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　　Many materials can meet the valve in a variety of different operating conditions of the use of requirements. However, the correct and reasonable selection of valve materials, valves can obtain the most economical service life and best performance. Now we are through the following aspects to tell you how to choose the correct valve material. Body material selection, steel and Nickel Alloy Body and bonnet material selection ASME B 16. 34 as shown in Table 1. The table is divided into three groups: Group 1 is Ferrous material; Group 2 is Austenitic stainless steel material; and Group 3 is nickel alloy material related to a particular pressure-temperature scale. In ASME B16.1 cast iron, ASME B16. 24 Copper Alloy, ASME B16. 42 Ductile iron contains other materials. Special attention needs to be paid to the notes in the table covering the temperature and heat treatment range. By consulting these temperature-pressure Gauges, the pressure-temperature ratings provide a basis for selecting reasonable mechanical strength requirements. In addition, the mechanism of material performance degradation in process fluid flow should also be considered, such as corrosion, stress cracking, low and high temperature, hydrogen-containing conditions. 02 General Rule for selection of Valve Internals "the selection of valve internals material is not only the seat sealing surface material, but is clearly defined in the internals table included in API 600 and API 602. Usually for all types of valves the internals include all parts in contact with the process fluid. If no specific requirements are defined, API standards require that the corrosion resistance of the internals must be at least the same as that of the body material. For Gate Valves, the internals are the shutoff and seat seal face, and the stem and upper seal seat. As noted in the API standard, a key factor in avoiding seat wear is to provide a different hardness for the mating pair unless both faces are hard. An internals table is provided in the reference standard. Some reasonable parts are in one or two surface wear-resistant surfacing, to better ensure that the sealing surface with long-term wear resistance, corrosion resistance and wear resistance. Other valve internals for, check valves, their internals are defined as shutoff and seat sealing surfaces. An internals table is provided in the reference standard. Some reasonable internals are hardfacing on one or two surfaces to better ensure that the seat will not fail under long-term wear, corrosion and abrasive service. For a ball valve, the closure is defined as the internal metal part of the valve, such as the ball, stem, and metal seat or seat support ring. These parts may have the same nominal chemical composition as the case and have similar mechanical properties and corrosion resistance to the case material. For Butterfly Valves, all materials in contact with process fluids shall be in accordance with the manufacturer's standard unless otherwise specified. Butterfly Valve internals are defined as body seat sealing surfaces, butterfly plates, Butterfly Plate and shaft connections such as keys, pins, screws, and all internal fasteners in contact with process fluids. The material of the shaft and sleeve is similar to that of the inner parts. For Plug Valves, the plug and stem materials are required to be at least as resistant to corrosion as the body materials. For Globe Valves, their internals are defined in API 602. The 03 seat sealing surface is a soft seat. Ball and butterfly valves often use an elastic, non-metallic seat to provide a seal. Both API 608 and API 609 contain PTFE and reinforced PTFE with pressure-temperature rating. Pressure-temperature ratings for other soft seat materials, as agreed by purchaser and manufacturer. Ptfe seats are often used in the temperature range 177 ~ 225 °C. 350 ~ 400F. For soft seat ball valves used in liquid service, medium-pressure relief requirements should be considered. 04 stem seal a fugitive discharge, usually used in the process pipeline stem packing for-kinds of flexible graphite. New Valve packing is typically designed by placing several pressed packing rings, one braided ring above the ring, and one braided ring at the bottom of the stacked packing. This braided material "wipe ring" helps to prevent the pressed packing ring from being squeezed out of the stuffing box. Compared with the asbestos materials previously used, the present valve packing technology effectively prevents the light hydrocarbon leakage and fugitive emissions. The key factors affecting the performance of flexible graphite fillers are the percentage of carbon, the density of fillers and the corrosion / oxidant resistance. The flexible graphite packing system for lifting stem valves is well discussed in MSS SP-120 to improve stem packing performance. The API 622 packing shall be considered. Although graphite materials are readily oxidized at temperatures higher than 343C 650F, they have been successfully used at temperatures greater than or equal to 538c. 1000F valve. Packing under dynamic loading and over-time working conditions may reduce its sealing performance due to "packing compaction" . This reduces the pressure on the gland and causes the packing to leak. This "dynamic loading" process may be a reasonable option, including a packing gland loaded by a disc spring, thus providing a means of maintaining the packing gland load to help extend the packing life. The stem of some quarter-turn valves may be sealed with an elastic o-ring. The operating temperature range of these seals may be low PT-FE. Bellows sealed valves have been used in steam and certain hazardous / toxic conditions to reduce potential packing leakage. The nominal size of the valve in Api 602 is NPS 1 / 2 ~ 2. API 602 includes design methods and test details for bellows stem seals in addition to conventional packing. 05 Bonnet Gasket API600 offers Several Bonnet gasket options: Metal Ring Gaskets, metal clad gaskets, flexible graphite composite metal toothed gaskets for class 150 only, ring joined, wound type gaskets greater than or equal to class 300 and reinforced flexible graphite gaskets for class 150 only upon purchaser approval. The valve cover gasket is reasonable when used at a temperature of 29 ~ 538C-20 ~ 1000F. API 602 requires cover Flange gaskets of the type of winding gaskets filled with flexible graphite, unless otherwise specified. Design requirements to prevent over-compression of gaskets. The API 603 Bonnet gasket design is similar to the API 600. Spiral wound gaskets with compression limit and flexible graphite packing pressure rating ≥300. The scale is allowed. API603 does not define a higher temperature requirement.