What is a Knife Gate Valve? – Function and Working

What is a knife gate valve?

Knife gate valve is designed mainly for on-off and isolation services in systems with high content of suspended solids. Knife gate valves are especially beneficial for handling slurry, viscous, corrosive, and abrasive media. The valves have a minimized pressure drop in the fully open position, they are easy to actuate, they have a relatively low weight, and are cost-effective.

Function Of a Knife Gate Valve

Knife gate valves are so-called because of their ability to cut through media-containing solids. They work by lifting a round or rectangular gate out of the path of the media. The sealing surfaces between the gate and seats are planar, so knife gate valves are often used when a straight-line flow of fluid and minimum restriction is needed.

Knife gate valves were designed for specific use in the pulp and paper industry. This was due to stringy pulp impinging between the wedge and seat of a normal gate valve and preventing flow shut-off.

How a Knife Gate Valve Works?

Because of these highly effective design characteristics, knife gate valves have become invaluable when it comes to applications that involve viscous fluids, slurry, and other systems where impingement is an issue.

Knife gate valves are used in a lot of processing plants today and come in large sizes which make it easier to handle thick flows of light grease, heavy oils, varnish, slurry, wastewater, and paper pulp. It’s important to note that these valves do have low-pressure limitations and are designed to seat the blade into an elastomer seal once the blade cuts through the substances it’s handling.

Thick liquids easily glide over these soft seals with no interference, however, when a solid mass or powder passes through the knife gate, the bulky, dry material ends up packing into the soft seals located at the end of the gate. When this happens, the seals eventually won’t close tightly enough. If this happens the seals will need to be replaced.

What is a Knife Gate Valve? - Function and Working

Where are knife gate valves used?

Knife gate valves are designed to work in some of the harshest environments, typically having a sharpened blade to cut through heavy liquids.

They are especially useful in wastewater applications where corrosion is an important issue. So, in addition to the valve design optimized for slurry media, it is beneficial to have a knife made of acid-proof stainless steel as this makes it less susceptible to damages caused by corrosion, and as a consequence, it needs less frequent maintenance or even replacement.

Knife gate valves should only be used for applications requiring a completely open or completely closed position and should not be used to regulate flow unless they are designed for it. Whenever fluid is forced against a partially closed gate, there will be a vibration, gradually eroding the disc and seat. In addition, the knife gate valves are designed to slowly open and close to safeguard against the impacts of water hammers.

When not to Use Knife Gate Valves?

These valves shouldn’t be used to regulate flow because whenever fluid is forced against a partially closed gate, a vibration takes place, gradually eroding the disk and seat. Consequently, knife gate valves should only be used completely closed or opened. In addition, these valves are designed to slowly open and close to safeguard against the impacts of water hammers.

What is the difference between knife gate valves and gate valves?

Both the wedge-type gate valves and the knife gate valves are primarily designed for on-off services where the valve is completely opened or completely closed. It is not recommended to use either of these to regulate flow because when fluid pushes against a partially closed gate vibration occur, and cavitation will eventually damage the seat and the body.

Also, both valve types are designed to open and close slowly in order to reduce the risk of water hammer.

Both types can be used in applications involving:

  • Slurries
  • Heavy oils
  • Non-flammable viscous fluids
  • Wastewater
  • Clean water

One of the differences between the gate and the knife gate valves is that the knife gate valve often has a sharpened disc to better cut through slurry/viscous media. Also, the knife gate valve has a short face-to-face length compared to a gate valve, which is dimensionally wider. As a consequence, a knife gate valve is light-weighted compared to a gate valve. These are the main differences and probably two of the biggest advantages of the knife gate valve.

Knife gate valves are typically found in wastewater treatment plants, chemical plants, mining, cement plants as well as in many other industrial applications while gate valves are typically found in drinking water mains, distribution networks, and drinking water pump stations.

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 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.