The effect of low temperatures on valve sealing materials

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With the development of modern industry, valves are widely used in various sectors of the national economy, including oil and chemical industries, power plants, long-distance pipelines, the nuclear industry, various low-temperature engineering applications, aerospace, and offshore oil extraction. Their performance and reliability have a direct impact on the safe and efficient operation of these industrial systems. Investigative studies show that a considerable number of valves are used in low-temperature conditions; in high-latitude and high-altitude areas, influenced by geographical factors, various types of valves operate for extended periods in relatively low-temperature environments. Our country is a vast ** with a wide range of landscapes, spanning several climate zones. In many regions such as the Northeast, North China, and the Qinghai-Tibet Plateau, the climate remains below 0°C for several months each year, or even throughout the year, with the lowest temperatures dropping below -50°C. The various valves used in the construction and development of these areas operate for long periods in low-temperature natural conditions, and these low-temperature environmental factors pose many specific technical challenges to the use of valve products. Only products that can withstand various harsh environmental conditions are qualified products and can thus deliver their intended performance. Working in low-temperature environments for extended periods has a significant impact on the sealing materials of valves. The elasticity and compressive set of sealing materials change at low temperatures; at certain low temperatures, they become brittle or even glassy. As a result, the joint between the valve stem and the valve body, the flange connections, as well as the internal sealing materials of the valve are affected by low temperatures, leading to leaks. To ensure the performance of the sealing material, the sealing surface requires not only sufficient strength, toughness, and workability at normal temperatures, but also the ability to resist embrittlement and good performance at low temperatures. Low temperature refers to temperatures below -29 to -196°C; the range from -196 to 269°C is considered ultra-low temperature, while petrochemical companies define temperatures below -20°C as low temperature. By simulating a low-temperature environment under laboratory conditions and conducting tests on valve sealing materials, it is possible to assess their suitability for such environments. This helps to determine their performance parameters in low-temperature conditions, thereby preventing leaks caused by gaps that may arise at low temperatures. It is of great importance for improving the leak prevention capabilities of valve products. Sealing material refers to a material that can withstand joint displacement in order to achieve airtightness, water tightness, and similar properties, and is inserted into the joints of components. Based on their form, they can be divided into fluid-sealing materials and non-fluid-sealing materials. Fluid-sealing materials are used in static sealing applications where there is no relative movement between the contacting surfaces of two components; this includes the sealing design between the valve stem and the valve body, as well as the sealing within the valve body itself. Packing gland sealing structures and bellows sealing structures are employed for this purpose ； Non-fluid seals, also known as gaskets, can be used in static sealing applications as well as in dynamic sealing applications where there is relative movement between the contact surfaces of two components. Gaskets are used in things such as the gaskets in the middle flanges of valves, as well as in the external connections of flanged valves. Seals are made of metallic materials (aluminum, lead, stainless steel, etc.), as well as non-metallic materials (rubber, plastic, ceramics, graphite, etc.) and composite materials (such as rubber-asbestos sheets), but rubber-based elastomeric materials are the most commonly used. Common valve seal material options are shown in Table 1-2. At a certain temperature, the sealing performance of rubber depends on its degree of recovery after stretching and compressing at that temperature. As the temperature drops, the recovery from these tensile and compressive deformations becomes increasingly slow; the material gradually hardens. After reaching a leather-like state, it eventually becomes as hard and brittle as glass, a process known as vitrification. This behavior has an adverse effect on the use of elastic sealing rings at low temperatures. The low-temperature properties of the sealing material depend on the base rubber. Sealing materials with good oil resistance generally perform worse at low temperatures than those with poor oil resistance. In valve sealing materials, the packing seal between the valve stem and the valve body components, the gasket used in the middle flange of the valve, and the external connections of flanged valves all represent external seals. Fillers are generally non-metallic materials, and their linear expansion coefficient is relatively higher than that of metallic stuffing boxes and valve stems. For gaskets assembled at room temperature, when the temperature drops to a certain level, their degree of contraction is greater than that of the gasket holes and the valve stem, which may lead to a decrease in the pre-tightening pressure and thus leakage ； The gasket material is mainly made of rubber or plastic materials, which harden and lose their plasticity at low temperatures, resulting in leaks.