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.
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.
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In this article, PIF spoke to leading Scottish valve distributors, BM Engineering Supplies, to explore the similarities and differences between gate valves and knife gate valves. Although these shut-off valves are both used in on/off applications involving viscous fluids, there are some noteworthy design variances to consider when deciding which is best suited your process application. Here, we put the two head to head to see which valve type is better.
Gate valves open by lifting a round or rectangle-shaped gate out of the path of fluids. The sealing surface found between the gate and the seats is planar. As such, gate valves will often be used when a straight line of flow of fluids is required with minimum restriction. This popular shut-off valve type features a flat fate closure which slides either in and out, or up or down between two parallel plates to open and close the valve. With this in mind, gate valves are often used for shut-off operations as opposed to flow regulation operations as on or off are the only two settings available.
One of the primary advantages afforded by using gate valves is that the pressure which drops can be very low when gate valves are fully open. While they are useful as on/off valves, they can also equally competent at performing bi-directional action. However, gate valves do require a large force to operate. Larger sized gate valves even require automatic actuators to operate. Gate valves are also not the quickest valve type to open or close. They also not the best suited for space-sensitive applications as they tend to take up more space compared to other valve types. In some cases, gate valves can also be prone to thermal expansion and shrinking when exposed to high-temperature fluctuations. As a result, this may provide applications with unwanted leakage.
Originally designed for the pulp and paper industry, knife gate valves perform exactly at their name implies. Stringy pulp would impinge between the wedge and sand seat of a normal gate valve to prevent flow shut-off. Knife gate valves feature a sharp edge to cut through the pulp and seal, a useful attribute which has seen this shut-off valve type become invaluable to applications which either deal with viscous fluids or ones which have a similar risk of impingement.
Sludge & Slurry
Paper & = Pulp
Wastewater
Heavy Oils
Knife gates are cheap, easy to actuate and light. They are advantageous in applications involving viscous fluids such as sludge and slurry because their blades can cut through thick liquids with ease. As such, they are generally specified in larger sizes for the handling of thicker flows in demanding applications. Despite this, knife gates are known for their low-pressure limitations. As such, this makes them a less desirable shut-off valve for applications which require cleanliness or optimal hygienic conditions.
As an established and trusted valve distributor to the Scottish process industry, BM Engineering Supplies stocks a comprehensive range of both gate valves and knife gate valves. For gate valves, BM Engineering is the Scottish partner for one of the UK’s market-leading valve wholesaler, Leengate Valves. Their range of gate valves is approved to ISO standards and other relevant industry governing bodies. While for knife gate valves, BM Engineering partners with Orbinox UK, one of the world’s leading manufacturers of knife gate valves.
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