Seeking advice on thickness measurement for atmospheric pressure storage tanks

For measuring the wall thickness of methanol storage tanks, first draw a schematic diagram of the tank along with the level of the methanol liquid. Then determine an appropriate number of measurement points and mark them; pay extra attention to marking those areas that are more prone to hazards. Finally, carry out the measurements as per the diagram and keep proper records. Take safety measures.

When measuring the wall thickness of a container, it is best to do so with no material inside the equipment, thereby ensuring both accurate measurements and safety. If measurements are taken while there is material inside the tank, it is essential to check the concentration of flammable gases in the surrounding area when grinding the measurement points, to avoid any risks during the grinding process. Moreover, since this is a tank with a capacity of 10,000 cubic meters, scaffolding is needed; measurements cannot be taken only at the bottom or top, otherwise it would be pointless.

Learning by borrowing a building* – is it done using an ultrasonic thickness gauge? What coupling agent to use, and how many points are generally selected?

I. Purposes of fixed-point thickness measurement 1. Thickness monitoring is primarily aimed at detecting uniform corrosion and erosion corrosion in equipment and pipelines; hydrogen corrosion, stress corrosion, etc. should be monitored through other testing methods. In high-temperature sulfur corrosion environments, thickness testing should be given priority to carbon steel, molybdenum alloy steel equipment, and pipelines. For newly installed units, as well as equipment and pipelines put into operation for the first time, the locations for thickness measurement should be determined prior to operation, and the original wall thickness data should be obtained. 2. Through monitoring over a period of time, the corrosion rate of the equipment during that period can be accurately assessed ； In conjunction with online corrosion monitoring, targeted monitoring of equipment and pipelines is carried out, along with research on corrosion patterns ； 3. Assess the remaining service life of the equipment, issue warnings regarding potential hazards associated with its use, ensure the safe operation of the device; in combination with other corrosion monitoring methods and anti-corrosion measures, this helps to extend the device’s operational life and improve the efficiency of the enterprise ； 4. By carrying out targeted thickness measurements, it is possible to understand the uniform corrosion pattern of the equipment, as well as the corrosion rates of all the devices and pipelines involved. This provides valuable guidance for formulating maintenance plans for the equipment, enabling scientific and rational maintenance efforts ； 5. Improve the management level of equipment and pipelines ； II. Managed Fixed-Point Thickness Measurement A. Create a layout diagram (or individual device diagram) for fixed-point thickness measurements at various devices in the workshop. B. The fixed thickness measurement points must have clear markings and numbering. At the thickness measurement points on the bare tube, a circle with a diameter of 3 cm can be marked using weather-resistant and temperature-resistant paint ； At the thickness measurement points on the equipment and pipes covered by insulation, removable insulation covers (boxes) should be installed and numbered. C. During the device maintenance, thickness measurements at all designated thickness measurement points of the device should be taken at room temperature. D. The locations for fixed-point thickness measurement should be determined based on the operational conditions of each device, the corrosivity of the medium, and the results of past corrosion inspections; these locations must cover all areas of the device where corrosion may occur. E. In cases where the corrosion thinning exceeds the designed corrosion margin, it is necessary to promptly verify the accuracy of the data. If the data is confirmed to be correct, the reasons should be analyzed and recommendations for handling should be provided. III. Principles for selecting points for fixed-point thickness measurement A. The following areas prone to corrosion and erosion should be given priority when selecting measurement points: a. Areas where pipeline corrosion and erosion are severe: elbows, tees, crosses, as well as pipe sections near nozzles, valves, control valves, pressure reducers, and orifice plates ； b. Areas with high flow rates (greater than 30 m/s), such as the outlet of heat exchanger tubes and behind the outlet valves of pumps ； c. Gas-liquid interface and liquid phase regions in naphthenic acid corrosion environments ； d. Gas phase and gas-liquid interface in a sulfur corrosion environment ； e. The downstream end of the fluid (including welds and straight sections of pipe) – areas prone to severe erosion ； f. Hot end of the same pipeline ； g. Fluid inlet pipe ends of heat exchangers and air coolers ； h. The gas-liquid interface at the towers, vessels, and reboilers, evaporators ； i. At the inlet of the shell side of heat exchangers and condensers ； j. Piping with a flow rate of less than 1 m/s (including water cooler tubes), areas where sediment is present and sub-scale corrosion is likely to occur ； k. Cecum and dead zone areas, such as: coagulation discharge pipes, sampling ports, regulator valve bypass lines, start-up and shutdown bypasses, line cleaning nozzles, etc. B. For pipes transporting highly corrosive media, when the length of the straight section is greater than 20 cm, three thickness measurement points are generally arranged longitudinally; when the length is 10–20 cm, two points are usually arranged, and when it is less than 10 cm, one point can be used. C. For pipes with moderate medium corrosion, one thickness measurement point is generally placed in the straight section of the pipe (the pipe segment between two elbows), and another thickness measurement point is placed at the elbow. D. Thickness measurement points should be installed wherever possible at locations on the pipeline that are prone to corrosion and erosion, such as elbows, tees, and other similar components. E. Considering the actual conditions on site, it is generally not advisable to place online thickness measurement points in locations that are difficult for the personnel responsible for measurement to access (except in areas where corrosion is particularly severe and requires special attention). F. For the placement of sensors in common pipe configurations such as elbows, tees, pipes following control valves or throttle valves, and collector pipes, refer to the examples shown in Figures 1–8. G. In principle, 4 thickness measurement points should be installed at the same cross-section of a pipe; these points are usually placed in areas where erosion and corrosion are likely to be severe, as well as near welds (mainly on the downstream side of the flow direction of the medium). H. The fixed thickness measurement points on the equipment should be determined based on temperature, medium, and equipment structure. IV. Determination of thickness measurement frequency: A. When the corrosion rate is between 0.3–0.5 mm/year or the remaining service life is between 1–1.5 years, measurements should be taken once a month. B. When the corrosion rate is between 0.1–0.3 mm/a or the remaining service life is between 1.5–2 years, measurements should be taken every two months. C. When the corrosion rate is less than 0.1 mm/a, it should be measured every three months. D. Areas with extremely severe corrosion (corrosion rate greater than 0.5 mm/a) or a remaining service life of less than 1 year should be monitored, and the thickness measurement frequency for such areas should be increased. E. A thickness measurement should be taken once more before restarting the deactivated equipment and pipelines. F. When there are significant changes in the sulfur content and acid value of the raw materials, the thickness measurement frequency should be adjusted accordingly. V. Thickness measurement methods and data processing: A. Ultrasonic thickness gauges are used for thickness measurement; the accuracy of these gauges should be no less than 0.1 mm, with the measurement error falling within the range of ±(H%+01) mm, where (H is the wall thickness in mm). B. Appropriate probes and couplants should be selected based on the temperature of the equipment and pipelines being tested. C. Before each thickness measurement, the thickness gauge should be calibrated at room temperature; simultaneously, the surface of the object to be measured should be treated to ensure that its material and surface condition are roughly identical to those of the calibration specimen. D. It is recommended to use the double-thickness measurement method, that is, measurements are taken twice in two directions perpendicular to each other at the probe separation surface (for pipeline thickness measurement, the probe separation surface should be perpendicular or parallel to the axis), and the minimum value is taken as the result. If the difference between the two thickness measurements is greater than 0.2 mm, the measurement should be repeated. E. For thickness measurement under medium to high temperature conditions (100°C–500°C), the measured values are higher than the actual values, and corrections should be applied. F. Thickness loss corrosion rate (mm/a) = Difference in thickness measured at two different times at room temperature (mm) / Time interval between those two measurements (a). G. Remaining life estimation: The average corrosion rate measured can be used to determine the remaining life of steel-based oil refining equipment and pipelines. The reliability of this remaining service life depends on the reliability of the thickness measurement data; it can only be applied to uniform corrosion, helps determine the inspection frequency, and should not be used as a basis for deciding whether a component is unusable. H. Relevant data on fixed-point thickness measurement, such as thickness measurement values, pipeline thickness maps, and basic pipeline parameters, should preferably be managed using computer software to ensure standardized and scientific data processing. VI. Points to Note for Thickness Measurement A. Use of the thickness gauge probe: During measurement, the probe must remain vertically aligned with the surface of the object being measured, making close contact with the area under inspection through the coupling agent. The probe should not be moved back and forth across the surface of the object being tested during use, as this can damage the contact surface of the probe. Shortens the probe's service life. When performing thickness measurement at high temperatures, the probe should not remain in contact with the hot area for too long; it is sufficient for the thickness gauge to give stable readings. When the thickness gauge gives unstable readings or no readings at all, we generally keep the probe in contact with the high-temperature area for no more than 10 seconds. After each measurement, the probe must be cooled to room temperature before taking a second measurement. Repeated measurements can be taken to ensure a stable reading on the thickness gauge. B. If the type or usage method of the coupling agent is chosen improperly, it will cause errors or fluctuations in the coupling signal, 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. C. Safety issues: a. Comply with all the rules and regulations of the manufacturing enterprise; when working on production facilities, pay attention to the signs on those facilities, identify areas that are toxic or hazardous, and wear appropriate gas masks during work. In high-temperature environments, use heat-resistant clothing and gloves. b. When using tools to remove the anti-corrosion coating or dirt from the surface of the detection components while the device is in operation, explosion-proof copper tools must be used. c. Under no circumstances is it permitted to step on small pipes such as the instrument tubing and purge steam pipes of the device when inspecting high-up areas. d. Stepping on or operating the valves on site is prohibited. e. It is prohibited to touch or press any instruments or switches installed on site. f. The inspectors should work in pairs, with one person carrying out the actual inspection and another person responsible for taking notes; the person taking notes also serves as a supervisor. g. It is recommended that testing be carried out by professional teams or personnel.