Analysis of the Reasons for the Distortion of the Indicating Value of the Thickness Gauge

In the practical application of ultrasonic thickness measurement, especially in the monitoring of in-service equipment, if the indication is distorted and deviates from the actual thickness, the hidden danger of pipeline (equipment) will be caused, that is, the pipe fittings are replaced according to the wrong data, resulting in a large amount of material waste According to the tracking application of ultrasonic thickness measurement in recent years, the phenomenon and reason of indication distortion are analyzed as follows: 1 The reason analysis of no indication display or indication flicker instability: this phenomenon often occurs in field equipment and pipeline detection, after a large number of phenomena and data analysis, the reasons are summarized as follows: (1) the roughness of the workpiece surface is too large, resulting in poor coupling effect between the probe and the contact surface, low reflection echo, and even unable to receive the echo signal Most of the equipment and pipelines in service are rusted on the surface, and the coupling effect is very poor (2) the curvature radius of the workpiece is too small, especially when measuring the thickness of the small diameter pipe, because the common probe surface is plane, and the contact with the surface is point contact or line contact, the sound intensity transmittance is low (poor coupling) (3) the detection surface is not parallel to the bottom surface, and the sound wave scatters when encountering the bottom surface, so the probe cannot accept the bottom wave signal (4) because of the inhomogeneous structure or coarse grain of castings and austenitic steels, the ultrasonic wave has serious scattering attenuation when it passes through them The scattered ultrasonic wave propagates along the complex path, which may annihilate the echo and cause no display (5) the contact surface of the probe is worn The surface of the commonly used thickness measuring probe is acrylic resin, which will increase the surface roughness and reduce the sensitivity, resulting in no display or flicker (6) there are a lot of corrosion pits on the back of the tested object Because there are rust spots and corrosion pits on the other side of the object to be measured, the acoustic wave is attenuated, resulting in irregular changes in the reading, or even no reading in extreme cases 2 Cause analysis of too large or too small indication in the actual detection work, the indication of thickness gauge is obviously larger or smaller than the design value (or expected value), and the cause analysis is as follows: (1) there is sediment in the measured object (such as pipeline), when the difference between the acoustic impedance of the sediment and the workpiece is small, the thickness gauge shows the wall thickness plus the sediment thickness (2) when there are defects (such as inclusions, interlayer, etc.) in the material, the display value is about 70% of the nominal thickness (at this time, the ultrasonic flaw detector shall be used for further defect detection) (3) the influence of temperature In general, the sound velocity in solid materials decreases with the increase of temperature Some experimental data show that the sound velocity decreases by 1% for every 100 ° C increase in hot materials This situation is often encountered for high-temperature equipment in service (4) laminated materials, composite (heterogeneous) materials It is impossible to measure uncoupled laminated materials, because ultrasound can not penetrate uncoupled space and can not propagate uniformly in composite (heterogeneous) materials For equipment made of multi-layer materials (such as urea high-pressure equipment), special attention shall be paid during thickness measurement The indication of the thickness gauge only indicates the thickness of the layer of materials in contact with the probe (5) the influence of coupling agent The coupling agent is used to eliminate the air between the probe and the object to be tested, so that the ultrasonic can effectively penetrate into the workpiece to achieve the purpose of detection If the type is selected or the use method is improper, the error or coupling mark will flicker and cannot be measured When the probe leaves the workpiece due to the excessive use of coupling agent, the indicator value of the instrument is the thickness of coupling agent layer (6) wrong choice of sound speed Before measuring the workpiece, preset its sound speed according to the material type or reverse measure the sound speed according to the standard block When one material is used to calibrate the instrument (the common test block is steel) and another material is measured, wrong results will be produced (7) The influence of stress Most of the equipment and pipelines in service have stress, and the stress state of solid materials has a certain influence on the sound speed When the stress direction is consistent with the propagation direction, if the stress is pressure stress, the stress effect will increase the elasticity of the workpiece and accelerate the sound speed; otherwise, if the stress is tensile stress, the sound speed will slow down When the stress and the wave propagation direction are different, the particle vibration track is disturbed by stress, and the wave propagation direction deviates According to the data, the general stress increases and the sound velocity increases slowly (8) effect of oxide or paint coating on metal surface Although the dense oxide or paint anti-corrosion coating produced on the metal surface is closely combined with the base material and has no obvious interface, the speed of sound propagation in the two materials is different, resulting in errors, and the error size varies with the thickness of the cover