MEASURING OXIDE SCALE ON INNER SURFACES OF BOILER TUBES

Abstract: In a method of ultrasonically detecting and measuring oxide scale (12) on an inner cylindrical surface of a fluid-containing boiler tube (10) in situ within a boiler, an ultrasonic pulse is directed into the tube (10) by a transducer (14), the time of flight of the ultrasonic pulse within the scale (12) is determined, and the determined time is correlated to the thickness of the scale (12).

Description: This invention relates to the detection and measurement of oxide scale on inner cylindrical surfaces of boiler tubes. Scale deposits on the water side of boiler tubes undesirably increase heat transfer resistance and have long been recognised as a cause of boiler tube failure. In the electric utility industry, a metallurgical technique is typically employed to measure scale thickness to predict the remaining life of high pressure boiler tubes. According to the metallurgical measuring technique, a sample of the boiler tube is removed from the boiler and a new section of tubing is installed in its place. The sample is sent to a metallurgical laboratory. A nickel coating is applied to the sample to prevent the scale from breaking loose during sample preparation. Next, small sections are cut from the tube sample, mounted, polished, etched and examined under a metallurgical microscope. Once measured, the thickness of the scale is used in an algorithm to predict the remaining life of the boiler tube. Metallurgical examination provides a very accurate measurement of scale thickness. However, such examinations inconveniently require physical removal and replacement of boiler tube sections and analysis of the removed tube sections at laboratory facilities each time a scale thickness measurement is desired. The overall procedure is very time-consuming and expensive as it directly relates to downtime, i.e. time loss in ability to generate power. At the present time, there are no known commercialised non-destructive techniques for measuring or detecting scale on the inner cylindrical surface of boiler tubes. The recovery and utility boiler industries have expressed a need for such techniques. Ultrasonics are presently utilised, in the inspection of boiler tubes, for example, for detecting surface and subsurface flaws and for the measurement of the thickness of a material or the distance to a flaw. Japanese Patent Application Publication No. JP-A-60 142 210, an English-language abstract appears at PATENTS ABSTRACTS OF JAPAN, Vol. 9, Number 312 (P-411)(2035), 7 December 1985, discloses a method of ultrasonically measuring the thickness of an inner layer of Zr forming part of a two-layer tube of which the outer layer is of Zr alloy. According to the method, the tube is immersed in a fluid medium and a line focus type probe is disposed in the fluid medium at a position spaced from the tube and energised to transmit an ultrasonic wave having a frequency of 20 to 100 MHz towards the tube. The ultrasonic wave is reflected at the Zr/fluid medium interface, the Zr/Zr alloy interface, and the Zr alloy/fluid medium interface. The thickness of both the inner (Zr) layer and outer (Zr alloy) layer are, it is said, measured accurately and surely from the time intervals (differences in times of flight) between the respective echoes (reflections). The invention provides a non-destructive ultrasonic method of measuring scale thickness in situ on inner cylindrical surfaces of in-service boiler tubes. The method uses the round trip time difference, referred to as the time of flight (ToF), between pulse echo reflections at the tube's metal/scale interface and scale/fluid interface as a basis for obtaining a reasonably reliable indication of scale thickness. More particularly, the invention provides a method of ultrasonically measuring oxide scale formed in the presence of steam on a cylindrical inner surface of a boiler tube within a boiler, the method comprising: establishing a relationship between the thickness of said scale on a cylindrical inner surface of a boiler tube to the time of flight of an ultrasonic pulse within the scale by first subjecting a plurality of samples of the boiler tube including scale of varying thicknesses to ultrasonic pulses to determine the time of flight within the scale, physically measuring the thicknesses of the scale on the respective ones of the samples, and then correlating the physical thickness measurements of the oxide scale with the time of flight of the ultrasonic pulse in the oxide scale; positioning an ultrasonic transducer in an orientation on the outer surface of an area of a boiler tube at which said scale is formed on the inner surface for directing transmission of an ultrasonic pulse into the boiler tube so that the centreline of the ultrasonic beam is perpendicular to the inner surface of the boiler tube; energising the transducer to transmit an ultrasonic pulse having a frequency of at least 50 MHz into the boiler tube; measuring a first time of flight representative of the time for the ultrasonic pulse to be transmitted to and reflected from the tube/scale interface; measuring a second time of flight representative of the time for the ultrasonic pulse to be transmitted to and reflected from the scale/tube interior interface; determining the difference between the second time of flight and the first time of flight; and determining the scale thickness from said difference and said relationship. The measurement or determination of the scale thickness is obtained by comparing or correlating the time of flight within the scale (the difference between the first and second times of flight) to the pre-established relationship; which may be a correlation curve or a formula. The relationship, which relates oxide scale thickness to the time of flight of the ultrasonic pulse within the scale, is provided or pre-established by subjecting a plurality of samples of the boiler tube to ultrasonic pulses and then actually measuring scale thickness, for example by physical or metallurgical means. Once the standard is determined by such techniques, further destructive testing of the boiler tubes is no longer necessary. A preferred feature of the invention is the use of an ultrasonic pulse of a frequency of 50 MHz for measuring scale as thin as 127 micrometres (5 mils). A preferred embodiment of the invention described in detail hereinbelow provides a rapid method of measuring the thickness of oxide scale formed in the presence of steam (hereinafter also referred to as "high temperature oxide scale") on the inside surface of boiler tubes without having to continously remove sections of tubing from the boiler. Because tube samples are not removed, more data points can be obtained in a shorter amount of time. The preferred method, moreover, permits the examination of many tubes in various locations of the boiler. The availability of additional data enhances the ability to accurately predict the remaining life of the boiler tubing. The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which like references designate like or corresponding parts throughout, and in which: Figure 1 is a schematic representation of an arrangement for ultrasonically determining the thickness of scale on an inside surface of a boiler tube in accordance with a method embodying the invention; Figure 2 is an oscillogram of pulse reflections characterising the response of a boiler tube containing scale; and Figure 3 is a graph depicting a correlation curve illustrating actual thickness of a scale in relation to time of flight of an ultrasonic signal within the scale. Figure 1 schematically illustrates an arrangement for determining the thickness of scale on an inner cylindrical surface of a boiler tube in accordance with a method embodying the invention. In Figure 1, a cylindrical boiler tube 10 is shown with a layer of scale 12 formed on its surface. A hand-held contact ultrasonic transducer 14, such as a model V214-BA hand-held transducer produced by Panametrics, Watham, Massachusetts, U.S.A., is positioned on the clean, outer surface of the boiler tube 10. To facilitate the transmission of ultrasound into and out of the tube 10, which is of steel, a low viscosity couplant 13 is used between the transducer 14 and the steel tube 10. The illustrated arrangement enables the ultrasound to penetrate heavy wall boiler tubing and measure the scale. The hand-held technique allows rapid repositioning of the transducer 14 to different locations on a boiler tube or to different boiler tubes. The transducer 14 is electrically connected via a coaxial cable 18 to a high-frequency pulser/receiver 16 which, in turn, is connected to a delayed time, pulse overlap oscilloscope 28 having a delayed time base and a pulse overlap feature for conveniently and accurately measuring a differential time of flight (ToF). The transducer 14 is a high frequency transducer. As used herein, high frequency is intended to refer to frequencies of 50 MHz or greater. In a preferred arrangement, the transducer 14 has a circular active element with a diameter of 6.35 mm (0.250 inches) and operates at 50 MHz. The element is attached to a cylinder of fused silica measuring 18.29 mm (0.720 inches) in diameter and 12.70 mm (0.500 inches) thick which produces a delay in the ultrasonic signal of 4.25 microseconds. A Panametric V214-BA transducer has been experimentally used with favourable results. A model PR 106 pulser/receiver, produced by Metrotek, Richmond, Washington, U.S.A., has been found to be suitable for use as the pulser/receiver 16. The high-frequency pulser/receiver 16 is a device preferably selected to produce a high frequency pulse of short duration with a wide (60 MHz) bandwidth. This enables an ultrasonic signal to be produced and received that is capable of resolving the energy reflected from both tube/scale and scale/fluid interfaces. A Panametric 5600 pulser/receiver has also been experimentally used with favourable results. In a preferred arrangement, the high-frequency pulser/receiver 16 consists of or comprises a power supply 22, a MetroTek PR 106 pulser/receiver 20, a decoupling device or decouplant 24 and a repetition rate generator 26. This arrangement operates at 200 volts with a rise time of 6 nano seconds and a bandwidth of 60 MHz. The oscilloscope 28 is preferably a high frequency delayed time base oscilloscope. A Tektronik 2236 oscilloscope having a bandwidth of 100 MHz and a delayed time base capable of 5 nanoseconds per division has been used on an experimental basis. The instrument has a pulse overlap capability which can be used for measuring the time of flight in the scale. The transducer 14 is positioned so that the inside surface of the tube 10 is normal to the ultrasonic beam. An ultrasonic signal is then generated and received by the high-frequency pulser/receiver 16. The signal is displayed on the oscilloscope 28. Figure 2 is an oscillogram which shows the signals produced on a cathode ray tube display formed by the pulse echo interface reflections of the ultrasonic wave. The impedance mismatch between the tube 10 and the scale 12 formed on the inside surface of the tube 10 causes the ultrasonic wave to be reflected. The amplitude of the reflected signal is related to the impedance ratio between the two materials. The amplitude of the reflected signal increases with greater impedance ratios. Similarly, a reflection occurs at the interface of the scale and fluid within the tube. Since the impedance ratio between the wall and the scale is very small, the signal amplitude is small. However, the impedance ratio between the internal scale and the fluid is larger and, thus, a larger signal is reflected and displayed on the cathode ray tube. Thus, as shown in Figure 2, a first time flight (ToF₁) to and from the tube metal/scale interface and a second time of flight (ToF₂) to and from the scale/fluid interface are determined. The difference between the first and second times of flight may be correlated via a correlation chart, as shown in Figure 3, or by formula, in order to determine the thickness of the scale. The following examples are illustrative and explanatory of portions of a method embodying the invention. EXAMPLE A series of boiler tube samples were cut from a radiant boiler that had run for 219,000 hours or approximately 30 years. The samples were cut from pendant tubes in a reheater section. Samples A to D were made of a ferritic steel with 2.25% chromium and 1.0% molybdenum, marketed under the name Croloy 2-1/4. The tubes had an outside diameter of approximately 50.8 mm (2 inches). A heavy-oxide coating on the outside of the tubes was mechanically removed. To measure the scale on the inner surface of the tubes, each sample was mounted on a V-block to stabilise the sample. A 50 MHz transducer was positioned on the surface of the sample and adjusted so that the inside surface of the tube was normal to the ultrasonic beam. The ultrasonic signal was generated and received using a high frequency pulser/receiver. The signal was displayed on a 400 MHz waveform processing oscilloscope, where time of flight measurements were made. With the system assembled as described, the time of flight measurements were rapidly obtained. After completing the ultrasonic measurements, the samples were marked and labelled to indicate locations for metallurgical measurements. The locations were selected so that the data points would be equally distributed over the entire oxide scale thickness range. The scale surface was nickel coated to protect the scale. Then, the samples were mounted, polished and measured. The results and the metallurgical measurements of the ultrasonic ToF measurements are shown in Table 1 below. Statistical analysis of the results give in Table 1 indicates that the ultrasonic and metallurgical oxide scale thickness have a correlation factor of 0.923 out of 1.00. Based on the linear least square curve fit technique, the ultrasonic and metallurgical results are related by the following equation: Oxide Thickness = (0.1619 x (ToF₂ -ToF₁)) - 1.42 where the oxide thickness is in mils (1 mil = 10⁻³ inches) and ToF is in nanoseconds. The oxide thickness in micrometres can, of course, be otained by multiplying the value in mils by 25.40. The ultrasonic test frequency depends upon the nominal value of the scale thickness to be measured. To resolve the interfaces, the scale thickness must be at least one wavelength of the ultrasound. The wavelength is determined by the velocity of sound in the material being inspected and the frequency of the transducer being used according to the relationship of velocity equalling the multiplication of the frequency and wavelength. Table 2 below illustrates the minimum scale thickness that can be resolved at various frequencies in steel where the velocity is conservatively approximated at 5939 m/s (2.338 x 10⁵ in/s). Since the velocity of sound in scale is not known and will vary in scales of different compositions, the time of flight technique does not produce an absolute or exact scale thickness. However, the time of flight data is related to actual scale thickness measurement established by physical techniques such as metallurgical examination. An actual scale thickness relationship (hereinafter also referred to as a "standard") is predetermined by subjecting a plurality of samples of the boiler tubes which include varying thickness of the scale to ultrasonic pulses to determine the time of flight within the scale by the techniques described herein. Thereafter, the scale on the samples is physically measured and a formula or correlation curve relating scale thickness to the time of flight of the pulses in the scale is established. The predetermined standard, i.e. curve or formula, is used in further testing, thereby obviating the need for future destructive tests. Thus, a standard can be established for a particular type of scale by a correlation, by formula or by a curve of the type shown in Figure 3. Testing has established that frequencies in the region of 5 and 10 MHz cannot be used to measure the thickness of oxide scale, although testing indicated that a highly damped 10 MHz transducer with a laboratory grade pulser/received and oscilloscope can detect but not measure the present of scale on the inner surface of a boiler tube when the thickness of the scale is greater than 0.178 mm (0.007 inches). In operation, the ultrasonic transducer is positioned on an elongate boiler tube within a boiler. The transducer may be placed directly in contact with the outer surface of the boiler tube or closely-spaced in relation thereto by the use of an appropriate couplant. The transducer is aligned so that the centreline of the ultrasonic beam which it will produce is directed perpendicularly to the inside of the boiler tube so that the incident beam will produce reflected beams that return to the transducer. The transducer is energised to transmit an ultrasonic pulse having a minimum frequency of 50 MHz into the boiler tube. The time of flight of the ultrasonic pulse transmitted to and reflected from the tube/scale interface of the oxide scale at the inner surface of the tube is measured. Similarly, the time of flight of the ultrasonic pulse transmitted to and reflected from the scale/fluid interface within the tube is measured. The difference between the two times of flight, i.e. the time of flight within the scale, is used to determine the thickness of the oxide scale. Most typically, this technique will be applied to areas of the boiler tubes that contain steam and which, accordingly, are more susceptible to a high temperature, relatively homogenised oxide scale formation. The ultrasonic transducer preferably utilises a contact technique which produces sufficient ultrasound to be transmitted into the boiler tube so that sufficient high frequency ultrasound can be reflected from the tube/scale interface and allow resolution from the reflection which will occur at the inside surface of the tube.

Claims: A method of ultrasonically measuring oxide scale (12) formed in the presence of steam on a cylindrical inner surface of a boiler tube (10) within a boiler, the method comprising: establising a relationship between the thickness of said scale on a cylindrical inner surface of a boiler tube and the time of flight of an ultrasonic pulse within the scale by first subjecting a plurality of samples of the boiler tube including scale of varying thicknesses to ultrasonic pulses to determine the time of flight within the scale, physically measuring the thicknesses of the scale on the respective ones of the samples, and then correlating the physical thickness measurements of the oxide scale with the time of flight of the ultrasonic pulse in the oxide scale; positioning an ultrasonic transducer (14) in an orientation on the outer surface of an area of a boiler tube (10) at which said scale is formed on the inner surface for directing transmission of an ultrasonic pulse into the boiler tube so that the centreline of the ultrasonic beam is perpendicular to the inner surface of the boiler tube; energising the transducer (14) to transmit an ultrasonic pulse having a frequency of at least 50 MHz into the boiler tube (10); measuring a first time of flight (ToF₁) representative of the time for the ultrasonic pulse to be transmitted to and reflected from the tube/scale interface; measuring a second time of flight (ToF₂) representative of the time for the ultrasonic pulse to be transmitted to and reflected from the scale/tube interior interface; determining the difference (ToF₂-ToF₁) between the second time of flight and the first time of flight; and determining the scale thickness from said difference and said relationship. A method according to claim 1, wherein the ultrasonic pulse has a frequency of 50 MHz. A method according to claim 1 or claim 2, wherein the transducer (14) is a hand-held contact transducer and the transducer is placed manually on the outer surface of the boiler tube (10). A method according to claim 1, claim 2 or claim 3, wherein said relationship is a formula relating scale thickness to the difference between the second time of flight (ToF₂) and the first time of flight (ToF₁). A method according to claim 1, claim 2 or claim 3, wherein said relationship is a correlation curve relating scale thickness to the difference between the second time of flight (ToF₂) and the first time of flight (ToF₁).

Inventors: LESTER, SAMUEL ROBERT
Applicants: THE BABCOCK & WILCOX COMPANY
Application year: 1987
Main classification: G01B17
IPC codes: G01N29, G01B17