A knife gate valve is a component that utilizes a blade to cut through clogging of heavy liquids. These valves were designed to work in some of the most corrosive, erosive and abrasive environments in the world.
Knife gate valves were originally designed for the pulp and paper industry. Stringy pulp would get stuck between the wedge and the seat of a normal gate valve and prevent flow shut-off. Knife gate valves were specially designed with a sharp edge to cut through the pulp and seal.
How a Knife Gate Valve Works
Because of this 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 is 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, waste water 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 its 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 replace.
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 hammer.
Knife Gate Value vs. a Gate Valve
The biggest difference between knife gate valves and gate values is that gate valves are manufactured to ANSI standards while knife gate valves comply with TAPPI standards. The gate valve is also flanged, wider dimensionally, ANSI pressure rated and its API leak tightness standards need to be met. Gate values are bi-directional, and widely used in fluid applications and they only come with metal seats. Another difference between a knife gate valve and an ANSI gate valve is within the packing gland area. A gate valve has a v-ring packing set that seals the shaft that is attached to the gate. Knife gate valves have a packing gland area that seals around the gate.
A knife gate valve has a thin profile in comparison to an ANSI gate valve. Knife gate valves are predominately uni-directional (some options are bi-directional) and feature a lugged or wafer body, without flanges. The knife gate valve seats are available in everything from resilient to the metal versions.
The most significant benefits of knife gate valves are the weight (16" usually less than 300#) and the cost. ANSI gate valves are usually over 1200# and they more expensive.