Purpose: The purpose of this Standard Practice is to outline the limitations and requirements of valves in which a nonmetallic, resilient diaphragm is used to separate the working parts of the valve from the line fluid and also functions in conjunction with other parts as valve closure-member. The use of a nonmetallic, resilient diaphragm causes valves described in this Standard Practice to be different from other commonly used valves for which genera1 and specific Standards and Standard Practices are in existence. These differences include pressure-temperature limitations, face-to-face dimension requirements, and specific service characteristics. This Standard Practice reflects industry practice in the design, manufacture, and application of this type of valve. Scope: This Standard Practice applies to valves for general liquid and gas service that effect valve closure by means of a resilient diaphragm sealing against a weir or a diaphragm acting in conjunction with a separate or integral disk-member sealing against a seat, having equal sealing capability in either flow direction, and having diaphragms being essentially made of elastomeric or plastic material or combinations thereof. This Standard Practice defines the following categories of valves: a) Class 125 or 150 Valves b) Standard Valves c) Valves for Special (Low Pressure) Applications The purchaser, when referencing the Standard Practice, shall also specifiy the appropriate category as listed above. This Standard Practice covers the following: a) Pressure-Temperature Ratings and Design Requirements b) Commonly Used Diaphragm Pressure Limitations c) Body, Bonnet, and Bolting Materials d) Commonly Used Face-to-Face Dimensions for Flanged Valves e) Body End Connections and Wall Thicknesses f) Actuator Application Requirements g) Tests h) Marking For specific requirements relating to the dairy industry, refer to 3-A SSI 54.02.