A gate valve, also referred to as as th sluice valve, is a valve that opens by lifting a barrier out for fluid to pass through. Gate valves need very little space along the pipe axis and barely restrict the flow of fluid when the gate is fully opened. The face of the gate is commonly wedge shaped in order to apply pressure on the sealing surface, but can be parallel as well.
Gate valves are designed for fully-open or fully-closed services. They are installed in pipelines as isolating valves, and should not be used as control or regulating valves. Operation of a gate valve is manipulated by either turning the gate clockwise to close (CTC) or clockwise to open (CTO) rotating motion of the stem. The gate moves upward or downwards on the threaded part of the stem when the stem is operated.
Gate valves are normally utilized during minimum pressure loss or when a free bore is needed. While fully opened, a standard gate valve has no obstruction in the flow path, which leads to an extremely low pressure loss; such design makes it possible to use a pipe-cleaning pig.
A gate valve is a multi-turn valve, which means that the operation of the valve is realized by threaded stem. Since the gate valve has to be turned multiple times to be in the closed position, the slow operation also avoids water hammer effects.
It is important to keep in mind not to confuse gate valve with a knife gate valve as knife gate valve does not have wedge, and belongs to a different category. Knife gate valve is often used to cut through materials.
Gate valves are used for a wide array of applications and are ideal for both above ground and underground installation. It is imperative to choose the right type of valve to avoid high replacement costs.
Gate valves are used to terminate the flow of liquids instead of ensuring flow regulation. When fully opened, the standard gate valve has no obstruction in the flow path, leading to a very low flow resistance.
The size of the open flow path typically changes in a nonlinear manner as the gate is moved. This means that the flow rate does not vary uniformly with the stem travel. Based on the construction, an open gate that is opened partially can vibrate from the fluid flow. Gate valves are mostly used with larger pipe diameters because of the less complicated construction compared to other types of valves large in sizes.
At high pressures, friction may constitute an issue. As the gate is pressed against its guiding rail by the pressure of the medium, it becomes more difficult to manipulate the gate valve. Large gate valves may be coupled with a bypass controlled by a smaller valve to be capable of reducing the pressure before operating the gate valve itself.
Gate valves without an extra sealing ring on the seat is used in cases where minor leaking of the valve is not a problem, such as sewer pipes or heating circuits.
Some of the common types of gate valves include:
● Solid Wedge Gate Valve:
Solid wedge gate valve is the most common and largely used disc type valve, known of its simplicity and strength. This type of gate valve can be installed in any position and is ideal for almost all fluids. It can be used in turbulent flow as well.
However, it does not compensate for changes in seat alignment due to pipe loads or thermal expansion. There, this type of disk design is most prone to leakage. Solid wedge is subject to thermal locking if used in high-temperature service.
● Flexible Wedge Gate Valve:
This type of gate valve is a one-piece solid disc with a cut around the edge. These cuts can vary in size, shape, and depth. A narrow and shallow cut on wedge perimeter will yield less flexibility, but retain strength. The design of a flexible edge gate valve enhances seat alignment and provides better leak tightness. It also improves performance in the event of thermal binding. These are mostly used in steam systems.
● Split Wedge Gate Valve:
This type of gate valve is also referred to as the parallel disc gate valves. The valve is comprised of two solid pieces, which are held together with the help of a distinct mechanism. Split wedge gate valves are spring loaded so that they are always contacting the seats and offer bi-directional sealing. This type of gate valve is specifically ideal for handling noncondensing gasses and liquids at normal and high temperature.
This type of gate valves allows freedom of movement of the disc, preventing thermal binding in spite of the fact that the valve may have been closed when a line is cold. This means that it does not create thermal binding when a line is heated by fluid thus expands.
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