Gate Valves VS Knife Gate Valves

 While gate valves and knife gate valves share many similar characteristics, there are some fundamental differences that set them apart.

While knife gate valves and gate valves share many similarities – they're both used in on-off applications, involving slurries and other viscous media – there are some noteworthy design variances to consider when weighing up which type of valve would be best suited for your application.

 

 

What Makes Them Similar?

Gate and knife gate valves are primarily for on/off service. Using these types of valves to regulate flow is not recommended because when fluid pushes against a partially closed gate vibration occurs, eventually wearing and eroding the seat and disk. For that reason, these types of valves should be either completely opened, or completely closed.

These valves are also designed to open and close slowly to protect from the effects of water hammer.

Both types of valves can be found in applications that involve:

• Slurries

• Heavy oils

• Non-flammable viscous fluids

• Wastewater

• Clean water

 

What Makes Them Different?

The differences between them are vast. First off, yes, the knife gate valve DOES have a sharpened disk (thank you very much). This is meant to sever any stringent solids that may keep the valve from closing and causing it to leak. But there's more that makes their designs quite different.

The biggest difference between these two types of valves is that gate valves are built to ANSI standards while knife gate valves adhere to TAPPI standards. Therefore, dimensionally the gate valve is wider, flanged, and ANSI pressure rated. It also has API leak tightness standards must be met. They're bi-directional, and commonly used in steam applications as well as fluid applications. The gate valve is only available with metal seats.

Another significant difference between an ANSI gate valve and a knife gate valve is in the packing gland area.  A gate valve will have a v-ring packing set that seals the shaft attached to the gate, while a knife gate valve has a packing gland that seals around the gate.

A knife gate has a very thin profile compared to its bulkier counterpart. It's also uni-directional (though bi-directional is an option on some), and comes in either wafer or lugged body, no flanges. The seats on the knife gate valve come in everything from metal to resilient types.

Two of the knife gate's biggest advantages knife are weight and expense. A 16” knife gate valve will typically weigh less than 300# while an ANSI gate valve will be in excess of 1200#.  The cost of the valve is similar to the weight difference as well.

 

 

How Do Gate Valves And Knife Gate Valves Work?

Gate valves open by lifting a round or rectangular gate (or wedge) out of the path of the fluid. The sealing surfaces between the gate and seats are planar, so gate valves are often used when a straight-line flow of fluid and minimum restriction is needed. They have a flat gate closure that slides in and out or up and down between two parallel plates to open or close the valve. Gate valves are used for shut-off, rather than flow regulation because they only have two settings: on and off.

While a knife gate valve performs exactly as the name implies. These types of valve were originally designed for the pulp and paper industry. Stringy pulp would impinge between the wedge and seat of a normal gate valve and prevent flow shut-off. Knife gate valves were designed with a sharp edge to cut through the pulp and seal. With this type of useful attribute, the knife gate valve has become invaluable in applications involving slurry, viscous fluids and other systems where impingement is an issue.

 

Gate Valve And Knife Gate Valve Applications

Gate valves are used in many industrial applications requiring shut-off valves, including the oil and gas industry, pharmaceuticals, manufacturing and automotive. They can cope in demanding environments, such as high temperature and high pressure environments. Common gate valve applications include in power plants, water treatments, mining and offshore applications.

Knife gate valves are advantageous in sludge and slurry applications because their blades cut right through thick liquids easily. They're generally specified in larger sizes for handling thick flows of heavy oils, light grease, slurry, paper pulp, varnish and wastewater to name but a few knife gate valve applications.

 

Advantages And Disadvantages Of Gate And Knife Gate Valves

One of the advantages of gate valves is that pressure drop across a gate valve is very low when it is fully open. They can also be used for bi-directional action and are useful as on-off valves. Gate valves do, however, need a large force to operate and large sized valves need automatic actuators. They're not quick to open or close, and take up more space, when compared to some other valves. Thermal expansion and shrinking can also lead to unwanted leakage in some gate valves that are exposed to high temperature fluctuations.

The advantages of knife gate valves are that they're cheap, easy to actuate, and light. One of the most notable disadvantages of knife gate valves is that they're known to have low-pressure limitations. This makes them less desirable for use in applications that require cleanliness or sanitary conditions.

 

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