Gate Valves, also known as knife valves are used when the fluid flow is to be controlled. Gate valves are used for linear motion flow and are equipped with a flat closure element that slides into the stream and shuts the flow. They are one of the most commonly used valves.
Gate valves can cut through slurries, and other viscous materials such as heavy oil, grease, molasses, varnish, heavy cream and more. They come in a variety of sizes to cater different volume of flow. These are not used under low pressure limitations. However, they can be used for throttling purposes but, excessive use for throttling can cause the disc and seat of gate valve to erode.
Gate valves are best suited for conditions where infrequent use of valves is required. They are designed to drop the pressure across the valve when it is fully opened and then stop the flow completely.
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Gate valve uses a flat insert that travels perpendicular to the flow stream. The diameter through the valve matches the diameter of the adjoining pipe, and since the gate lies entirely outside the flow stream, when the valve is fully open, the gate valves have very low pressure drops.
Gate valves are used for making hot tapes, for on/off service and are useful for handling heavier fluids such as slurries. Once the gate is open, it is out of the way of the slurry flow path and slices through it to close the flow. They are also used in viscous liquid service. Gate valves are not a good choice in applications requiring cleanliness or sanitary conditions.
Types of Gate Valves
There are two basic types of gate valves
- Double Disc Type Gate Valves have parallel seats against which the wedge is driven when the gate is closes. This results in a tight seal between the disc and the seat. Variation of this design includes valves that depend on fluid pressure to force the disc against the seat.
- Wedge Type Gate Valves have a disc that is in the shape of a wedge that seats against two inclined seats. Solid wedges are used in high flow or turbulent applications such as steam service. Solid design also minimized the vibration and chatter. Split wedge discs are more flexible and may be used where pipeline strains may distort the valve seats.
Gate Valve Stem
There are two types of gate valve stems;
- Rising Stem Gate Valve are preferred because the position of the gate is readily discernible by observing the position of the valve stem. If stem is up, the gate i open. If stem is down, the valve is closed.
- Non-Rising Stem Gate Valves are generally only used in tight spaces and where the extended stem is not an option. It is usually prudent to post a sign to not rely on the stem position to determine valve gate closure. The rising stem design keeps the threads out of fluid contact whereas the non-rising stem has thread internals in contact with fluid. This may result in corrosion, erosion of accumulation of deposits on the stem threads making it difficult to operates years after installation.