A Guide To Using Ultrasound For Leak Detection And Condition Monitoring. By Thomas J. Murphy and Allan A. Rienstra

Transcription

1 A Guide To Using Ultrasound For Leak Detection And Condition Monitoring By Thomas J. Murphy and Allan A. Rienstra

2 Presents... You have received a chapter of HEAR MORE by Thomas J. Murphy and Allan A. Rienstra The use of airborne ultrasound by a wide array of manufacturing has led many to dub ultrasound a predictive maintenance tool for the masses. It is a technology with mass appeal, a wide range of applications, and a cost entry point that makes it accessible to practically anyone. In this book, the authors guide you through the technology step by step, with each chapter dedicated to an application and how the technology applies to that application. You will learn how the inspection should be carried out, along with real-life examples of how these applications are currently being applied. To order this book, go to The MRO-Zone Bookstore website:

3 Chapter 1: Introduction Knowledge of the existence of sound beyond the range of human hearing has been around since the late 18th century when it was discovered that bats use hearing rather than vision to navigate. In 1881, Pierre Curie reported his discovery of the piezoelectric effect a discovery which was vital to the world of vibration and ultrasound. The large majority of sensors (and transmitters in the case of ultrasound) used in vibration and ultrasound applications use piezoelectric crystals as the transducer which converts motion into voltage or vice versa. Without piezoelectricity, we would be left with moving coils, moving magnets, and little else. After the sinking of the Titanic in 1912, there was great interest in developing a technique which could be used to detect icebergs. This stimulus gave us the hydrophone. A hydrophone is basically an underwater microphone. It detects sound travelling through water and converts the acoustic pressure into a voltage which can be processed or listened to. The next logical stage of development for the hydrophone was to use an ultrasound sensor. By 1916, this ultrasonic device was being used to hunt submarines. In the very early days, this was accomplished by listening for the noise they made as they travelled through the water. The parallel development of sonar (Sound Navigation and Ranging), first patented just after the sinking of the Titanic, copied the navigation of bats, using hydrophones to generate sounds and pick up the reflection of those sounds to calculate depth and bearing. Medical applications of ultrasound began to be reported in the 1930 s. You are probably aware of the use of ultrasound in obstetrics. However, medical and biomedical applications go far beyond this small area and include its use in the areas of soft body tissue inspection and repair, heart, kidney and liver, muscles, 5

4 Hear More ligaments and tendons. Because ultrasound is live and available in real time, it is often used to guide interventional procedures such as fine needle operations. The development of portable ultrasound equipment for airborne leak detection started in the late 60s and early 70s. This gave birth to the use of this technology as a maintenance tool. When compared with the two big PdM players of infrared and vibration, ultrasound has a much larger range of application both inside PdM and beyond airborne/contact, rotating/non-rotating, predictive, energy, quality, safety, industrial, marine and power transmission are just some of the many areas of application. Despite almost forty years of sales, marketing, and technical development, ultrasound has been quite slow to gain acceptance. The rise in popularity of airborne ultrasound for use in predictive maintenance programs in the last ten years can be attributed to three factors: ease of use, versatility, and low implementation cost. Once considered a companion technology to core predictive tools such as vibration and infrared analysis, you now see the emergence of stand-alone ultrasound inspection programs as standard practice for maintenance departments around the globe. Ultrasound is now considered a front-line defense system in the everyday battle for manufacturing uptime. The use of airborne ultrasound by a wide array of manufacturing, from mining to power generation and from waste management to food production, has led many to dub ultrasound a predictive maintenance tool for the masses. It is a technology with mass appeal, a wide range of applications, and a cost entry point that makes it accessible to practically anyone. The purpose of this book is to inform. We want to open your eyes to the vast range of possibilities for using this technology, and along the way correct a few false truths. To accomplish our objective, we chose to lay out this book in much the same manner we would teach the technology to an aspiring ultrasound inspector. As you work your way through the wonders of this technology, you will no doubt realize that it can be used every day of the week for a different 6

5 Introduction type of job. Therefore, each chapter is dedicated to an application, with a focus on how the technology applies to that application. You will learn how the inspection should be carried out, and then read one, two, or three examples of how these applications are currently being applied in interesting ways, by interesting people your peers and colleagues, in interesting factories both stationary and moving, around the globe. 7

6 Chapter 2: Principles of Sound In this chapter we will discuss: What is sound What is ultrasound How sound and ultrasound are measured How the properties of sound change with frequency Sound and the behavior of sound, acoustics, is a vast and complex subject. Attempts in the past to simplify and ignore some basic fundamentals have resulted in misconceptions, mistakes, and errors. In a world where the old saying, A bad workmen always blames his tools! is commonplace, it is not surprising that ultrasound doesn t work is heard too often. Since acoustical terms are used in many walks of life, frequently multiple words, from different sources, are used for the same parameter. For example, in music you might refer to the pitch of a sound. Pitch is the musical term for frequency. So let s go back to the basics. Sound Sound is a mechanical wave. It is not an electromagnetic wave like light or radio. It requires a medium through which to travel and cannot exist in a vacuum. Sound is a pressure wave. It is a longitudinal wave. The wave propagation is along it like the motion of a slinky or a worm. 9

7 Hear More LONGITUDINAL PRESSURE WAVE Energy Wave Particle Movement Direction This motion produces areas of higher and lower pressure which is referred to as compressions and rarefactions. It is these pressure fluctuations that you can hear and measure. AREAS OF COMPRESSION AND RAREFACTION Compression Rarefaction If a transducer were used to detect fluctuations in pressure as the sound wave impinged upon the device, it would detect a high pressure which corresponds to the compression region and a low pressure corresponding to the rarefaction. 10

8 Principles of Sound A Sine wave, or sinusoidal sound, is one in which there is only one frequency present also known as a pure tone. Connect this pressure transducer to an oscilloscope and you would see the repeating increase and decrease in pressure of the sound wave. AREAS OF COMPRESSION AND RAREFACTION IN A SINE WAVE Pressure Time C = compression R = rarefaction In a rather human-centric manner, sound is generally split into three ranges: The audible range which you can hear: 20-20,000 Hz The infrasound range: below 20 Hz The ultrasound range: above 20,000 Hz (which is of interest to us) Ultrasound Ultrasound is nominally all sound above 20 khz. Medical applications of ultrasound often work at several megahertz, but for our applications, you will normally be using a frequency range around the khz region. We need to quantify sound both in terms of the frequency content and the amplitude. 11

9 Hear More Frequency One parameter that is used is wavelength. The wavelength of a sound is a measurement of length and is the distance between two repeating points in a signal. WAVELENGTH AND PERIOD The frequency of sound is measured in units of Hertz (Hz) and ultrasound in units of thousands of Hertz (khz). Another term used is period. The period of a signal is the reciprocal of its frequency. Where frequency is how many times something happens per second, period is how many seconds it takes for something to happen once. 60Hz means 60 times per second. At 60Hz, an individual event would take place in 1/60 th of a second. 12

10 Principles of Sound Amplitude and Decibels The measurement of amplitude in the case of sound is slightly more complicated than the measurement of amplitude in vibration. This is because in sound a decibel scale is used. If this were not complicated enough, there are actually two decibel scales one is used for the measurement of acoustical sound and a slightly different one for ultrasound, which is actually a measurement of voltage rather than sound itself. The decibel scale is a ratio scale. You do not measure an absolute value. Instead, you scale that value comparatively against some other reference point. This is similar to the measurement of acceleration in the world of vibration. The majority of the world does not measure acceleration in the correct SI unit of m/s², or even the correct imperial unit of ft/s², but instead uses the unit of g, thereby comparing all acceleration levels to the reference value of earth s gravity. A signal of 120m/s² would nominally be said to be 12g. In radio, and mobile telephony, the measurement of signal strength is the dbm, a comparison between the actual signal strength and what that signal strength would be if you were 1 metre from the antenna. In the world of electronics and ultrasound, the dbµv is used all amplitudes are compared with a reference value of 1µV. A signal of 120µV would nominally be described as 120x bigger than our reference 1µV. One key point should fall out of this discussion. It is absolutely meaningless to quote a ratio without a reference, and it is equally meaningless to quote a db value without its reference. Simply saying something is so many dbs conveys little or no useful information without that reference. 20dBV and 20dBµV are different by a factor of 1 million, a difference which is not conveyed if you simply say 20dB. It is equally false to infer that the higher the unreferenced db reading, the more sensitive the system is. If you measure 20dBµV and then change the reference to 1nV instead of 1µV, your 20 dbµv would instantly become 80dBnV. Is your 13

11 Hear More measurement chain in any way, shape, or form more sensitive? No, you have merely changed the scale you are using to give you a bigger number. If this were not confusing enough, we now add more confusion by making this scale logarithmic. The expression used is: db = 20log 10 (V 1 /V 0 ) where in our case V 0 is 1µV Examples: A ratio or factor of 10 (i.e. x10) h The 20log 10 (10) = 20 x 1 = 20dB A ratio or factor of 100 (i.e. x100) h The 20log 10 (100) = 20 x 2 = 40dB A ratio or factor of 1000 (i.e. x1000) h The 20log 10 (1000) = 20 x 3 = 60dB Note the additive property of logs: x1,000 = x10x100 = 20dB + 40dB = 60dB Another common ratio is x2 log 10 (2)=0.301 So a doubling of the amplitude of an ultrasound signal would become: 20log 10 (2) = 20 x = 6dB Similarly: x4 = x2x2 = 6+6 = 12dB Finally, take an example of 52dB: 1. 52dB = 40dB + 12dB 2. Where 40dB = x And 12dB = x4 4. So, 40dB+12dB is the same as x100x4 which = x So, 52dB is a ratio or factor of

12 Principles of Sound It is very important to keep in mind that you NEVER multiply or divide dbs. They are logarithmic values and should only be added or subtracted. To say, for example, that 36dBµV is twice as big as 18dBµV is nonsense. The difference between these two values is 18dBµV which corresponds to a ratio of 7.9. So the voltage amplitude of a signal of 36dBµV is not 2 times higher than the voltage amplitude of a signal of 18dBµV, it is, in fact, 7.9 times higher! Let s apply this to a bearing: 1. In January, bearing A was measured with an ultrasound detector and the value was 10 dbµv. 2. In April, that same bearing measured 62 dbµv dBµV 10dBµV = 52dBµV (factor of 400). 4. Therefore, from January to April, the ultrasonic signal from bearing A increased by a factor of 400 probably the bearing is already broken. As stated earlier, when you multiply in normal numbers, you add the exponents. It is therefore reasonable, without going through the proofs, that when you divide, you subtract. So -6dB is ½ and -52dB would be a reduction by a factor of 400, i.e. 1/400 th. Some Simple Log Voltage Relationships: 2x = 6dB 10x = 20dB ½ = -6dB 100x = 40dB 4x = 2x2x = 6dB+6dB = 12dB ¼ = -12dB One final absurdity, which has crept in to so many mechanical measurement methods used in predictive maintenance, is the insistence on measurement to one or more decimal places. For example, 60dBµV corresponds to 1,000µV while 59.9dBµV corresponds to µV. Is this difference significant? To achieve repeatability in a mechanical measurement in ultrasound (just like in vibration) of better than 15

13 Hear More 10% requires great luck, skill and/or control. ±5% in voltage terms corresponds to ±0.4dB, and a more realistic ±10% in voltage terms corresponds to ±0.8dB. Achieving measurement repeatability of 0.5dBµV or so is quite acceptable. Look-up Table of db to Factor Converstions: db Value Factor of Using this table, you could now say that our earlier example of 120µV equates to a decibel value of roughly 42dBµV. 16

14 Check Out Our Website! Thank you for downloading a Chapter of HEAR MORE by Thomas J. Murphy and Allan A. Rienstra Purchase this book, go to The MRO-Zone Bookstore website:

15 Hear More: A Guide To Using Ultrasound For Leak Detection And Condition Monitoring By Thomas J. Murphy and Allan A. Rienstra