Gate Valves

A gate valve is used to stop or start the flow of fluid in a pipeline by inserting or removing a rectangular gate in the path of the flow. It is operated by a threaded stem that connects a handwheel to the gate.

It is an on/off valve, designed to completely shut off flow or allow full flow in the open position. It is not normally used to regulate flow or for throttling, as a partially open gate can suffer from vibration and chatter and the gate and seat may be subjected to excessive wear. When flow needs to be regulated in a pipeline, a plug valve or globe valve should be used instead.

Gate valves are suitable for most fluids including steam, water, oil, air and gas and they are used more often with larger pipe diameters, as they are less complex to construct than other large sized valves.

They are typically used in refineries and petrochemical plants where pressures are low and temperatures high and are used less in oil and gas production because of those facilities’ high operating pressures and severe marine environmental conditions.

Different types of gate valves have different applications including:

Full-bore through conduit gate valves – have a full bore design to prevent solids from entering the body cavity and to allow cleaning and monitoring drones to pass through them.

Wedge gate valves – have a wedge-shaped instead of parallel seat that seals against both high and low fluid pressures and on both sides of the gate.

Knife gate valves - have a tapered knife-like edge on the lower surface for cutting through highly viscous slurries such as paper pulp.

Rising stem gate valves – popular because the position of the valve (open or closed) can be noted just by looking at the position of the stem (preferred in applications where monitoring must be done from a distance or when the line cannot be shut down to test it).

Gate valves and ball valves are both used to control the flow of liquids and gases. The main difference is in how they open and close the valve, with a gate valve moving a gate up and down and a ball valve rotating a ball 90 degrees.

The gate valve is operated by a threaded stem connected to a handwheel and requires several turns to open or close the valve. The ball valve’s stem is attached to a handle, which when turned 90 degrees, rotates the ball to open or close the valve. This difference gives each certain advantages and disadvantages, making both more suited to certain applications:

• Because a ball valve handle only requires a 90 degree turn, it is more suitable for applications where emergency shut off is a desirable feature.
• Fast shut off can also cause water hammer, so the gate valve’s slower closing speed is preferable in situations where damage to a delicate system needs to be avoided.
• The position of a ball valve’s handle indicates whether the valve is open or closed, while it’s much harder to tell with a gate valve handwheel. This makes ball valves better for applications where monitoring from a distance is required.
• The ball valve’s handle requires more room to operate than a handwheel, making a gate valve preferable in tight installation applications.

Other differences include:

Cost - ball valves are more expensive

Sealing - gate valves are more prone to leaks

Flow capacity - gate valves can provide larger flow capacity

Design – ball valves are harder to clean, making gate valves preferable for applications involving slurries and viscous media

Pressure – gate valves are better for high pressure applications

Durability - ball valves are more durable and require less maintenance than gate valves.

Gate valves are not suitable for variable flow control or throttling as the seating is perpendicular or at right angles to the direction of flow. Repeated movement of the wedge (gate) near the point of closure under high velocity flow, may create a drag on the seating surfaces and causing galling or scoring on the down stream side. A slightly opened edge may cause turbulent flow with vibration and chattering of the wedge.

The gate valve may be used in domestic and commercial plumbing applications. It works by turning a handwheel which raises or lowers an internal wedge shaped gate to start or stop the flow of water through a pipeline.

Gate valves are purely on/off valves and should never be used to regulate or throttle the flow of water. That’s because a gate that is partially opened or closed can be subjected to vibrations and wear damage.

In plumbing, gate valves are commonly used as shutoff valves on main and branch water supply lines, where the water only needs to be shut off infrequently. This is because if they are used too frequently, their parts can become corroded and they can get stuck in an open or closed position.

While ball valves are gradually becoming more popular in plumbing, gate valves still offer a number of benefits including:

Slow opening and closing – the handwheel takes several turns to open or close the gate, which reduces the likelihood of damaging water hammer (shock waves) from occurring if the valve is closed too quickly.

Minimal flow resistance – because the gate retracts entirely into the body of the valve, this allows for maximum flow capacity.

Less expensive price – gate valves are less expensive than ball valves, which last longer but are initially more expensive to buy.

Large capacity – due to their simple design, gate valves are easier to manufacturer in larger sizes and pipe capacities, making them more suited to large water management systems.

Easy installation – because a handwheel takes up less room than a ball valve’s operating handle, a gate valve can be installed in much tighter spaces.

Easy cleaning – gate valves are easier to clean than ball valves, making them a better choice for use with slurries and viscous fluids.

A gate valve opens by lifting a gate out of the path of the fluid in a pipeline and closes by replacing the gate in its path. The face of the gate can either be parallel or more commonly, wedge-shaped, which applies more pressure to the seal at the bottom.

Gate valves are typically used as shut-off valves, fully blocking or unblocking the flow rather than regulating it. This is because a partially open gate can be subjected to vibrations and wear damage.

The gate is operated by a threaded stem connected externally to a handwheel. This will either be a rising or non-rising stem, depending on which end is threaded.

A rising stem is attached to the gate and they rise and lower together as the valve is operated. The rising stem shows at a glance the position of the valve, making it useful for applications where inspection from a distance is necessary.

A non-rising stem is attached to and rotates with the handwheel and because the gate's motion is concealed inside the valve, it is harder to tell the position of the gate. However, non-rising stem valves are useful when installation space is limited.

When fully open, the gate slides up into the body of the valve (the bonnet), so there is no obstruction in the flow path and very little flow resistance.

Gate valves can have either a screw-in, union, or bolted bonnet. The screw-in bonnet provides a durable, pressure-tight seal, while a union bonnet is best in applications where frequent inspection and cleaning is required. And a bolted bonnet is designed for larger valves and for higher pressure applications.

Gate valves are typically made from cast iron or steel and they are more often used with larger pipe diameters as they are easier to construct in large sizes than other types of valves.