How Does Knife Gate Valve Work?

Knife gate valves are similar in design to gate valves and were initially developed for use in the pulp and paper industry. As the name implies, these valves can cut through thick, porous media that may contain a high concentration of suspended solids (such as dense, sewage sludge) which had caused problems with valves not seating completely and thus leaking.

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A knife gate valve works by lifting a plate with a planar or “knife” edge from the path of flow that can literally cut through various impediments during closing thereby creating complete closure against a soft sealing surface. This soft sealing surface generally dictates use in rather low-pressure systems and is not designed to handle high pressure. Both gate and knife gate valves are primarily designed for isolation or on/off service rather than a throttled or partial-flow application.

Knife gate valves are a specific type of isolation valve that is typically used for media that are not hazardous such as wastewater. The bore of a knife gate valve is full and smooth so the use of a cleaning pig for maintenance is possible. While there is no pressure loss across the valve, they are susceptible to “chatter” or vibration during opening or closing caused by fluid flow against the gate. This is not a significant issue as the appropriate application for a knife gate valve is isolation, not flow restriction, so the normal operating position is fully open or fully closed. A thin (typically) stainless-steel gate is raised and lowered into the bore by a spindle or rising screw and, unlike other gate valves, lifts outside of the valve body when raised. The guide and seal are located around the circumference of the bore and provide a bi-directional, bubble-tight seal.

Knife gate valves are available with bodies made of cast iron, cast steel, stainless steel, fabricated steel, or exotic alloys depending on the specific application. While the gate is usually made of stainless steel, the yoke can be of similar materials to the valve bodies and may include a brass or bronze bearing. The seat seals can be elastomers or flexible metal. Reference the diagrams below to identify the different parts that are common. Bore sizes from DN 50 to DN 600 are common with knife valve dimensions of DN + being available.

All valves are designed to limit or stop the movement of a liquid in a flow stream but there is a myriad of designs and applications that depend on the specific use case. Therefore, there is a slight difference between a gate valve and a knife gate valve in construction. In a typical wedge gate valve, as the name implies, the wedge serves as a “gate” to move into the flow stream and seal off or reduce the flow by impeding the path. A knife gate valve is a specific design of a valve that uses a knife edge that can enter the flow stream and, if necessary, use the edge of the gate to separate any materials that might prevent the valve from sealing completely.

As described above, the knife gate can be raised using a spindle or a rising screw and the actuating mechanism can be manual or pneumatic/electrically activated. The open/closed state of the valve can be observed from the position of the gate – if it is visible above the valve body then it is open and likewise, if it is not visible then the valve is closed.

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Many of the advantages of regular gate valves also apply to knife gate valves; they are easy to actuate by mechanical, electrical or pneumatic means, and they are lightweight which makes them easier to transport, store, install and maintain (knife gate valves tend to be even lighter than resilient- or metal seated gate valves). They are also very cost-effective. Because of these attributes, knife gate valves have seen extensive use involving slurry, viscous fluids, and other applications where impingement is an issue and are used not only in pulp and paper production but are also widely used in municipal wastewater pumping stations and sewage treatment plants. Hawle produces valves and fittings mainly for municipal water supply and sewage, but our knife gate valves can also be used for other non-volatile, non-flammable viscous fluids.

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 optimised 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 hammer.

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 neither of these to regulate flow because when fluid pushes against a partially closed gate, vibration occurs, 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: 

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