Electromagnetic acoustic boiler tubes inspection with robotic device

Corrosion of boiler tubes remains an operational and economic limitation in municipal waste power plants. The understanding of the nature, mechanism, and related factors can help reduce the degradation process caused by corrosion. The chlorine content in the fuel has a significant effect on the production of gaseous components (e.g., HCl) and condensed phases on the chloride base. This study aimed to analyze the effects of flue gases on the outer surface and saturated steam on the inner surface of the evaporator tube. The influence of gaseous chlorides and sulfates or their deposits on the course and intensity of corrosion was observed. The salt melts reacted with the steel surface facing the flue gas flow and increased the thickness of the oxide layer up to a maximum of 30 mm. On the surface not facing the flue gas flow, they disrupted the corrosive layer, reduced its adhesion, and exposed the metal surface. Beneath the massive deposits, a local overheating of the inner surface of the evaporator tubes occurred, which resulted in the release of the protective magnetite layer from the surface. Ash deposits reduce the boiler’s thermal efficiency because they act as a thermal resistor for heat transfer between the flue gases and the working medium in the pipes. The effect of insufficient feedwater treatment was evinced in the presence of mineral salts in the corrosion layer on the inner surface of the tube.

Eddy Current Array Inspection of Zlin 142 Fuselage Riveted Joints

Foreign objects impact damage characteristics of aluminum/composite hybrid drive shaft

In Japan, pulverized-coal-fired thermal power plant boilers are operated with a high two-stage combustion ratio to reduce NOx emissions. This combustion ratio forms a strongly reducing atmosphere containing H2S at the burner zone. Water-wall tube materials, such as low-alloy steel, sulfidize severely in such an atmosphere. In the past few years, wastage of the tubes observed in limited areas. However, the sulfidation area tends to extend and/or to shift along with changes in combustion conditions and coal properties. As a result, large-area inspections are necessary to identify the sulfidation areas. Several methods for identifying sulfidation areas have been developed, but these methods have various drawbacks, including (a) long inspection times to adequately assess large areas and (b) a need for descaling, which can act to accelerate tube wastage. Accordingly, a new approach is required for quick identification of sulfidation areas in large commercial boilers without any need for wall descaling. Toward this end, we investigated coal ash chemistry, scale structure/composition, and combustion conditions at several pulverized-coal-fired boilers in Japan. The results show that severe sulfidation occurs in areas where coal-ash deposits contain a comparatively high Zn content. In a reducing atmosphere with sufficient sulfur potential, only a limited amount of Zn contained in coal is sulfidized to form ZnS. At the combustion temperature (about 1300 °C), ZnS is present as a gaseous phase. It presumably solidifies on the tube surface, the temperature of which is about 500 °C. Continuous deposition of ZnS acts to increase the Zn content in the coal ash on tube surfaces. A relatively high level of Zn on the tube surface can be an “indicator” for a sulfidation area. With this as an indicator, the entire region of actual boilers can be easily evaluated without descaling and within several hours by the use of a handheld X-ray fluorescence analyzer.

With the ongoing emphasis on the efficient use of energy, much effort is put on the development and application of various heat recovery technologies. Heat exchangers play an important role in energy transfer and utilization and are widely used to recover waste energy from different kinds of heat sources in many fields such as energy and power engineering, petroleum and chemical engineering, air conditioning and refrigeration, aviation and aerospace, etc. In the heat exchangers for waste heat recovery, the gas-side fluid mediums usually are unclean and corrosive. The problems of fouling, erosion and corrosion of heat exchangers in the middle and low temperature flue gas-side caused by the high ash content, viscous and corrosive components are inevitable, and how to effectively solve the problems has been the subject of much research in recent years. In this paper, the researches of fouling, erosion and corrosion characteristics in middle and low temperature flue gas heat exchangers are reviewed. The causes and influence factors are discussed. Firstly, the fouling mechanisms of heat exchangers are introduced and the existing theoretical models, empirical models, and numerical methods of particle transport, impact and deposition are reviewed. The ash fouling characteristics of 6-row tube heat exchangers are presented and the effects of flue gas velocity, particle diameter, spanwise tube pitch, longitudinal tube pitch, tube shape and geometry conditions on fouling rate are discussed. Both decrease of the longitudinal tube spacing and increase of spanwise spacing can effectively reduce the deposition of particles. The geometry of elliptical tubes can fulfill this requirement for fouling reduction with smaller longitudinal spacing and larger spanswise pitch. The feasibility of using oval tube is explored for the fouling reduction. Compared with the circular tube arrangement, the deposition rate of the elliptical tube was decreased by 68.8%. Secondly, the existing erosion mechanisms, theories and methods are introduced. Different erosion models, including the model advantages and disadvantages are also summarized. The dynamics behaviour of the entrained solid particles in the flow and the deformation of the tube surface due to erosion are presented. The effects of particle diameter, particle mass flow rate and fluid Reynolds number are discussed. Among the three parameters, fluid Reynolds number has the most important effect on the erosion of tube surface. Erosion mainly occurs at the first part of the single tube surface. Thirdly, the existing mechanisms, theories and methods of corrosion characteristics of heat exchangers are introduced. Then a coupling numerical model, developed in author’s previous research, is discussed to predict the condensation characteristics of sulfuric acid vapor on heat exchanger surfaces. The corrosion risk can be reduced by decreasing the water vapor concentration and increasing the flue gas temperature. The effects of operating parameters and geometry conditions on the sulfuric acid condensation and corrosion characteristics are also discussed. A correlation of Sh number of sulfuric acid versus fin geometries for a 10-row tube bundle is provided. In order to reveal the anti-corrosion characteristics of different materials and the coupling mechanisms between the fouling and dewpoint corrosion, the online water-cooled test system and laboratory static sample test system on the corrosion characteristics of heat exchangers are introduced. Some dewpoint corrosion resistant steels are presented. The mechanism of dewpoint corrosion is revealed, the relationship between the thickness of corrosion layer and wall temperature is discussed and the coupling mechanisms between the fouling and dewpoint corrosion are also discussed. The corrosion products are composed of the ash and acid reaction products in the outer layer, iron sulfate in the middle layer, and iron oxide in the inner layer. The innermost layer is the main corrosion layer. Finally, the research needs and prospect of fouling, erosion and corrosion research are discussed. Hope to solve the problems of designing the anti-fouling, anti-erosion and anti-corrosion heat exchangers, and promote the development of efficient utilization technologies in middle and low temperature flue gas heat exchangers.

Modern requirements for the quality of manufactured and operated metal products are largely ensured by non-destructive testing methods, among which ultrasonic testing is the most advanced one. One of the promising ultrasonic quality assessment methods is the electromagnetic-acoustic (EMA) method, as it does not require special surface preparation of the object under test (OUT), does not imply the use of contact liquids, and offers high testing productivity. However, it is generally believed that the EMA method has low sensitivity. Some specialists suggest that the sensitivity of the EMA method can be increased by using pulsed magnetization of the OUT. Nevertheless, a comprehensive approach to address the technical and technological challenges has not been found in the existing publications. Therefore, the authors of this paper conducted relevant studies and analysed modern information sources, which only partially address some solutions related to the development of methods and measuring instruments, testing, and diagnostics using ultrasonic electromagnetic-acoustic transducers (EMATs) with pulsed magnetization. These developments potentially allow increased EMAT sensitivity by improving the signal-to-noise ratio of the ultrasonic signal. As a result of the analysis, a new EMA method of non-destructive ultrasonic testing was developed. It defines the operations, parameters, and sequence of actions required during pulsed magnetization of the OUT. The necessity to determine optimal relative timing between the magnetizing pulses and the short high-frequency pulses supplying the EMAT coil was established, which makes it possible to significantly (up to 40%) improve the efficiency of the transducer when assessing the quality of ferromagnetic materials. To study the potential of the new EMA method, a special test bench was created. It included an EMAT, a generator of powerful high-frequency current pulses, a generator of powerful magnetizing current pulses, an amplifier for received ultrasonic pulses (with parameters adjustable over wide ranges), and a digital dual-beam oscilloscope. Experimental results using this test bench demonstrated that the new EMA method and its supporting equipment improve the signal-to-noise ratio of the received ultrasonic signals compared to traditional EMATs based on permanent magnets. In other words, the efficiency of the EMAT has been enhanced.

The main causes of damage to the water-wall tubes of hot-water boilers are discussed. The quality of the system water, design factors, and furnace conditions contribute to the damage. Transverse fatigue and corrosion-fatigue cracks on the outer and inner surfaces of water-wall tubes are described. The results of standard and special high-cycle fatigue tests on damaged metal are analyzed. The most important recommendations on the prevention of damage to the water-wall tubes of hot-water boilers are formulated.

Electrochemical corrosion monitoring has been used to evaluate fireside corrosion of coal fired boilers materials under low NOX operations. The reducing environments which are prevalent under these firing conditions can lead to the deposition/formation of non-protective iron sulfide films on the boiler tubes, resulting in excessive corrosion attack. In order to mitigate the boiler tube corrosion problems, it is common practice to weld overlay the boiler tubes with alloy C276. Corrosion of typical boiler tube material and C276 has been monitored electrochemically using a combination of Linear Polarisation Resistance (LPR), Harmonic Distortion Analysis (HDA), Electrochemical Current Noise (ECN), and Solution Resistance (Rs) measurements. The electrochemical corrosion monitoring instrumentation used in this work has been designed to operate under extreme conditions, with embedded algorithms to minimise the effects of electrical interference and provide simplified data analysis. Corrosion measurements for the field trials were updated at five minute intervals to provide a continuous record of the changes in corrosion behaviour. Electrochemical probes, using a two electrode configuration, have been designed specifically for this application where access is restricted. The probes are designed to be inserted into the webbing between the boiler tubes, where they will experience similar temperatures and heat flux as the boiler tubes. Results of both laboratory tests, and field trials in an operating boiler are presented.

The article deals with progress in electromagnetic methods used for quality evaluation of conducting materials. The term "electromagnetic methods" covers the following areas: magneto-inductive methods, magnetic leakage flux probe method, magnetometer principle and eddy-current methods. For the aim of numerical cracks recognition the sensitivity analysis with finite elements was shown.

Experimental study on dew point corrosion characteristics of the heating surface in a 65 t/h biomass-fired circulating fluidized bed boiler

Changes of Physical Properties of Silesian Granite Due to Heat Loading

The role of oxygen in the corrosion of internal surfaces of boiler-furnace-wall tubes has been discussed widely without arriving at consistent conclusions in respect to the maximum concentration of oxygen that can be tolerated and the exact cause of the initial pit formation at the tube surface. The present paper reviews the various theories that have been proposed to explain the severe, although local, corrosion of furnace-wall tubes in high-pressure boilers. Results of field tests of the oxygen record in utility-type boilers are presented. Metallurgical significance of failures and welding techniques also are discussed.

Summary form only given. A new method of ion implantation and plasma coating has been tested to achieve a surface modification of a dielectric thin material which is not suited for plasma based ion implantation (PBII) method. A capacitor of 500 nF charged up to 10 kV was discharged into a 3 cm wide, single-turn coil which surrounds a glass tube of 20 mm O.D. and 16.6 mm I.D. The coil current having 15 kA peak and period of 1.54 micro-sec induced a electric field which had the maximum value at the inner surface of the glass tube. Before applying the oscillating electric field, nitrogen gas was filled to 10 Pa and weekly ionized by a capacitively coupled 10 kHz voltage source. The plasma current initiated at first and the plasma column contracted at third period of the coil current. The plasma column was compressed from 16.6 mm to 4 mm in diameter within 500 nsec by a Lorentz force caused by the axial magnetic field and the azimuthal plasma current. The coil current continued to flow for 20 micro-sec, so the plasma contracted and expanded repeatedly. Nitrogen ion was confirmed by spectrum analysis. PTFE (polytetrafluoroethylene) rod with 2 mm in diameter was placed at the center of the tube. The surface showed hydrophilic after treatment by the pinch plasma. Ion implantation was done by this compressed kinetic energy to the target. By using this method, one can easily produce and deposit metallic materials on the surface of the rod, because the large electric field at the inner surface of the tube vaporises and ionizes the metal films placed at the wall. During the contraction and expansion period, the metallic plasma interacted with the rod repeatedly. After several shots, the metal was deposited and implanted into the rod surface

Nondestructive Testing (NDT) methods provide scientists a way to understand the properties of samples without causing them damage. For an electrochemical device, it is crucial to investigate the behavior of it before, during, and after its operation. Therefore, NDT methods are indispensable for studying such devices.Literature highlights the effectiveness of elastic-wave-based testing methods in the evaluation of electrochemical devices. The Acoustic Emission (AE) method was applied to respectively study the behavior of Solid Oxide Fuel Cells (SOFCs) and Ni-MH batteries, while the Ultrasonic Testing (UT) method was applied to study Li-ion batteries. In our previous research, the signals representing cracking and delamination in SOFCs were successfully distinguished by their peak amplitude, wave shape, and AE energy [1]. The same technique was further used for the evaluation of mechanical damages in SOFCs during the operation [2]. In another study, similarly, the pulverization of the Ni-MH battery electrodes was observed by the AE technique, and frequency peak property was included in the discussion to classify the signals [3]. The UT method was employed to study the relationship between the state of charge of Li-ion batteries and the profiles of ultrasonic waves at different frequencies. The evolution of wave velocity, attenuation, and amplitude were discussed [4]. Based on these researches, elastic-wave-based testing methods are adopted by us.In our present study, the multi-frequencies in-situ ultrasonic monitoring technique was employed to analyze the behavior of the commercial Ni-MH batteries during the operation. Continuous ultrasonic waves at three different frequencies were separately sent into the specimens during charge cycles, and their waveforms were recorded and compared. Specific wave patterns were found near the end of the charge and discharge state, especially from the wave profiles at 750 KHz. The result showed that the evolution of waveform is frequency-dependent, which was also suggested in the literature [4]. Potential explanations for these wave patterns were inferred, including phenomena such as crystal structure evolution, internal pressure shift, and reverse of electrodes.Based on this finding, a new ultrasonic testing/monitoring method considering frequency analysis is also designed. In this new method, signals consisting of elastic waves at a wide range of frequencies with equal intensities are designed to be applied to various electrochemical devices. The application of this method will be included, while the frequency-dependent property of the testing of electrochemical devices will be assessed in combination with the present findings in this study.

Assessing the load carrying capacity of concrete anchor bolts using non-destructive tests and artificial multilayer neural network