Material selection for valves in high-temperature conditions

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High-temperature operating conditions mainly include sub-high temperature, High Temperature Level I, High Temperature Level II, High Temperature Level III, High Temperature Level IV, and High Temperature Level V, which will be introduced below respectively. 1. Sub-high temperature: Sub-high temperature refers to a working temperature for valves in the range of 325–425°C. When the medium is water and steam, WCB, WCC, A105, WC6, and WC9 are primarily used. When the medium is sulfur-containing oil products, C5, CF8, CF3, CF8M, and CF3M, which have resistance to sulfide corrosion, are primarily used. They are commonly used in the atmospheric and vacuum distillation units as well as the delayed coking units of refineries, where valves made of CF8, CF8M, CF3, and CF3M materials are not used to resist corrosion by acidic solutions, but rather in applications involving sulfur-containing oils and oil and gas pipelines. Under these operating conditions, the maximum allowable operating temperature for CF8, CF8M, CF3, and CF3M is 450°C. 2. High temperature Class I: Valves with an operating temperature of 425–550°C are classified as High Temperature Class I (abbreviated as PI class). The main material of PI-class valves is a high-temperature Class I medium-carbon chromium-nickel-rare-earth-titanium high-quality heat-resistant steel based on CF8 as specified in ASTM A351. Since the PI grade is a specific term, it encompasses the concept of high-temperature stainless steel (P) here. Therefore, although high-temperature steels WC6 (t≤540℃) or WC9 (t≤570℃) can also be used when the working medium is water or steam, and high-temperature steel C5 (ZG1Cr5Mo) can also be used in the presence of sulfur-containing oils, they cannot be classified as PI grade in this case. 3. High temperature Class II: Valves with an operating temperature of 550–650°C are classified as High Temperature Class II (abbreviated as P II). Class PⅡ high-temperature valves are primarily used in the heavy oil catalytic cracking units of refineries; they include wear-resistant gate valves with high-temperature linings that are employed in components such as three-spin nozzles. The main material of PⅡ-class valves is a \"high-temperature Class II medium-carbon chromium-nickel rare-earth titanium tantalum strengthened heat-resistant steel\" based on CF8 as specified in ASTM A351. 4. High-temperature Class III: Valves with an operating temperature of 650–730°C are classified as High-temperature Class III (abbreviated as P III). Class PⅢ high-temperature valves are mainly used in large heavy oil catalytic cracking units in refineries. The main material for Class PⅢ high-temperature valves is a \"high-temperature Class III medium-carbon chromium-nickel-molybdenum rare-earth titanium tantalum strengthened heat-resistant steel\" based on CF8M as specified in the ASTMA351 standard. 5. High-temperature Class IV: Valves with an operating temperature of 730–816°C are classified as High-temperature Class IV (abbreviated as PⅣ Class). The upper operating temperature limit for Class PⅣ valves is set at 816°C because the highest temperature specified in the ASME B16.134 pressure-temperature class standard adopted for valve design is 816°C (1500υ). Furthermore, when the operating temperature exceeds 816°C, the steel approaches the forging temperature range; at this point the metal is in a state of plastic deformation, possessing good ductility, and it is difficult for it to withstand high operating pressures and impacts without deforming. The main material of PⅣ-class valves is CF8M as specified in ASTM A351, which is a \"high-temperature Class IV medium-carbon chromium-nickel-molybdenum rare-earth titanium tantalum strengthened heat-resistant steel\". Heat-resistant stainless steels such as F310 (with a C content of ≥0.1050%) and F310H specified in the CK-20 and ASTM A182 standards. 6. High-temperature Class V: Valves whose operating temperature is above 816°C are referred to as Class PV. High-temperature Class PV valves (used as shut-off valves, rather than control-type butterfly valves) require special design measures, such as thermal insulation linings or cooling with water or gas, in order to ensure their proper operation. Therefore, no upper limit is specified for the operating temperature of PⅤ-class high-temperature valves; this is because controlling the operating temperature of such valves relies not solely on the material used, but rather on special design techniques, and the basic principles behind these design techniques remain the same. For Class PⅤ high-temperature valves, appropriate materials that can meet the requirements of such valves can be selected based on their working medium, operating pressure, and the special design methods employed. In PⅤ class high-temperature valves, the vanes of flue gate valves or butterfly valves are usually made from the HK-30 or HK-40 superalloys specified in ASTM A297 standard; these materials can resist corrosion in oxidizing and reducing gases at temperatures below 1150°C, but they cannot withstand impacts or high-pressure loads.