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Gate Valve Performance and Characteristics
A gate valve, also known as a gate, is a valve that opens by lifting a round or rectangular gate/wedge in the path of a fluid.
The distinguishing feature of a gate valve is that the sealing surface between the gate and the valve seat is flat, so gate valves are often used in a straight line with little restriction of fluid.
The gate faces can form a wedge or they can be parallel. In simple terms, the gate valve principle is primarily used to allow or stop the flow of liquids. Still, typical gate valves are not used to regulate flow unless specifically designed for that purpose.
Because of the gate valve's ability to perform through liquids, gate valves are typically used in the petroleum industry for thick fluids, and a specialized valve often called a knife valve is used for cutting fluids. In an open gate, the flow path is amplified in a highly non-linear manner relative to the opening. This means that the flow rate varies unevenly with the stroke of the valve stem. Operating a gate valve while partially opening the gate will oscillate the fluid flow.
Most of the variation in flow occurs at relatively high fluid velocities causing the disc and seat to wear and eventually leak shut if used to regulate flow. Typical gate valves are designed to be fully open or closed.
Gate valves are characterized by having a rise or fall when the valve comes down. Stem rise provides a visual indication of valve position. Since the valve stem is connected to the valve seat, the valve stem and disc rise and fall together to operate the valve. A non-rising stem may have a pointer thread on the upper end of the stem that shows the position of the valve since the gate travels up or down without raising or lowering the stem. Stems are used underground or where vertical space is limited.
The cap is the valve body sealing cover. Gate valves may have a screw or bolt bonnet, union. The screw cap is simple and provides a durable, pressure seal. Union's caps are suitable for applications requiring frequent inspection and cleaning. It also replenishes energy in the body. Bolted bonnets are used on larger valves and high-pressure applications. Another type of pressure sealing bonnet structure in gate valves.
Gate valves have flanged ends that are drilled to comply with pipe flange size standards. Gate valves are usually made of cast iron, ductile iron, cast steel, carbon steel, stainless steel, alloy steel, and forged steel.
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