What Is A Gate Valve Used For?
- 01 What is a gate valve?
- 02 Design & parts of a gate valve
- 03 Types of gate valves
- 04 Gate valve working principle
What is a gate valve?
A gate valve is the most common valve type found in water supply systems. It represents a linear-motion isolation valve and has a function to stop or allow the flow. Gate valves got their name from the closure element sliding into the flow stream to provide shutoff and, therefore, acting like a gate. Gate valves are used to isolate specific areas of the water supply network during maintenance, repair works, new installations, as well as to reroute water flow throughout the pipeline.
Design & parts of a gate valve
A gate valve has a simple design and can be applied in many low pressure-drop services, which makes it one of the most common valves in use today. Gate valves are designed as full-port valves. This means that the valve port is of the same size as the inner diameter of the connecting pipe. Full-bore gate valve passes the fluid flow without any obstructions to a flow and do not cause a pressure drop in a pipeline. This also allows to clean the pipe using a cleaning pig.
A resilient seated gate valve with a non-rising spindle consists of the following parts:
- ductile iron valve body
- seating area
- stem (spindle)
- spindle bearing
The body is the largest element of the gate valve. Since the spindle remains in the valve body during rotation, it allows an economical bonnet construction. The valve bonnet itself is joined to the body with bolts, which allows cleaning and maintenance. As the gate valve closes, the wedge travels down until it presses the valve seat, which would mean a complete shutoff. During the opening the wedge slides towards the upper part of the valve body.
A wedge gate valve construction did not alter a lot for the last 100 years. However, the design of some elements of the gate valve has undergone some modifications. Most of the changes are aimed at increasing the service life of the gate valve, improving the sealing characteristics, using superior quality materials and epoxy coatings for a better corrosion protection. At the same time, the modern gate valve body has a compact sleek design, allowing installations in limited space conditions. There are already 7 generations of Hawle gate valve models which have been developed in the last 70 years.
Types of gate valves
Based on construction, gate valves for water supply systems can generally be divided into the following types:
- By wedge: parallel gate valves and wedge gate valves
- By seat: metal seat or resilient seated gate valves
- By stem: with rising or non-rising stem.
Parallel or wedge-shaped gate valves
Parallel slide gate valves feature a flat, parallel-faced gate-like closing element, fitting between two parallel seats. The so-called knife gate valve is a type of parallel gate valves.
Wedge gate valves are so called because of a wedge-shaped gate. The wedge has ribs on both sides and is guided by the slots in the gate valve body. The wedge guides serve the following purposes:
- to transfer the axial loads imposed on the wedge by the medium to the valve body
- to enable a low-friction movement of the wedge in radial direction
- to prevent the wedge from rotation while travelling between open/closed valve positions (serving as an anti-twist lock).
Most gate valves used in water distribution systems are wedge gate valves.
Metal seated or resilient seated gate valves
Wedge gate valves can have metal or resilient seats. In metal seated gate valves, a wedge slides towards a groove in the invert of the gate valve body, which would often trap solids contained in the fluid. For this reason, resilient seated gate valves provide a tighter shut-off and are more preferred in water distribution systems.
In resilient seated gate valves, a wedge, mostly made of ductile iron, is encapsulated in resilient material (elastomer) and therefore provides a tight seal. The seating takes place between the rim of the wedge and the valve body. This does not necessitate a groove at the bottom of the valve. Therefore, resilient seated gate valves can seal around small solids, carried by the water flow. All Hawle gate valves are resilient seated where the valve gate is vulcanized with high-quality elastomer and suitable for application in drinking water supply. For example, in the latest Hawle gate valve model E3 both wedge and the nut are fully coated with elastomer, which allows the highest degree of corrosion protection.
Rising stem or non-rising stem gate valves
The difference of rising to non-rising stems is that they may be either fixed (rising stem) or threaded (non-rising stem) to the gate. In rising stem gate valves, the rotating stem rises as the valve is opened. Some clear disadvantages of the rising-stem valves are:
- due to a bulky design they require a greater amount of space when opened as the spindle travels upwards
- they are not suitable for underground or buried installations.
How does a non-rising stem gate valve work?
In these types of gate valves, the threads on the stem remain inside the valve body. The valve gate travels up and down the stem threads once the valve stem is rotated. Non-rising spindle gate valves have an advantage in installations where space is limited, for example, in pumping stations. All Hawle gate valves have a non-rising stem and are therefore suitable for buried service.
Gate valve working principle
Gate valves are normally operated by a handwheel, a valve T-key (wrench) or an actuator. The wheel is attached to a valve stem and transfers rotational energy to it. During the gate valve opening, the rotation of the handwheel turns the threads of the gate stem into the gate and vice versa for closing. This energy moves the gate valve wedge downwards or upwards. In underground gate valve installations, an extension spindle is attached, which allows operation without direct access to the valve itself.
Generally, gate valves are installed in the areas of a pipeline where they will longer remain open or closed, as they generally require sometime to switch between these two positions. However, when the valve needs to be operated quite often or even remotely, motorised gate valves are used (i.e. equipped with an electric actuator).
How to tell if a gate valve is open or closed?
In rising stem gate valves, the stem position tells whether a gate valve is open (outside of the valve body) or closed. In non-rising stem gate valves, it is not easy to determine if the gate valve is fully open, closed or somewhere in between at first sight. A marker of the closing direction (cc for clockwise closing, acc for anti-clockwise closing) can be helpful to determine the position of the wedge. A position indicator is a special device, which is mounted on the gate valve stem and helps a user quickly determine a position of the gate valve.
Most of the gate valves close by a clockwise direction, however, in some countries, utilities use anti-clockwise closing gate valves. Hawle produces both versions of gate valves, which allows to meet local standards of such countries as Australia or New Zealand.
Can you throttle a gate valve?
Since gate valves are isolation valves, they must be used either in the “fully open” or “fully closed” position and must not be deployed to regulate the water flow. Why a gate valve is not suitable for throttling a flow? When throttling a gate valve, turbulent flow rates through the valve body can cause vibration and cavitation, damaging the wedge and the seat, which will eventually lead to leakages and worse sealing capacities.
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