What Are Gate Valves?
Gate valves are valves that operate by lifting of a round or rectangular gate out of the path of the fluid. This valves 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. Click here to read about other types of valves such as Globe Valves that are used on various types of ships for different purposes.
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. The screw-in type bonnets have the simplest arrangement but provide a reliable pressure tight seal. Union type bonnets are used in applications that require frequent overhauling and inspection. The main advantage of union type bonnets is that they provide robustness and structural strength to the valve. Bolted bonnets are generally used is large sized gate valves and in applications where pressure and load is expected to be higher.
One more type of bonnet is a pressure seal bonnet which is used for applications with pressure higher than 12 Mpa. The unique feature of this bonnet is that with the increase in the internal pressure the quality of the seal improves, which is contrary to the conventional valves where the increase in internal pressure makes the valves susceptible to leaks.
The threaded stem of the valve can be a rising or a non-rising type. The rising types provides a visual indication of the position of the valve, i.e open or closed. The non rising stem type is incapable of such indication and is generally fitted where there are space constraints.
Just like any other valves, Gate valve too have flanged ends in order to connect them to the pipelines. The valve can be made of stainless steel, gun metal , cast iron or cast carbon steel.
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.
References & Image Credits
Marine Auxiliary Machinery 7th Edition, 1995 by McGeorge