The Nature of a Gate Valve
A gate valve consists of an actuator---such as a lever, a hand wheel or an electric motor---a housing, which holds the valve mechanism, and a gate---a rectangular or circular piece of metal that acts like the gate on a fence. When the gate is raised, liquids or gases can pass through the gate.
When the gate is closed, the flow stops.
The best-known example of a gate valve is a household faucet: when you turn the handle, you raise or lower the gate and the water flows or stops.
Opening the Gate Valve
Most gate valves are operated by a handle, whether the handle is a lever or a hand wheel, like the knob on a faucet. If you turn the handle counterclockwise, the gate raises.
If you turn the handle clockwise, the gate drops.
Gate valves are used in many industrial settings and, in such a setting, the valve may be controlled by an electric motor operated either on command from a human or computer, or automatically, perhaps by a timer.
Gate valves should not be used to restrict flow; that is, they should not be partially opened. Using a gate valve to restrict flow puts unnecessary strain on the gate and the supporting structures, particularly in a high-pressure setting.
This strain will eventually cause the mechanical failure of the valve.
Ideally, when you open a gate valve using a hand wheel, you should open the valve all the way by turning the hand wheel counterclockwise until it stops, then turn the hand wheel 1/4 turn clockwise. This will prevent the gate from being jammed by the pressure from the flow of the liquid.
Closing the Gate Valve
Closing a gate valve is slightly more than a matter of turning the hand wheel clockwise until you can't turn it anymore, then leaning into it with a crowbar and your full weight. Instead, you should close the gate valve only until the flow of liquid stops---whether you can see it or a flow gauge indicates the flow has ceased.
Applying excessive pressure to a gate valve to make it close "all the way" has the effect of bending the gate, its seat or seal and doing permanent damage. If a gate valve drips or leaks, the correct course of action is to put the valve out of service until it can be repaired or replaced.