What is a knife gate valve?
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.
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.
Design & parts of a knife gate valve
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 1000+ being available.
Working theory of knife gate valves
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.
Advantages of a knife gate valve
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.
Types and applications for knife gate valves
The Hawle Austria Group manufactures high-quality knife gate valves for all different municipal needs. Depending on the application, Hawle offers 4 models of knife gate valves.
Hawle knife gate valves are manufactured in the EU and meet the European standards for wastewater. If you have any further questions regarding Hawle knife gate valves or looking for a suitable solution for your water application, please use our contact form to get in touch. We are happy to advise you!