A quick guide to gate valves
Gate valves are generally used for all sorts of applications and are suitable for installation both above-ground and underground. It is necessary, not least for underground installations, to select the correct type of valve to avoid high replacement costs with non-slam check valve.
Gate valves are designed for an operation that is completely open or fully closed. They are mounted as insulating valves in pipelines and should not be used as valves for control or regulation. A gate valve operation is performed by either clockwise closure (CTC) or clockwise opening (CTO) of the stem rotating motion. The gate moves up- or downward on the threaded portion of the stem while working the valve stem.
- Reducing the torque requirement of the valve and allowing one-man operation in order to control the pipeline differential pressure
- A continual flow is permitted with the main valve closed and the by-pass open, preventing potential stagnation.
- Filling of pipelines postponed.
Swing check valve
When minimal pressure loss and a free bore is required, gate valves are often used. A typical gate valve has no obstruction in the flow path when completely open, resulting in a very low-pressure loss, and this design allows a pipe-cleaning pig to be used. A gate valve is a multi-turn valve, meaning that a threaded stem is used to control the valve. As the valve has to turn several times to go from open to the closed position, water hammer effects are also prevented by the slow operation.
It is possible to separate gate valves into two main types: parallel and wedge-shaped. A flat gate between two parallel seats is used by the parallel gate valves, and the knife gate valve built with a sharp edge on the bottom of the gate is a common type. Two inclined seats and a slightly mismatched inclined gate are used in the wedge-shaped gate valves.
Gate valves with a metal seating wedge were commonly used until the resilient seated gate valve was introduced to the market. In order to ensure tight closure, the conical wedge shape and angular sealing devices of a metal-seated wedge need a depression in the valve bottom. Sand and pebbles are thus embedded in the bore. Regardless of how thoroughly the pipe is flushed upon installation or repair, the pipe structure will never be fully free from impurities. Eventually, any metal wedge would therefore lose its capacity to be drop-tight.
Growing stems are attached to the gate, and as the valve is worked, they rise and lower together, giving a visual indicator of the position of the valve and making it possible to grease the stem. The threaded stem is rotated around by a nut and moved. This form is only suitable for above-ground installation.
In the gate, non-growing stems are threaded, and rotate within the valve with the wedge growing and lowering. Since the stem is held inside the valve body, they take up less vertical space. To indicate valve location, AVK offers gate valves with a factory-mounted indicator on the upper end of the stem. Non-rising stem gate valves are ideal for installation both above ground and underground.