All About Gate Valves

 

A wedge type gate valve.

Image credit: al7/Shutterstock.com

Gate valves, or sluice valves, are mainly used to block or permit fluid flow through pipes. They are not good at regulating flow for which other valve designs, such as globe valves, are better suited. Ball valves are another on/off valve type unsuited to throttling. Gate valves offer a size advantage over these other types in terms of how much space must be allotted between pipe flanges to accommodate the valve. For information on other types of valves, please see our Valves Buyers Guide.

Valve design

Gate valves consist of three major portions, the valve body and seats, the gate (or disc) and stem, and the packing and bonnet. In operation, the body and seats remain stationary while the threaded stem rotates to lift the gate off its seats. Stem rotation is accomplished manually through a handwheel or automatically via a valve actuator. The bonnet houses the disc while the valve is open and provides a location for the stem packing where the stem exits the valve.

As with some other valve types, a distinction is made between rising stem and non-rising stem designs. Rising stem styles give a quick visual indication as to whether the valve is opened or closed. In non-rising stem designs, the stem threads into the disc, raising or lowering it as the stem turns while remaining fixed axially. Gates can be straight-sided or tapered discs. Wedge gate valves, also called tapered gates, are most common as they provide stronger sealing action and they can be solid, flexible, or split. Flexible gates accommodate some distortion of the seats arising from pipeline flex.

When a gate valve is fully opened, the gate disc is completely withdrawn from the flow path into the bonnet. For this reason, the pressure drop through gate valves is minimal. When closed, the gate disc seals against seats in the valve body which may be integrally cast or, more likely, welded or threaded into place. Metal seats are most common and seats are generally lapped after installation. In certain applications, the seats may be hardened. Seats on both sides of the gate permit the valve to be used bi-directionally. Where some leakage is acceptable, the gate seals without an extra sealing ring.

Gate valves should not be used in other than fully opened or fully closed positions as partially opened gates can expose the valve internals to accelerated and uneven wear and vibration. Another problem with using a gate valve in regulating service is the gate does not open the valve passage uniformly, making it difficult to produce a consistent increase in flow for a given turn of the stem.

Gate valves are not quarter-turn devices such as ball valves but instead close slowly through multiple revolutions of their handwheels. This can be a benefit in the prevention of water hammer. Acme threads are commonly employed on the stems, with multi-lead threads sometimes used for faster operation. The unthreaded portion of the stem is usually finished to provide a good sealing surface for the packing. Packing is adjusted by tightening a gland nut or nuts, though some designs use springs – so-called live loading – to provide constant pressure on the seal for better reduction of fugitive emissions.

Applications

Gate valves are used in wastewater plants, power plants, and process plants for shut-off and isolation service. They overshadow ball valves in larger applications because of the mechanical advantage a threaded stem offers over a quarter-turn lever. Some very large valves must incorporate a means of pressure reduction before the gate can be moved. Their simple design makes them an economical solution as pipe diameters increase beyond 2 inches.

Material selection for gate valve bodies runs the gamut, with cast iron and steel common for larger valves and stainless steel, forged steel, bronze, etc. widely available in smaller sizes. Non-metallic options such as plastic gate valves are also available. Specifying material for the body generally includes all components under pressure, while “trim” refers to the components apart from the body, including the seats, the disc, the stem, and, if applicable, the bellows. Larger sizes are identified by an ASME class pressure rating and ordered with standard bolted or welded flanges. Sizing a gate valve is straightforward as the design precludes any significant pressure drop through the valve.

Non-rising stem designs are popular where vertical space is limited – aboard ships, for example. Rising stem designs offer a fast, visual confirmation of a valve’s status, though the exposed stem can be subject to corrosion. Non-rising stem valves often use an indicator to verify the gate position. Neither design has much impact on the performance of the actual valve.

Gate valves are routinely automated using electrical rotary actuators, and their operation can be sped up with the use of hydraulic or pneumatic linear actuators. The effort required to open and close large gate valves manually can be reduced through the use of geared actuators.

Some building codes require that on/off valves be installed upstream of certain equipment, on water heater inlets, for example. Here, a ball or gate valve is acceptable while a globe valve is not. At one time, gate valves were predominant in residential plumbing systems, where they had a tendency to “freeze” open from disuse. New plumbing systems mostly use lever-actuated ball valves in their place, as the ball has a much lower tendency to stick. The additional space required for the lever can require some creativity in retrofits.

A special gate valve known as a knife gate valve is used to control the movement of slurries and viscous liquids. Sliding gate valves are used for dispensing dry and bulk materials from hoppers.

Considerations

A gate valve makes economic sense when compared to other valve types intended for the same purpose. They provide positive shutoff, low pressure loss, and bi-directionality in a compact envelope. Their downsides include slow actuation, an inability to regulate flow, and susceptibility to vibration.

Summary

This article presented a brief discussion of gate valves including the different types and applications. For more information on related products, consult our other guides or visit the Thomas Supplier Discovery Platform to locate potential sources of supply or view details on specific products. More information on valves, in general, can be found at the Valve Manufacturers Association website, www.stvvalves.com.