tailieunhanh - Root Cause Failure Analysis Episode 8

Tham khảo tài liệu 'root cause failure analysis episode 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 17 CONTROL VALVES Conưol valves can be broken into two major classifications process and fluid power. Process valves control the flow of gases and liquids through a process system. Fluidpower valves control pneumatic or hydraulic systems. Process Process-control valves are available in a variety of sizes configurations and materials of construction. Generally this type of valve is classified by its internal configuration. Configuration The device used to control flow through a valve varies with its intended function. The more common types are ball gate butterfly and globe valves. Ball Ball valves see Figure 17-1 are simple shutoff devices that use a ball to stop and start the flow of fluid downstream of the valve. As the valve stem turns to the open position the ball rotates to a point where part or all of the hole machined through the ball is in line with the valve-body inlet and outlet. This allows fluid to pass through the valve. When the ball rotates so that the hole is perpendicular to the flow path the flow stops. Most ball valves are quick-acting and requừe a 90 turn of the actuator lever to fully open or close the valve. This feature coupled with the turbulent flow generated when the ball opening is only partially open limits the use of the ball valve. Use should be limited to strictly an on off control function . fully open or fully closed because of the turbulent flow condition and severe friction loss when in the partially open position. These valves should not be used for throttling or flow control. 202 Control Valves 203 Figure 17-1 Ball valve. Ball valves used in process applications may incorporate a variety of actuators to provide direct or remote conưol of the valve. Actuators commonly are either manual or motor operated. Manual values have a handwheel or lever attached directly or through a gearbox to the valve stem. The valve is opened or closed by moving the valve stem through a 90 arc. Motor-controlled valves replace the handwheel with a .