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Mechanical Science HandbooksMechanical Science Handbooks 20000 Part 4

Tham khảo tài liệu 'mechanical science handbooksmechanical science handbooks 20000 part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | TYPES OF VALVES DOE-HDBK-1018 2-93 Valves Port Patterns Ball valves are available in the venturi reduced and full port pattern. The full port pattern has a ball with a bore equal to the inside diameter of the pipe. Valve Materials Balls are usually metallic in metallic bodies with trim seats produced from elastomeric elastic materials resembling rubber materials. Plastic construction is also available. The resilient seats for ball valves are made from various elastomeric material. The most common seat materials are teflon TFE filled TFE Nylon Buna-N Neoprene and combinations of these materials. Because of the elastomeric materials these valves cannot be used at elevated temperatures. Care must be used in the selection of the seat material to ensure that it is compatible with the materials being handled by the valve. Ball Valve Stem Design The stem in a ball valve is not fastened to the ball. It normally has a rectangular portion at the ball end which fits into a slot cut into the ball. The enlargement permits rotation of the ball as the stem is turned. Ball Valve Bonnet Design A bonnet cap fastens to the body which holds the stem assembly and ball in place. Adjustment of the bonnet cap permits compression of the packing which supplies the stem seal. Packing for ball valve stems is usually in the configuration of die-formed packing rings normally of TFE TFE-filled or TFE-impregnated material. Some ball valve stems are sealed by means of O-rings rather than packing. Ball Valve Position Some ball valves are equipped with stops that permit only 90 rotation. Others do not have stops and may be rotated 360 . With or without stops a 90 rotation is all that is required for closing or opening a ball valve. The handle indicates valve ball position. When the handle lies along the axis of the valve the valve is open. When the handle lies 90 across the axis of the valve the valve is closed. Some ball valve stems have a groove cut in the top face of the stem that shows the .