A knife gate valve is a special type of gate valve, featuring a very sharp sized and machined disc to open and close the flow of very viscous fluids, slurries, thick grease, heavy oils, varnish, wastewater, and paper pulp. The sharp steel blade of such valves cuts clogged heavy liquids or solid/liquid media and seals the passageway tightly. Knife gate valves are used, mainly, for mining operations or heavy oil processing.
Knife gate valves were originally introduced within the pulp and paper industry, where standard gate valves could not properly operate due to the nature of the fluids conveyed during the paper production process.
Knife gate valves should never be used as modulating valves (to regulate the flow) as the fluid flowing through a partly closed valve would generate a vibration and erode both the disk and the seat. Therefore, knife gate valves should be used completely closed or opened like any other type of gate valve (globe valves are recommended to regulate the flow). Lastly, to protect the valve against the impact of the water hammer effect, knife gate valves feature a very slow opening and closing speed.
(Source: AVK Youtube Channel)
TYPES OF KNIFE GATE VALVES
A Knife gate valve can be manufactured in materials from ductile iron to stainless steel and in sizes between 2 and 24 inches (generally with cast bodies) with low-pressure ratings (< 300 lbs). There are many different variants of knife gate valves, such as the soft-seated (resilient type, with elastomer seats) and the metal seated (the seat and the disc generate a metal-to-metal seal), the slide gate and bonneted types.
Soft seat knife gate valve.
Metal seated knife gate valve (left) and resilient, soft-seated knife gate valve (right side).
GATE VALVE VS KNIFE GATE VALVE
The main difference between a knife gate valve and a standard gate valve is that gate valves comply to API and ANSI/ASME standard, whereas knife gate valves are manufactured according to the TAPPI standard. Other differences are:
- A standard gate valve is available with flanged, butt weld and socket weld connections (knife have lugged or wafer connections mainly)
- A standard gate valve has a V-ring packing set that seals the shaft that is attached to the gate. Knife gate valves feature instead of a packing gland area that seals around the gate
- Gate valves are bidirectional, whereas knife gate valves are generally uni-directional
- A knife gate valve has a smaller profile than ANSI gate valve, which is more bulky and refined
- Knife gate valves are lighter and cheaper than API and ASME gate valves
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