How to Protect Thermometers from Corrosive Media?
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In industries such as chemical industry, metallurgy, medicine, and energy, thermometers usually need to work in corrosive media. As a basic measuring instrument, thermometers, once corroded, will not only lead to inaccurate measurements, but may even lead to equipment failure, production accidents, and heavy losses. Therefore, this article will systematically explain how to protect thermometers from damage by corrosive media from aspects such as corrosion sources, material selection, structural protection, and later maintenance.
The reason why corrosive media poses a threat to thermometers is that they are highly chemically active and can react electrochemically or chemically with metals or other materials, causing surface oxidation, perforation, stress cracking and other damages. For example, chloride media can induce pitting and crevice corrosion, and sulfide gases can easily react with metals to form brittle substances; strong acids (such as concentrated sulfuric acid, nitric acid) will quickly dissolve most alloy materials, and strong alkali may corrode some non-metallic materials such as ceramics or glass linings, causing their surfaces to fall off. Understanding the concentration, operating temperature, pressure, redox characteristics and presence of impurities of the corrosive medium is the first step to prevent the thermometer from being corroded.
Match the material of the wetted part of the thermometer to the type of medium
The temperature sensing probe of the thermometer and the parts in direct contact with the medium are the parts most susceptible to corrosion in the entire instrument system. Therefore, appropriate corrosion-resistant materials must be selected based on the corrosion type and severity of the specific medium. For example, 316L stainless steel can resist general neutral or weakly acidic environments, but is not suitable for high chlorine concentration situations; Hastelloy can remain stable in strong acid and chlorine-containing environments; titanium has good salt resistance and is suitable for seawater or electrolytes containing high chloride ions; polymer materials such as polytetrafluoroethylene (PTFE) are inert to most organic solvents and strong acids and alkalis. When selecting materials, you should not just look at the label "corrosion resistance", but should refine it to specific parameters such as media type, operating temperature, and whether it contains impurities, so as to achieve "material selection based on needs and accurate matching."
Using plug-in protective sleeve
In working conditions where the medium is extremely corrosive and it is difficult to replace the thermometer frequently, installing a protective sleeve (Thermowell) on the thermometer has become a key structural protection method. The protective sleeve is an intermediary component specially used to physically isolate the thermometer from the process medium. The outside is exposed to the medium; while the inside is a cavity structure for inserting the temperature sensing element.
In terms of materials, the protective sleeve can be made of 316L stainless steel, titanium alloy, Hastelloy, Monel alloy, zirconium alloy, etc. according to the medium characteristics, or even a composite structure with surface spraying of polytetrafluoroethylene or ceramics.
In terms of structure, the protective sleeve can be selected from straight, stepped or tapered structures according to the flow rate and installation environment. Due to the gradual shrinkage of the tip, the tapered structure can effectively reduce the impact of the fluid on its front and reduce the periodic vibration caused by the Karman vortex phenomenon. It is the first choice in high flow rate situations.
For thermometers with relatively high accuracy, in order to prevent the medium from penetrating into the temperature-sensing element through micro-cracks or capillary channels, inert gas (such as nitrogen, argon) or filling liquid (such as silicone oil) can be injected inside the protective structure. The principle of this is that the inert gas can form a positive pressure barrier to prevent the reverse penetration of external corrosive gas; and these filling fluids are highly chemically inert and will not easily react with surrounding materials or media.
The installation location of the thermometer has a direct impact on its anti-corrosion performance. If the temperature sensing element is facing the high-speed fluid in the pipeline, it will easily accelerate the corrosion of surface materials due to erosion and turbulence. Therefore, when installing a thermometer in a corrosive medium, priority should be given to an area with a stable flow rate as the installation point, and avoid locations close to the pump port, elbow, throttle valve, etc. If conditions permit, a deflector or baffle can be installed to act as a buffer and improve its corrosion resistance.
