Abbas Mohimi

Cracks and welds detection approach in solar receiver tubes employing electromagnetic acoustic transducers

There is a significant rising in development of new concentrated solar plants due to global energy demands. Concentrated solar plant requires to improve the operational and maintainability in this industry. This article presents a new approach to identify defects in the solar receiver tubes and welds employing a simple electromagnetic acoustic transducer. The absorber tubes in normal working conditions must withstand high temperatures, which can cause the tubes to deteriorate in areas such as welding, or it can cause hot spots due to defects or corrosion. A proper predictive maintenance program for the absorber pipes is required to detect defects in the tubes at an early stage, reducing corrective maintenance costs and increasing the reliability, availability, and safety of the concentrated solar plant. This article presents a novel approach based on signal processing and pattern recognition for predictive maintenance employing electromagnetic acoustic transducers. Hilbert transform is used to obtain the envelope of the signal that is smoothed by wavelet transform. It reduces the probability of detecting false-positive alarms. The algorithm uses the distance of the sensors from the edges to perform a self-identification of signal events. The events are located using two possible ways of ultrasound propagation, forward and reverse, and the time of flight of each echo. The algorithm correlates the theoretical events with events found experimentally. These echoes could come from different paths due to the electromagnetic acoustic transducer that generates forward and reverse shear waves. The main novelty in this approach is that the detection and location of the defect is determined considering two echoes that come from the same defect, but they arrive at the sensor flowing by different paths. The results are obtained with a double validation by matching the echoes that meet certain conditions. It increases the accuracy of the inspection and reduces false alarms. The approach has been tested and validated in an experimental platform that simulates the concentrated solar plants.

High Temperature Shear Horizontal Electromagnetic Acoustic Transducer for Guided Wave Inspection.

Guided Wave Testing (GWT) using novel Electromagnetic Acoustic Transducers (EMATs) is proposed for the inspection of large structures operating at high temperatures. To date, high temperature EMATs have been developed only for thickness measurements and they are not suitable for GWT. A pair of water-cooled EMATs capable of exciting and receiving Shear Horizontal (SH0) waves for GWT with optimal high temperature properties (up to 500 °C) has been developed. Thermal and Computational Fluid Dynamic (CFD) simulations of the EMAT design have been performed and experimentally validated. The optimal thermal EMAT design, material selection and operating conditions were calculated. The EMAT was successfully tested regarding its thermal and GWT performance from ambient temperature to 500 °C.

Assessment of Material Properties of Gallium Orthophosphate Piezoelectric Elements for Development of Phased Array Probes for Continuous Operation at 580°C

In this paper, the thickness extension mode gallium orthophosphate single crystal elements were characterised using the impedance analyser. Impedance characteristics of piezoelectric elements were investigated at temperatures from 25°C up to 580°C at first and then at a constant temperature of 580°C for a period of 25 days. The resonant and anti-resonant frequencies extracted from the impedance characteristics, capacitance (measured at 1 kHz), density and dimensions of the gallium orthophosphate elements were used to calculate electromechanical, piezoelectric and elastic properties of these elements at high temperatures as a function of time. The tested gallium orthophosphate elements proved to possess very stable efficiency and sensing capability when subjected to high temperature. The results are very encouraging for proceeding with development of phased array probes using gallium orthophosphate, for inspection and condition monitoring of high temperature pipelines in power plants at a temperature up to 580°C.

High Temperature Dielectric, Elastic and Piezoelectric Coefficients of Shear Type Lithium Niobate Crystals

In this paper shear type lithium niobate has been studied. The impedance-frequency characteristics were measured at high temperatures. This was carried out by placing the samples inside a furnace and performing in-situ impedance measurements at up to 600°C. The characteristic frequencies, capacitance, density and dimensions of samples were used to calculate the dielectric, elastic and piezoelectric coefficients. Prototype transducers were built using shear type lithium niobate crystals. The initial ultrasonic experiments show that it can be used at temperatures up to 450°C to transmit and receive guided wave signals at 70 kHz. This will enable design and manufacture of high temperature transducers for continuous guided wave monitoring of power plants.

Long Range Ultrasonic inspection of aircraft wiring

Inspecting complex aircraft wiring by means of ultrasonic non-destructive testing has been addressed. This paper discusses the progress on the development of software and hardware that enable a novel technique of Long Range Ultrasonic Testing - a subset of ultrasonic non-destructive testing to be implemented for the inspection of complex aircraft wires insulation. A representative aircraft wire was modelled to identify appropriate wave modes that can be utilized for the inspection. The modelling work was validated via laser interferometry. The sensor array was driven with the Teletest® pulser-receiver unit used in laboratory conditions to produce data for signal processing. Signal processing algorithms that combine baseline subtraction and anti-correlation algorithms were deployed to detect features as well as defects on cable insulation. Further work on validating the hardware, software and system integration is planned.