Knife Valve

Knife Gate Valves (KGVs) were introduced to the world to be used in the pulp and paper industry in 1927. When the market demanded a low-cost valve for pulp stock operations, the specialized knife gate valve design was born. Knife gate valves have been used all over the world since then, with several variations; the Push-Through Knife Gate was introduced in the United States, whereas the Guided Shear Gate was designed in Switzerland. While we’ve adopted the word “knife gate” as a general term for each version of this valve, you might be surprised to learn that only the Guided-Shear Gate and its higher-caliber variant, can really “cut” through media like a knife.

In this article, we will describe, understand how the Knife Gate Valve works, distinguish with gate valve, and explain its strengths and limitations.

Knife gate valve

Table of Content

1. What is a knife gate valve
2. Knife gate valve classification
3. How does a knife valve work
4. Features and benefits of knife gate valve
5. Knife gate valve maintenance guide
6. What is the difference between gate valve and knife gate valve
7. Summary

What is a knife Gate Valve?

Knife gate valves are engineered specifically for on-off and isolation operations in systems with a high dissolved solid particle.  Knife gate valves are extremely useful when dealing with slurry, abrasive, corrosive, and viscous media. The valves have a low-pressure drop in the fully open position, are simple to operate, light in weight, and cost-effective.

Knife gate valves are intended to work in the harshest conditions, with a sharpened blade to cut through heavy liquids.

They are extremely helpful in wastewater applications where corrosion is a major concern. In addition to the valve design that is optimized for slurry media, having a knife gate valve composed of acid-proof stainless steel is advantageous because it is less vulnerable to corrosion damage and therefore requires less frequent maintenance or even replacements.

Knife gate valves should always be used for applications that require a fully open or completely closed position and should not be used to regulate flow unless specifically designed for it. When fluid is pushed against a partly closed gate, a vibration occurs, gradually eroding the disc and seat. Furthermore, the knife gate valves are intended to open and close slowly to protect against the effects of the water hammer.

Knife Gate Valve classification.

1. By body Material.

Knife gate valves are constructed from a variety of materials. Cast carbon steel, cast iron, ductile iron, gunmetal, nickel, alloy steels, stainless steel, and forged steels are examples of common materials used. The material used in gate valves is primarily determined by the fluid service and the design temperature.

• Carbon Steel knife gate valve is a strong material of high hardness, tensile strength, and impact value. It is frequently used in industrial plants for high temperature/pressure operations.
• Despite its high cost, stainless steel knife gate valve has higher corrosion resistance, heat resistance, low-temperature resistance, and mechanical strength.
• Lined valves are inherently thermally stable, chemically resistant, and have excellent mechanical properties. Because of its superior workability, it is possible to provide valve wet parts with fine lining without fear of pinholes. It is also recommended for food processing because it is colourless and does not require any additives or pigments.

2. By Connection Type.

• The end connection of the valve is primarily determined by the requirement and suitability of the valve, such as wafer type knife gate valve and lug type knife gate valve are used with less costly valves, flanged type end is the most common of all kinds, and a gasket is used for sealing. 

Lug type knife gate valve

3. By Sealing method. Metal seat/ Resilient seat

• Prior to the introduction of the resilient seated knife gate valve, metal seated knife gate valves were commonly used. To ensure tight closure, the conical wedge construction and angular sealing devices of a metal seated wedge necessitate a dip in the valve bottom. Sand and pebbles are still trapped in the bore. Regardless of how well the pipe is flushed during installation or repair, the pipe system will never be totally free of impurities. As a result, any metal wedge would lose its ability to be drop-tight over time.
• A resilient seated knife gate valve has a simple valve bottom that allows sand and pebbles to pass freely through the valve. If impurities pass through the valve as it closes, the rubber surface may close around the impurities. As the valve shuts, a high-quality rubber compound contains impurities, which are flushed away when the valve is opened again. The rubber surface would return to its original shape, ensuring a watertight seal.

4. By stem Structure. Rising stem/Non-rising stem

• Knife gate valves are controlled by a threaded valve stem that connects the actuator (hand or motor) to the gate. They are categorized as rising or non-rising stems based on which end of the stem is threaded.
• Rising stems are attached to the gate and rise and lower together as the valve is operated, providing a visual indicator of the valve’s location. The nut-shaped actuator rotates around the threaded stem to lift or lower it.
• The non-rising stem valves are threaded at the gate end and are attached to and rotate with the actuator. Because the motion of the gate is concealed within the valve, they often come with a pointer threaded to the upper end of the stem to demonstrate the location of the valve. Non-rising stems are used underground or in places where vertical space is limited.

How does a knife valve work?

A knife gate valve is a component that uses a blade to cut through heavy liquid clogging. These valves were created to function in some of the world’s most corrosive and abrasive conditions.

Knife gate valves have become essential in applications involving viscous fluids, slurry, and other systems where impingement is an issue due to their extremely effective design characteristics.

Knife gate valves are used in many processing plants today and available in large sizes to accommodate high-density flows of light grease, heavy oils, varnish, slurry, wastewater, and paper pulp. It should be noted that these valves have low-pressure constraints and are intended to seat the blade into an elastomer seal after the blade has cut through the substances being handled. When a solid mass or powder passes through the knife gate, the thick, dry material ends up packing into the soft seals situated at the gate end. When this happens, the seals will eventually fail to close tight enough. If this situation occurs, the seals must be replaced.

Features and benefits of knife Gate valve.

Knife gate valves are full bore valves that allow liquids of any viscosity to pass through easily, and there are nobody cavities below the gate where the medium can accumulate. The valves are self-cleaning since particles are forced off the gate when the valve is opened, and gate scrapers and deflector cones for impure or abrasive media can be supplied for additional packing gland protection. Furthermore, the top packing gland is normally replaceable, allowing the sealing to be replaced without disassembling the valve.

Knife gate valves have a basic design that makes for easy maintenance and low-cost installation. These valves can be bi-directional, allowing for installation with no limitations on flow direction. A protected seal, high-quality materials, and a complete, plain bore result in excellent performance and long service life.

Knife gate valve maintenance guide.

Knife gate valves are being used for providing isolation in a variety of applications, as explained earlier. It is one of the simplest valve designs, and with only one moving part, the preventative maintenance schedule is simple & quick.

1. Make the packing adjustments once a month.
2. Lubricate the stem and stem nut every 3 months. This can be done with a greasing machine at the grease fitting at the top of the yolk.
3. Also, lubricate the gear controller once a year.

What is the difference between Gate Valve and Knife gate valve?

The key difference is in the types of applications or media that these two Valves can handle. Knife Edge Valves can handle slurries, powders, or granules much more efficiently than Gate Valves, which require clear media to function properly. Secondly, Gate Valves are not designed to handle highly viscous media, whereas Knife Edge Gate Valves are.

Another difference between knife gate valves and gate valves is that gate valves are designed to ANSI standards, while knife gate valves are manufactured to TAPPI standards. Gate valves are bi-directional, extensively used in fluid applications, and only have metal seating.

One other distinction between a knife gate valve and a gate valve is their packing gland. A v-ring packing set on a gate valve seals the shaft that is connected to the gate. A packing gland region seals around the gate in knife gate valves.

Knife Edge Valves are thus suitable for industries such as process plants, cement, paper, stringy food products, and so on.

Summary.

Knife gate valve self-cleans material from the seat on each stroke of the valve blade, which is built into the design and improves overall seat life. The design aids in the elimination of problems, allowing for higher production and lowering labor and equipment costs.

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