Gate Valve

GATE VALVE A gate valve, also known as a sluice valve. Gate valves are valves that operate by lifting of a round or rectangular gate out of the path of the fluid. This valve facilitates only unidirectional flow, but unlike globe valves they give a full bore flow to the fluid. The round or wedge shaped ring is known a gate as it moves in a perpendicular direction to the flow of the fluid. Typical gate valves should never be used for regulating flow, unless they are specifically designed for that purpose. On opening the gate valve, the flow path is enlarged in a highly nonlinear manner with respect to percent of opening. This means that flow rate does not change evenly with stem travel. Also, a partially open gate disk tends to vibrate from the fluid flow. Most of the flow change occurs near shutoff with a relatively high fluid velocity causing disk and seat wear and eventual leakage if used to regulate flow. Typical gate valves are designed to be fully opened or closed. When fully open, the typical gate valve has no obstruction in the flow path, resulting in very low friction loss. Gate valves are characterized as having either a rising or a nonrising stem. Rising stems provide a visual indication of valve position. Nonrising stems are used where vertical space is limited or underground.

Construction The bonnets of gate valves are made of cast iron and thus require extra care during overhauling. The valves or the gates are circular or wedge shaped discs and are attached to a threaded spindle working in a nut. The salient feature of gate valves is that the sealing surfaces between the seat and the valve are planar. The valves and seats might be tapered or parallel at the sides. The bonnets might be a screw-in, bolted or a union.type depending on the requirement and the space where the valve has to be fitted. It is of utmost importance that the bonnets are absolutely leak proof. Screw-in bonnet is the simplest, offering a durable, pressuretight seal. Union bonnet is suitable for applications requiring frequent inspection and cleaning. It also gives the body added strength. Bolted bonnet is used for larger valves and higher pressure applications. Another type of bonnet construction in a gate valve is pressure seal bonnet. This construction is adopted for valves for high pressure service, typically in excess of 15 Mpa (2250 psi). The unique feature about the pressure seal bonnet is that the body - bonnet joints seals improves as the internal pressure in the valve increases, compared to other constructions where the increase in internal pressure tends to create leaks in the bodybonnet joint. Gate valves normally have flanged ends which are drilled according to pipeline compatible flange dimensional standards. Gate valves are typically constructed from cast iron, ductile iron, cast carbon steel, gun metal, stainless steel, alloy steels, and forged steels.

Usage of Gate Valves Gate valves are used in applications where a full unrestricted flow of the fluid is desired. As the gate can be opened to the full bore it provides very less frictional loss. Gate valves are not suited for regulating flow as the gate arrangement allows the valve to remain in either fully closed or open position. In order to increase tightness of the seal some gate valves are provided with twin discs which can be pressed against the seat with the help of a spring when closed. In case the valve requires a change in direction, a full bore angle valve can be used. Steam circuits on ships are best suited for the use of gate valves due to the characteristics described above. Since steam causes heating and deformation of the material that it passes through; normally such valves do not have a solid gate, rather they have a flexible gate made out of two circular plates with a hub in-between to account for any change of dimensions in the valve body.