tailieunhanh - Industrial Machinery Repair Part Episode 1 Part 9

Tham khảo tài liệu 'industrial machinery repair part episode 1 part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 184 Control Valves Figure Butterfly valves provide almost unrestricted flow Cross-flow Figure Three globe valve configurations straight-flow angle-flow and cross-flow Control Valves 185 View A View B Figure Globe valve When the valve is open as illustrated in View B of Figure the fluid flows through the space between the edge of the disk and the seat. Since the fluid flow is equal on all sides of the center of support when the valve is open there is no unbalanced pressure on the disk to cause uneven wear. The rate at which fluid flows through the valve is regulated by the position of the disk in relation to the valve seat. The globe valve should never be jammed in the open position. After a valve is fully opened the handwheel or actuating handle should be closed approximately one-half turn. If this is not done the valve may seize in the open position making it difficult if not impossible to close the valve. Many valves are damaged in the manner. Another reason to partially close a globe valve is because it can be difficult to tell if the valve is open or closed. If jammed in the open position the stem can be damaged or broken by someone who thinks the valve is closed. Performance Process-control valves have few measurable criteria that can be used to determine their performance. Obviously the valve must provide a positive seal when closed. In addition it must provide a relatively laminar flow with minimum pressure drop in the fully open position. When evaluating valves the following criteria should be considered capacity rating flow characteristics pressure drop and response characteristics. Capacity Rating The primary selection criterion of a control valve is its capacity rating. Each type of valve is available in a variety of sizes to handle most typical 186 Control Valves process-flow rates. However proper size selection is critical to the performance characteristics of the valve and the system where it is installed. A valve s capacity must .