In case of manually operated valves, human force is used to actuate the steam which in turn moves the closing element. A manually operated valve can be operated by any of the following three mechanisms :
1. Wheel type- These valves are operated by rotating the wheel. The wheel is connected to the stem. When the wheel is rotated, it moves the stem accordingly and closes or opens the valve.
2. Lever Handle Type- This type of operation is used in valves which have 90 degree operation (plug, ball, and butterfly.) When the lever is parallel to the pipe or flow, the opening is aligned to the flow direction and allows the fluid to flow through it. When perpendicular moved to the pipe/flow, the flow is stopped.
3. Gear Operated- Gear Operated valves have a wheel which transmits energy to actuator through a gearbox. Gears offer the advantage of reducing operating torque.
These valves are operated automatically, without requiring human power. They offer many advantages over manually operated valves, precise control, and ability to apply large torque being a couple of them. Automatic operation can either be electric or pneumatic.
1. Electric Actuation -
This type of valve is actuated using a DC or AC motor. Electrically actuated valves are well suited for water hammer prone situations compared to solenoid valve. These valves need a back-up power source in case of intermittent energy supply.
2. Pneumatic Actuation -
These valves are actuated by using compressed air. It is well suited for applications where explosion proof actuators are required by law. The speed of operation is significantly reduced if the valve is located at a large distance from the compressor.