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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:
- wedge
- ductile iron valve body
- seating area
- stem (spindle)
- spindle bearing
- bonnet
- packing.
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.
Gate valves may not be used as often as they were in the past since ball valves started becoming more and more popular, but for certain applications, gate valves are perfect for the job. Irrigation applications still use gate valves as there are still strong benefits that gate valves can offer that Industry.
In this post, we will take a look at the difference between gate valves and knife valves and explore why gate valves are still popular in certain applications. Let's get started.
Gate Valves and Knife Valves
Gate valves and knife valves are linear motion valves, which include a flat closure element called a gate. The gate inside the valve slides up and down by turning a hand wheel counter clockwise to open and clockwise to close. For knife valves, you simply slide the gate by pushing or pulling the handle. Both valves move the gate into the flow stream to stop the flow of fluid completely. To explore what the markings found on gate and ball valves mean, check out our blog post, How to Understand Ball Valve Markings.
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Gate and Knife valves are designed to minimize pressure drop across the valve. In the fully opened position, the diameter of the opening that the liquid passes through is equal to that of the pipe and the direction of flow is not changed.
Applications & Strengths
Gate valves can be found anywhere an economical shut off is required. Gate valves are ideal for any application that involves slurries as the gate can cut right through. Gate valves are also common in applications using liquids like heavy oils, light grease, varnish and other non-flammable liquids.
Irrigation Applications
Gate valves are perfect for irrigation systems where high flow rates are required. Gate valves are slow closing as it takes several revolutions to open or close the gate, therefore flow is started and stopped slower than ball valves. Gate valves are extremely common in Irrigation applications because the speed of closing is not as essential as strength to support a high flow rate.
Gate Valve Limitations
Most gate valves have metal to metal seats which doesn't create a positive seal and can sometimes cause leaks, whereas ball valves shut tight. Gate valves also have a tendency to seize up if not used over a certain period of time, leading to leaks at the stem packing.
As seen in the picture above, gate valves tend to be bulkier compared to ball valves. They stick out more, which is not ideal for tight spaces. For some applications, ball valves would be the better choice as they can offer a leak free seal. For more information on ball valves, check out our post, What the Heck is a Ball Valve?
Throttling (Restricting Flow)
Gate valves can be used wherever throttling capabilities are desired however, this is not typically recommended as erosion of the seat and disc occur due to the vibrations of the disc resulting in the valve leaking over time.
In Summary:
Although gate valves might not be the most favoured valve on the market, they are still a suitable option for certain functions. Be aware of location and the situation for which you require a valve to determine which style of valve will work the best for you.
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