. Frequency describes how rapidly the spring changes back and forth and amplitude describes how large the motion is. Work Force x Distance = Work By understanding the relationship of Force x Distance = Work of a particular machine you can more efficiently and accurately perform airborne ultrasonic inspections. Work is the result of a force moving an object, the distance an object is displace and the amount of force required to displace the object. Whether the object is raised, hoisted or pushed work is being performed. Each piece of equipment has its unique set of circumstances to make them work. Most people understand that a gear rotates and is intertwined with another gear which will rotate as a result of the push coming from the primary gear. By understanding the physical characteristics of the equipment and the nature of the work, the airborne ultrasonic inspection can get an audible picture of the condition of the test equipment. Sources of Power Work/Time = Power In addition to understanding force, motion and work the ultrasonic inspector should understand the source of energy that makes the subject test equipment work. When performing inspections familiarity with the source of power for a piece of equipment, whether it is electricity, pneumatics, hydraulics, steam or internal combustion. Steam and hydraulic power sources are usually powered by electricity. Internal combustion engines are typically run by gasoline, natural gas, or diesel fuel. Mechanical Efficiency Friction is the largest factor contributing to efficiency losses in mechanical equipment. Other causes are heat, slippage, and the requirement to drive other equipment such as pumps, fans, gear trains, etc. The process of friction creates a source of ultrasound. Airborne/Structure borne ultrasonics is capable of detecting ultrasound caused by

32 friction. By understanding the function of the test subject, the airborne ultrasound inspector is able to discriminate good ultrasounds from anomalies in the test subject. Laws of Friction The amount of friction is independent of the area of the contacting surface as long as their elastic limits are not exceeded. This law states that the area of the object contacting the surface will not affect the amount of friction unless the surface is deformed. The amount of friction is proportional to the normal force between the surface. If a load requires 200 pounds of force to move it and the load is doubled, the force required to move the load must be doubled. Bearings A good, healthy bearing is round, adequately lubricated and emits relatively low amount of ultrasound. However, ultrasound frequencies are present in bad bearings which allows the ultrasonic detector to identify wear, evidence of fatigue, embedded particles, scoring and improper clearances. Mode Conversion In the above diagram, the bearing has a fault in its outer race. The anomaly is caused by the weakening of the metal below the surface of the outer race. As a fault develops in a bearing s components, the defect begins below the surface. Ultrasound emitted from the fault is initially a longitudinal wave. As the longitudinal wave hits the inside of the bearing housing the wave enters a mode conversion and some ultrasound is turned into shear, surface and plate waves. By using the contact probe on the bearing housing, there is little acoustic impedance. The ultrasound in the form of plate and surface waves travel up the contact probe and then heterodyned to an audible frequency. The ultrasonic sound waves produced are heard as crackling or repetitive clicking. Data Collection Techniques

33 In order to improve quality, repeatability and test reliability attention must be given to data collection. Several measures can be taken to optimize the data collected and reduce the change in test variables. First, mark a spot on the bearing where data will be collected. This increases the reliability and repeatability of the data collected. Second, collect data with the ultrasonic device at the same angle every time. Ultrasonic instruments are extremely sensitive and changing the approach angle can result in a wide range of data. Finally, apply the same amount of pressure on the contact probe each time data is collected. Pressure affects the sound intensity and the measured db level of the ultrasonic device. Using the same pressure allows the db level of the bearing to be used for trending and comparative analysis. When performing mechanical inspections the procedure requires the use of the contact probe. The probe is extremely sensitive to ultrasound and may need to be protected from competing ultrasounds. It is a good idea to cover the contact probe with the focus cone to eliminate some environmental borne ultrasounds. This will eliminate most noise picked up by the contact probe. Inspecting Bearings Ultrasonic inspection and monitoring of bearings is a great technology to use with vibration analysis to detect early bearing failure. Ultrasonic warnings appear before a temperature rise. Ultrasonic inspection of bearings is useful in detecting: Metal Fatigue Failure Lubricant problems Beginning of fatigue failure Brinelling of bearing surfaces When a roller bearing in raceways begin to fatigue, a subtle deformation starts to occur. The metal deformation creates an increase in the emission of ultrasonic sound waves. Changes in amplitude indicate incipient bearing failure before temperature and vibration changes. Ultrasonic inspection based on detection and analyses of changes in bearing resonance frequencies provide subtle detection capability when conventional methods are not able to detect very slight faults. As a ball passes over a fault pit in the race surface, it creates an impact. A structural resonance by one of the bearing components vibrates or rings by this repetitive impact. The sound created is observed as an increase in amplitude in the bearing s monitored frequencies. Fatigue failure begins as FLAKING, the term used to describe the bearing as it begins to fail from stress. In flaking, the failure starts as a crack and from there deteriorates. As the bearing deteriorates, pieces will begin to fall off the bearing, this condition is called SPALLING. The environment that surrounds the bearings can be a source of bearing failure. High temperature can cause failure in bearings due to expansion of bearings due to heat causing the bearing to seize. A high temperature can also cause lubricant to fail. Contaminants such as dirt, moisture, or corrosion can cause excessive wear as well. Two basic methods can be used to diagnose a bearings condition: First, the comparative method. Using ultrasonics to compare the same type of bearing under the same load conditions, sound and db level can be used to ascertain condition. For example, in looking at a conveyor system that uses hundreds of bearings of the same type. An average running db can be determined as well as characteristics of sound that describe a good bearing. Using ultrasonics allows for quick, easy analysis of the bearing system using both quantitative and qualitative measures. Second, the historical method. Monitoring a component over time and recording its db and temperature level allows thorough analysis. By analyzing bearing history, wear patterns can be detected, providing for early detection and

34 correction of bearing failure. A demodulated envelope waveform can also be used for trending bearings. An excerpt from the paper The SonicScan Output What is it? by Brent Van Voorhis details what a demodulated envelope waveform is. This is specific to CSI s SonicScan ultrasonic detection device, but accurately describes using a envelope waveform. The SonicScan Output What is it? Brent Van Voorhis The Output The output signal from the SonicScan is not the translated audio signal or the raw ultrasonic signal. The output signal is an envelope of the ultrasonic signal. Essentially, this envelope signal is an amplitude demodulation of the raw ultrasonic signal. The periodicity (or repetition rate) of the ultrasonic emissions, and their amplitudes, convey the important information about the health of the machine. The envelope output of the SonicScan enhances the detection of impact type noises, such as those that might be caused by a chip in a bearing race, or a cracked gear tooth. A single ultrasonic noise burst (or ring) will produce a ramp-like pulse at the output of the envelope detector. A series of rings or bursts will likewise produce a series of pulses from the envelope detector. Please refer to Figure 1. Figure 1 This is a laboratory generated simulation of an idealized machine impact noise. The top time domain trace is the raw 40 khz unprocessed ultrasonic impact signal, the bottom trace is the output of the SonicScan envelope detector. Please note that the burst is only about 500 microseconds in duration, yet has been easily captured by the SonicScan. Time Domain Observing the output waveform of the envelope detector in the time domain, will yield a wealth of signal detail in such applications as reciprocating engines, air compressors, and pumps. Obviously, the use of a tachometer to reference the waveform timing would be of great value in many applications. Figure 2 is an example of a time domain waveform of a centrifugal coolant pump.

35 Figure 2 This is a time domain waveform of a centrifugal coolant pump with one defective impeller vane. The fault signal was appearing once per revolution. A nearby pump (not shown) in good operating condition had a barely discernible signal at the running speed. Frequency Domain An FFT can be performed on the SonicScan envelope detector output using a CSI 2120 or other FFT analyzer. This would be of value, for example, in monitoring bearings and gear boxes. Since the output of the SonicScan is already an enveloped waveform, do not use the PeakVu or the Demodulation settings of the CSI The results obtained with the SonicScan are often similar to the spectral content obtained when using the CSI 2120 in the PeakVu mode with an accelerometer. Normally, Fmax settings of not more than 1000 to 2000 Hz (usually less) are used, since the running speeds and bearing signatures will be at rather low frequencies on most machinery. A good rule of thumb is to set Fmax to not more than 30 times the running speed (frequency) of the machine being monitored. Figure 3 is the same pump of Figure 2 shown in the frequency domain. Figure 3 This is the signature of the defective pump of Figure 2 shown in the frequency domain. The fault frequency and two harmonics are clearly visible. The output of the envelope detector is a linearized log of the input signal. The amplitude (in decibels) of the detected ultrasonic signal is output as a scaled voltage. This output voltage will change about 18 millivolts per decibel change of

36 the input ultrasonic signal, and is quite linear over about an 80 decibel range. The envelope waveform can be thought of as nearly instantaneous (positive) DC values that correspond at any point in time to the decibel level of the ultrasonic Slow Speed Bearings Monitoring slow speed bearings is possible with ultrasonic instruments. In extremely slow speed bearings, less than 30 RPM, it is often necessary to disregard the meter and listen to the sound of the bearing. Most often no sound will be heard as grease will absorb most of the acoustic energy. If a sound is heard, usually a crackling sound, there is some indication of a fault. On most other slow speed bearings, it is possible to see a baseline and monitor with procedures previously described. Lubrication Seventy-five percent of all bearing failures are due to lubrication problems. Inadequate lubrication and over lubrication are major problems in all industries. Normal bearing loads cause an elastic deformation of the elements in the contact area, which give a smooth elliptical distribution. Bearing surfaces are not perfectly smooth. For this reason, the actual stress distribution in the contact area will be affected by a random surface roughness. With the presence of a lubricant film on a bearing surface, there is a dampening effect on the stress distribution and the acoustic energy produced will be low. When lubrication is reduced to a point where the stress distribution is no longer present, the normal rough spots will make contact with the face surfaces and increase the acoustic energy. These normal microscopic deformities will begin to produce wear and the possibilities of small fussers may develop which contributes to the prefailure condition. Therefore, aside from normal wear, the fatigue or service life of a bearing is strongly influenced by the relative film thickness provided by an appropriate lubricant. The proper amount of lubrication is very important. If a bearing is over lubricated the bearing can be pushed excessively by the lubricant causing additional wear of the bearing. On the other hand, if there is not enough lubricant, the bearing will rub on the solid surface. This causes friction and wear on bearings. Either case is detrimental to the life of the bearing. Ultrasonics can be used in conjunction with a plant s lubrication program to properly lubricate bearings. To avoid lack of lubrication: As the lubricant film reduces, the db level and sound of the bearing will increase. When lubricating, add just enough to return the bearing to baseline readings. Some lubricants will need time to work uniformly over the bearing. Lubricate a little at a time. Bearing Analysis Using the SonicScan 7000 Many predictive maintenance teams do not have a large budget with which to work. The result is a lack of training and technology for monitoring equipment. Other predictive maintenance teams are tasked with monitoring a tremendous amount of equipment, which does not allow much time for analysis. What maintenance departments need is an affordable, easy-to-use analyzer with adequate diagnostic capabilities. To address these needs CSI developed the SonicScan SonicScan 7000 has several features that enable quick and accurate bearing analysis, such as dual processing paths and autoranging, threshold alarms, multiple data parameters, a demodulated envelope output, and a wireless connection to a handheld personal computer (H/PC).

37 SonicScan s dual processing paths enable the operator to adjust the volume level to a preferred comfort level while maintaining the integrity of the incoming signal. Additionally, the autoranging feature automatically adjusts the headphone volume so that background noises are minimized and any impact noises are accentuated. SonicScan has two threshold alarms: a db alarm and a temperature alarm. The db alarm allows the user to, based on previous experience, set a threshold to warn when the sound output of the bearing increases beyond historical levels, indicating a change in condition. The temperature alarm works the same way the db alarm feature does, alarming when the surface temperature exceeds a temperature threshold determined by previous experience. The four data collection parameters, Peak, Peak Hold, Average and Peak Factor, are used to determine the type and severity of a bearing problem. Typically, as data is collected, the Peak parameter, providing an instantaneous db level, or the Peak Hold parameter, which provides the highest detected db level, is displayed. When monitoring bearings, either mode is sufficient for comparative analysis. For advanced analysis, use the Average mode and the Peak Factor to characterize the signal. The Average mode performs a linear average of the measured db values, providing a db level where the majority of the energy is found. The Peak Factor mode determines the difference between the Peak Hold value and the Average db value. Figure 4 Component Problem The corresponding db levels measured on the bearing in Figure 4 are: Peak Hold: 36 db Average: 0 db Peak Factor: 36 db These modes are used as a diagnostic tool, for comparative analysis. If the db level in the Peak Factor mode exceeds that of the Average mode, meaning that the average noise level for the bearing is relatively low, there are distinct peaks in the signal that increase Peak Factor value. In this case the bearing potentially has a component fault. Figure 1 illustrates this - note the overall level of the energy is considerably lower than the higher peaks. When listening to a bearing displaying these characteristics, a distinct, repeated, clicking sound is heard. Figure 5 Lubrication Problem The corresponding db levels measured on the bearing in Figure 5 are: Peak Hold: 60 db Average: 43 db Peak Factor: 17 db

38 In a bearing with a suspected fault, if the db level in the Average mode exceeds that of the Peak Factor mode, meaning that the general noise level of the bearing is high, the bearing has a possible lubrication problem. This is seen in Figure 5, note the general random energy creates a higher average value, as there are no significant peaks. When listening to a bearing with these characteristics, a random grinding noise should be heard. Figures 4 and 5 are demodulated enveloped waveforms recorded from SonicScan using a 2120 machinery analyzer. Using MasterTrend/RBMware and a 2120 machinery analyzer an ultrasonic route can be set-up for periodic bearing monitoring. Figure 6 - Faulty bearing: Spectrum and Waveform taken with the SonicScan 7000 and a 2120 Figure 7 Good bearing: Spectrum and Waveform taken with the SonicScan 7000 and a 2120 Figures 6 and 7 show the recorded with SonicScan and a 2120 on a bearing that has a fault in the outer race and a bearing that is in good condition. While both bearings exhibit ultrasonic energy in the waveform, the faulty bearing shows a repeated spiked pattern in the energy indicating an anomaly in the bearing. Ultrasonic monitoring works very well along side vibration analysis to detect and confirm problems.

39 Figure 8 - Trend plot Figure 8 is a Trend plot for a bearing taken with the SonicScan and a The condition of the bearing appears to be deteriorating rapidly. This trend plot shows the condition of the bearing from June of 1988 to February of The bearing fault appeared in the vibration data in August. Finally, a hand held portable computer (HPC) can be used in the ultrasonic data collection process. The SonicHPC enables trending of the db level in all four data collection parameters and of the temperature. This feature also allows a more thorough analysis of bearing condition. SonicHPC enables a route based ultrasonic program that is easy-touse and provides adequate diagnostic capabilities. Figure 9 - H/PC Data Input Screen Figure 9 is the data screen from the SonicHPC application, showing the current Peak Hold value and displaying the previous measured db level. Using ultrasonics to monitor bearing condition is not a new concept, but the quantitative process of ultrasonic monitoring is. Using the different parameters, integrating with the 2120, or communicating with the SonicHPC, the SonicScan 7000 provides the capabilities and accessories for quickly and accurately diagnosing bearing conditions in an affordable and easy-to-use tool. Section 6 Valves

40 Valve Analysis Structure borne ultrasound is generated when sound vibrations transfer energy in metal through a series of particle displacements. When inspecting valves, ultrasound originates in the fluid within the piping. When inspecting valves, much of the ultrasound originates in the fluid in the containment. Energy is transferred through particle displacement from the fluid to the containment. During this process, mode conversion occurs. In order to detect the ultrasound, the contact probe is used. In piping, ultrasound is created by the fluid passing through the piping. Energy is transferred from the fluid through a series of particle displacements that eventually effect the particles in the wall of the pipe. By using the contact probe on the pipe, we minimize acoustic impedance with the metal to metal contact. The ultrasound, in the form of plate and surface waves, travel up the contact probe and is received by the transducers. The sound is then heard and provides details on a substances flow. Valve Failure Modes Valves either enable or inhibit gases or liquids flowing through a system. Valves fail similarly to Steam Traps - open or closed. Some valves, when open, create very little disturbance in the flow that can be detected ultrasonically. This must be taken into account when diagnosing a valve s condition. Failed Open When a supposedly closed valve does not seal completely the valve is said to have failed "open". In this case, there is considerable ultrasonic noise downstream as the valve creates disturbances or eddies in the flow of the gas or liquid. These eddies can be detected ultrasonically to determine if a valve closed properly. Failed Closed If no sound is heard downstream from a supposedly open valve, the valve may have failed "closed." Again, it must be noted that some valves, when open, create very little disturbance in the flow that can be detected ultrasonically. Testing Valves When testing valves it is important to be certain on the position where the valve is supposed to be. Also, the line behind the valve must be full and pressure must exist on the line. Listen to the inlet and outlet side of the valve. And note the db level and sound characteristics for each. Adjust the position of the valve.

41 Testing On/Off Valves Listen to the inlet and outlet side of the valve again. Compare the db levels and sound characteristics after the change. Based on the supposed position of the valve, its operation can be determined. Valves typically operate two ways, either modulating or are an on/off design. Examples of on/off valves would be gate, globe, butterfly and ball. These valves are used in flow or no-flow applications. Modulating valves, such as globe or V- ball, are used for controlling flow at all times. The testing procedure for both valves is similar, but testing personnel must identify the type of operation of the valve. On/Off valves are checked for position open or closed or for leakage in the closed position. The first step in testing a valve for leakage is to visually confirm if the valve is closed or in the off position. The valve is then tested for leakage. First, contact two points upstream of the valve, test points A and B. Second, the downstream side is then contacted at two test points, C and D. The sound intensity of the baseline is compared to test point C. Point D is measured to insure there is no other ultrasonic sound downstream of the valve. If Point D is higher than Point C, ultrasonic sounds down from the valve may be interfering with the readings. If D is less than C, the valve test is valid. Modulating valves are tested in the same manner as On/Off valves. The valve is tested the same to insure proper seating of the valve. The valve can also be tested during set-up to insure proper stroking of the valve. First, test the valve seat or leakage. If the valve is not initially seated, the control valve will not function properly or the valve stroke will be off. The testing procedure to set-up, or test the valve stroke is to listen to the valve discharge at test point C. As air pressure is increased, ultrasonic signal strength should also increase. Section 7 Electrical Electrical Inspections

42 Ultrasonic emissions from electrical apparatus have a very distinctive sound due to their unique characteristics. This section focuses on ultrasonic emissions caused by electric anomalies such as corona, tracking and arcing. Each of these problems has their own sound characteristics and coincide with a level of severity to equipment. By identifying one electric ultrasonic emission from another, the inspector is able to evaluate the condition and severity of the test equipment. Theoretically, ultrasound can be used in low, medium and high voltage systems. Low voltage applications typically display less ultrasonic phenomena, so in reality, it is used more often in 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. Most often this sound will be heard as a crackling noise, frying sound, or as a buzzing. Ultrasound bounces and penetrates any size opening, allowing testing of enclosed systems that are energized. Typical applications include: insulators, cable, switchgear, bus bars, relays, contractors, junction boxes. In substations, components such as insulators, transformers and bushings may be tested. The airborne ultrasound inspector must always consider the safety when performing inspections. It is extremely important that the inspector understand the electrical apparatus that is being inspected. Failure to do so can lead to serious injury or death. Physics of Electricity In order to understand electrical inspection, the primary forces in electricity must be understood. The fundamental forces that control every electrical circuit are voltage, current and resistance. All matter is composed of atoms, which are made up of subatomic particles, some of which are negatively charged electrons and positive charged protons. Usually an atom is stable and has no net charge, meaning the electrons and protons are equal in number and balanced. When electrons are pulled free from atoms, there is an excess of negative charge in one area and an excess of positive charge in another. This happens when electrons are transferred between two objects. This results in static electricity. Coulomb s Law states positive and negative electric charges attract one another and like charges repel one another with a force that is proportional to the amount of charge and diminishes with the square of the distance. This is illustrated in the diagram above. When negative and positive electrons are separated a potential for electricity exists. The amount of work that it would take to move a charge to another point of travel is called voltage. Voltage is the force that pushes the electric current through the electrical circuits. It is the same as pressure is to a water system the more pressure, the faster the water will flow, meaning, the higher the voltage, the more current will flow through the electrical system. A volt is a measure of the potential energy of the source pushing electrons through the wire. Ionization

43 The illustration above shows the collision process. This creates positive ions and free electrons and are able to create negative ions or oppositely charged ions under certain conditions. High temperatures can make atoms or molecules active enough to collide. Electrons of an electric discharge such as lightening or an electric arc produce ions. Ultrasonics detect ionization in the air due to the turbulence that is created from the ionization process. Detectable electrical conditions that are created by this turbulence are corona and arcing. Electric Current Electric current is created when separated electric charges are allowed to move. The current is the rate of flow of the electric charge. An electric current consists of charged particles, typically electrons, moving through a conductor. One unit quantity of electrical charge is called a coulomb. Current, which is measured in amperes (amps), is the rate of flow of electrical current or intensity (I) of current flow. Electricity is measured by a flow of electrons through a conductive material. For example, 12 amps would have 4 times as many electrons flowing through a circuit than one with 3 amps. Watts is the electrical unit for power and is calculated by voltage (E) times current (I). Forty (40) volts with 4 amps of current equal 160 watts. Resistance Resistance is the resistance to the flow of electricity and is measured in ohms. It is a characteristic of a conductor, which are influenced by the type of material, the temperature of the conductor and the diameter of the conductor material. As the temperature of the conductor increases, the resistivity of the conductor increases. As an example, plastic has a very high resistance (an insulator) and copper has a very low resistance (a conductor). A resistance factor of 20 ohms will restrict twice the amount of current than a resistance factor of 10 ohms. Conduction of Electricity by Air Air is generally a good insulator. When electricity jumps between components, air becomes a conductor. Lightening is a good example of an electrical current caused by arcing breakdown of the atmosphere. Hundreds of thousands

44 amperes of electrical current rapidly heats the air in its conduction path. This rapid expansion of air acts like an explosion to generate ionized air. The air in the path of a lightening stroke is a conductor. The ionization produces strong ultrasonic components. Air suddenly changes from an insulator to a conductor at a the sparking voltage. Corona When voltage on electrical conductors, such as high-voltage transmission lines, exceeds threshold value, the air around them begins to ionize to form a blue or purple glow. The glow or electrical discharge around the component is called corona. Corona is a condition that is present before flashover occurs. The air between layers of insulation becomes charged when electrical stress exceeds the insulation value of the air. Corona occurs to form Ozone and Nitrogen Oxides. These combined with moisture produce Nitric acid, which is destructive to most dielectrics and certain metallic compositions, resulting in corrosion. Humidity and moisture contribute to this condition and are an important factor. The more humidity, the more potential for corona since it will help conduct electricity around insulators and pot heads. Therefore, it is important when testing to consider and possibly record atmospheric humidity for trending purposes. The more humidity present, the more corona present. This does not mean the corona is destructive. Corona can be a serious problem for operators of high voltage equipment. Corona sees a path to ground, causing several problems. Aside from energy loss with alternating current on high voltage lines, corona can cause surface deterioration and breakdown on solid insulation surfaces, serious chemical effects, or the loss of a transformer. Common equipment surveyed for corona discharge are: transformers, cables and switchgear. Since corona is strictly discharging to the air and there is no ground, there is no heat build-up. Because of this, corona is not detectable by infrared. Corona discharge has a constant, buzzing sound that is easily detected with ultrasonics. Tracking Tracking is often called baby arcing. It follows the path of damaged insulation. Tracking differs from corona in that it starts almost silent and builds in intensity until it reaches flashover. At this point it discharges and begins the process again. Tracking occurs when contaminants such as dust, dirt or moisture are present allowing electricity to follow a path. Tracking begins a path to the ground. Most often, tracking does not generate heat initially. As the condition worsens, heat will be produced. When using ultrasonics to detect tracking, listen for a buzzing or crackling that increases in amplitude and then stops. The sound will continuously repeat signaling tracking is present. Arcing

45 An arc occurs when electricity flows through space. Lightening is a good example of arcing. Arcing is very abrupt. It begins at full intensity, and ends quickly. Arcing always develops a path to the ground and is often accompanied by heat as well as sound. This is cause for serious concern as failure is occurring. The ultrasonic noise associated with arcing is a crackling, buzzing, or crackling. Visual Inspection Electrical faults corona, arcing and tracking provide visual evidence of their presence usually. Usually there is some form of discoloration. Visual inspections are crucial parts of all ultrasonic inspections and can aide in identifying many problems. Important Factors in Electrical Inspections Sonic Deflection Sonic deflection is a problem when conducting electrical inspections. Electrical inspections can be thought of as electrical leak inspections. When conducting electrical inspections follow the same procedures as in leak detection. Move back and forth, alter the scan angle and direction when possible to confirm the source of an anomaly. Sonic deflection can be a source of confusion and can lead to false readings. Attenuation Attenuation of ultrasound caused by ionization in air during corona, tracking and arcing is caused by a loss of energy. The energy peaks and valleys decrease in height as the wave loses energy. Sound Quality In the interpretation of the site location, it is important to understand sound quality for diagnosis. This includes understanding the difference in sound pattern of arcing, tracking and corona discharge. It also ties into understanding the difference of a normal 60-cycle hum and an electrical emission, as well as the difference between mechanical vibrations and electrical emissions.