Discussion on fixed-point thickness measurement technology

Generally, in the first quarter, we conduct a thickness measurement on pipelines and containers that are prone to corrosion, such as the gas lines in the absorption and stabilization system as well as the liquid separation tanks. The instrument used for this is a small portable measuring device.

We usually do it once a year, mainly on the elbow sections of the pipelines.

I know there is a type of PIG that can measure the thickness of all pipelines comprehensively; it just isn’t available in China yet. I’m in contact with foreigners in hopes of obtaining their technology

Usage: Ultrasonic thickness gauge. Basic principle of the ultrasonic thickness gauge: It measures thickness based on the principle of reflection of ultrasonic pulses. When the ultrasonic pulses emitted by the probe pass through the object being measured and reach the interface between different materials, these pulses are reflected back to the probe. The thickness of the material being measured is determined by accurately measuring the time it takes for the ultrasonic waves to travel through the material. Any material that allows ultrasonic waves to propagate through it at a constant speed can be used to measure using this principle. Thickness gauges designed based on this principle can perform accurate measurements on various types of sheets and machined parts. They can also be used to monitor various pipes and pressure vessels in production equipment, assessing the degree of thinning caused by corrosion during their use. It can be widely applied in various fields such as petroleum, chemicals, metallurgy, shipbuilding, aviation, and aerospace. Usage tips: (Taking Pengxiang Technology’s PT900 ultrasonic thickness gauge as an example) 1. General measurement method: (1) Perform two thickness measurements at one point using the probe; the cutting surfaces of the probe should be at 90° to each other in these two measurements, and the smaller value obtained shall be taken as the thickness of the workpiece being measured. (2) 30mm multi-point measurement method: When the measurement values are unstable, multiple measurements are taken within a circle with a diameter of about 30mm, centered on one measurement point, and the minimum value is taken as the thickness of the workpiece under test. 2. Precise measurement method: Increase the number of measurements around the specified measurement points, and the thickness variations are represented by isochrones. 3. Continuous measurement method: Use the single-point measurement method to take consecutive measurements along the designated route, with an interval of no more than 5 mm. 4. Grid measurement method: A grid is drawn in the designated area, and thickness measurements are recorded at each point. This method is widely used in the corrosion monitoring of high-pressure equipment and stainless steel linings. 5. Factors affecting the reading of an ultrasonic thickness gauge: (1) Excessive surface roughness of the workpiece results in poor coupling between the probe and the contact surface, leading to low reflected echoes, or even no echo signals being received. In the case of surface rust, on operating equipment and pipelines where the coupling effect is very poor, the surface can be treated using methods such as sanding, grinding, or rubbing to reduce its roughness. This also helps to remove oxide layers and paint, revealing the metallic luster, thereby enabling a good coupling effect between the probe and the object being inspected through the use of a coupling agent. (2) The radius of curvature of the workpiece is too small; especially when measuring the thickness of small-diameter tubes, since the surface of the commonly used probe is flat, contact with a curved surface occurs as point or line contact, resulting in a low sound intensity transmission rate (poor coupling). A probe designed for small pipe diameters (6 mm) can be used, allowing for more accurate measurements of curved surfaces such as pipes. (3) The detection surface is not parallel to the bottom surface; sound waves are scattered upon hitting the bottom surface, preventing the probe from receiving the bottom-wave signals. (4) In castings and austenitic steels, due to uneven microstructure or large grains, severe scattering and attenuation occur as ultrasonic waves pass through them. The scattered ultrasonic waves travel along complex paths, which may cause the returning waves to be obscured, resulting in no signal being detected. A coarse-grain dedicated probe with a lower frequency (2.5MHz) can be used. (5) There is some wear on the probe contact surface. The surface of common thickness measurement probes is made of acrylic resin; prolonged use increases the surface roughness, which leads to a decrease in sensitivity and thus inaccurate readings. Optionally, use 500# sandpaper to polish it smooth and ensure parallelism. If it remains unstable, consider replacing the probe. (6) There are numerous corrosion pits on the back side of the object under test. Due to rust spots and corrosion pits on the other side of the object being tested, sound waves are attenuated, resulting in irregular variations in the readings; in extreme cases, no readings are obtained at all. (7) If there are deposits inside the object being measured (such as pipes), and the acoustic impedance of these deposits is not significantly different from that of the workpiece, the thickness gauge will display a value that is the sum of the wall thickness and the thickness of the deposits. (8) When there are defects inside the material (such as inclusions, interlayers, etc.), the displayed value is approximately 70% of the nominal thickness; in such cases, an ultrasonic flaw detector can be used to further detect the defects. (9) Effect of temperature. In general solid materials, the sound speed decreases as the temperature rises. Experimental data show that for heated materials, the sound speed drops by 1% for every 100°C increase in temperature. This is a common issue encountered with high-temperature operating equipment. Special high-temperature probes (300–600°C) should be used; ordinary probes must not be employed. (10) Laminated materials, composite (heterogeneous) materials. It is impossible to measure uncoupled layered materials, as ultrasonic waves cannot penetrate uncoupled spaces nor travel at a constant speed in composite (heterogeneous) materials. For devices made of multi-layered materials (such as urea high-pressure equipment), special care must be taken when measuring thickness, as the reading given by the thickness gauge only indicates the thickness of the layer in contact with the probe. (12) Effect of the coupling agent. A coupling agent is used to remove the air between the probe and the object being inspected, allowing ultrasonic waves to penetrate the workpiece effectively for detection purposes. If the type or usage method is selected improperly, it will cause errors or the coupling indicator to flash, making measurement impossible. Since the appropriate type should be chosen based on the application, a low-viscosity coupling agent can be used when applied to smooth material surfaces ； When used on rough surfaces, vertical surfaces, and top surfaces, a coupling agent with high viscosity should be used. High-temperature couplants should be used for high-temperature workpieces. Secondly, the coupling agent should be used in appropriate amounts and applied evenly. Generally, it should be applied to the surface of the material being tested, but when the measurement temperature is high, it should be applied to the probe. (13) Incorrect sound speed selection. Before measuring the workpiece, preset its sound speed based on the type of material, or determine it by measuring it using a standard block. When the instrument is calibrated using one material (with steel being the common test block) and then used to measure another material, incorrect results will be obtained. It is necessary to correctly identify the material and select the appropriate sound speed before measurement. (14) Effect of stress. Most in-service equipment and pipelines are under stress. The stress condition of solid materials has a certain influence on the speed of sound; when the direction of the stress is consistent with the direction of propagation, if it is compressive stress, this stress causes an increase in the elasticity of the material, thereby increasing the speed of sound ； Conversely, if the stress is tensile, the sound speed decreases. When the stress is not aligned with the direction of wave propagation, the vibration path of the particles during wave propagation is disturbed by the stress, causing the wave to deviate from its propagation direction. According to the data, as stress generally increases, the sound speed increases slowly. (15) Effect of oxide layers or paint coatings on metal surfaces. The dense oxide layer or paint coating formed on the metal surface is closely bonded to the base material, with no distinct interface; however, the speed of sound varies between these two materials, which leads to errors. Moreover, the magnitude of these errors changes depending on the thickness of the coating.