【Haichuan Chemical Valve Management】Series Posts--What is \"low-temperature brittleness\" of valves? In winter, the valves break as soon as they’re touched; it’s not a quality issue

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Proper management of valves is the foundation for stable operation. Everyone is welcome to engage in discussions and share insights on this series of posts: 【HaiChuan Chemical Valve Management】 series of posts: https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=5719309 ---------------------------------------------------------------- Disclaimer: The content contained in this article is intended solely for technical exchange and reference purposes only; it does not constitute any form of professional engineering advice, design basis, or operational guidance. ----------------------------------------------------------------------------------- What is the \"low-temperature brittleness\" of valves? In winter, valves break as soon as they are touched; it’s not a quality issue. Repair workers in factories in the north have probably encountered this problem: when trying to inspect valves in winter, the moment a wrench is applied and the valve is gently touched, it cracks. It’s not a problem with quality, but rather \"low-temperature brittleness\" at work. What is low-temperature brittleness? Metal materials become brittle at low temperatures; this is a physical property. Carbon steel, which has good toughness at room temperature, may break just from a single strike at temperatures of minus twenty to thirty degrees. This phenomenon is called \"low-temperature brittleness\" or \"cold brittleness\". Why? Because the crystal structure of the metal changes at low temperatures. Metals with a face-centered cubic structure (such as austenitic stainless steels) exhibit good low-temperature toughness, while metals with a body-centered cubic structure (such as carbon steel and low-alloy steel) tend to become brittle at low temperatures. Which valves are most sensitive to cold? Gray cast iron valves are a major area affected by low-temperature brittleness. Cast iron itself has a high carbon content, many impurities, and poor toughness; it becomes extremely brittle when the temperature drops below minus ten degrees. They crack as soon as they fall to the ground during transportation, and they break with just a tap from a wrench during installation; cast-iron valves in winter are nothing but \"porcelain dolls\". Cast steel valves are better than cast iron, but they also become brittle at low temperatures. The impact toughness of carbon steel valves decreases significantly below -20°C. Stainless steel valves (304, 316) have good low-temperature toughness and can still be used at -196°C, but they are expensive. Minimum operating temperature for different materials: Gray cast iron: generally not lower than -10°C; some standards require not lower than -29°C, but this needs to be verified. Ductile iron: It’s better than gray cast iron; it can still be used at around -20°C. Carbon steel (WCB): usable above -29°C; special heat treatment is required for lower temperatures. Low-temperature carbon steel (LCB): usable at -46°C. Stainless steel (304/316): usable at -196°C. What should be noted when operating valves in winter? If the valve has frozen, never try to twist it forcefully! Forcing it can cause the valve stem to break or the valve body to crack. The correct method is to thaw first, then proceed. Thawing can be done by pouring hot water on it, using steam, or with a heat gun. After thawing, check the valve body for any cracks; only proceed with operations once it is confirmed to be intact. When operating valves in winter, do so slowly and avoid using force. When turning on and off valves, apply even force; do not push or pull them harshly. Avoid conducting pressure tests at extremely low temperatures; the water temperature should be kept above 5°C. Material selection advice: If your system is located outdoors in the north, where temperatures can drop to minus 20 or 30 degrees in winter, you need to be careful when choosing materials. Ordinary carbon steel valves pose a greater risk at temperatures below -20°C; it is recommended to use low-temperature carbon steel (LCB/LCC) or stainless steel instead. Gray cast iron valves should be avoided in outdoor environments with low temperatures; if their use is necessary, proper insulation must be provided. Safety valves and critical shut-off valves should be made of low-temperature materials to ensure sufficient toughness at the lowest design temperature. The special design of low-temperature valves: In LNG (Liquefied Natural Gas) applications, where temperatures drop as low as -196°C, valves made of stainless steel or special alloys are necessary. Low-temperature valves feature a long-neck valve cover design that extends the packing box from the low-temperature area to the normal-temperature area, preventing the packing from freezing. The materials of the valve stem and the valve body must have matching coefficients of thermal expansion to prevent jamming at low temperatures. A small reminder: Factories in the north had better conduct a thorough inspection of outdoor valves before winter arrives. Check for cracks and rust. Check whether the insulation is in good condition. For valves that have not been moved for a long time, move them slightly when the temperature is not too low, to prevent them from being unable to be moved when the time comes in winter. Don’t wait until it’s minus 20 degrees to fix the valves; it will be difficult to do anything at that temperature.