Knife Edge Gate Valve 

What is a knife edge gate valve? 

A knife edge gate valve is a type of gate valve which is meant for closing and opening fluid flow as well as isolation services. These valves have the word “knife” in their name because they can cut through slurries. Knife edge gate valve manufacturers design these valves for use in different types of media such as heavy slurries, abrasive, corrosive, and viscous media. These valves have a low-pressure drop when operating in the fully open position. Knife edge gate valves are of low weight and they are cheaper relative to other gate valves. Knife edge gate valve manufacturers design these valves with sharp edges which makes them suitable for use in slurries and to enhance tight shut-off. With the sharp edges, these valves can cut through slurries and thus avoid valve blockage. The knife gate valve employs a knife gate to close and or open fluid flow by moving the gate downwards or upwards respectively. 

Figure: knife edge gate valve.

How does a knife edge gate valve work? 

Knife edge gate valves work by raising or lowering the gate to open or close the fluid flow. The lowering and or raising of the gate is enhanced by the use of a handwheel or actuator. The handwheel is connected to the gate via a component known as a stem. To open a knife edge gate valve, the handwheel is rotated in the counterclockwise direction. This makes the stem which has threads to rotate in the same direction. As the steam rotates it moves upwards linearly which then moves the gate. To close a knife edge gate valve, the handwheel is rotated in the clockwise direction to rotate the stem in the same direction. This makes the stem move linearly downwards together with the gate. As such, the gate moves down to block fluid flow and also cut any solid material available. Knife edge gate valve manufacturers design the knife gate with strong materials to help in cutting the slurries as well as highly viscous fluids. This helps to prevent valve from clogging. 

Types of knife edge gate valves 

Through conduit knife edge gate valve 

This is a type of knife edge gate valve with two body seats and a bottom and top packing gland. Seats are responsible for sealing the valve when it is fully closed. Through conduit knife edge gate valve is known for high fugitive emission and through leakage. As such, knife gate valve manufacturers recommend these valves for use in fluids with high fiber contents. These valves are also used in mineral processing industries where scales are expected since these scales help to prevent fluid leakage. 

Figure: Through conduit knife edge gate valve.

Hopper Shape Knife edge gate valve

This is a type of knife edge gate valve which is unidirectional. Hopper shape knife edge gate valve manufacturers design these valves to handle bulk media. These valves have special bodies suitable for easy passing of fluid and ideal for use as outlet valves. Hopper shape knife edge gate valves are used in applications such as mining, food and beverages, chemical industries, and power plants. Hopper shape knife edge gate valve manufacturer design these valves with the seat in the reverse position that is the seat in the upstream direction to prevent seat wear, blockage, and difficulty when closing. 

Figure: Hopper shape knife edge gate valve.

Wafer knife edge gate valve 

This is a knife edge gate valve that has the knife gate placed between flanged end connections. These valves are bi-directional. Wafer knife edge gate valve manufacturers use this design to make the valve non-clogging when cutting media flow. Features of wafer knife edge gate valves are wafer design, a low-pressure drop with a high flow rate, and a two-piece body connected using screws. Wafer knife edge gate valves are used in applications such as chemical plants, mining, power plants, viscous fluids, and wastewater transportation. 

Figure: Wafer knife edge gate valve.

Rising stem knife edge gate valve 

This is the type of knife edge gate valve in which the stem rises upwards when the valve is opening. When a rising stem knife edge valve is closing, the stem moves downwards. When the valve is opened the stem rises above the handwheel which helps the valve operator to know that the valve is open. When the valve is closed the stem moves down thus it is easy to know when a rising stem knife edge gate valve is open or closed. The stem has threads that enhance upwards or downwards linear movement. One of the main challenges of a rising stem knife edge gate valve is that it occupies a lot of space so it cannot be used where space is limited. 

Figure: Rising stem knife edge gate valve.

Non-rising stem knife edge gate valve 

This is a knife edge gate valve that does not have upward stem movement. Unlike the rising stem type, the non-rising stem type can be used in applications where space is limited. Non-rising stem knife edge gate valve manufacturers design the valve with an indicator that shows when the valve is opened or closed. 

Figure: Non-rising stem knife edge gate valve.

Metal seated knife edge gate valve 

A metal seated knife edge gate valve is a type of valve which has seats made of metallic materials. Such metallic materials are bronze, stainless steel, carbon steel, and brass among others. Metal seated knife edge gate valve is suitable for use in high-pressure and high-temperature applications. This is because the metallic materials have high mechanical and thermal strength thus they cannot be deformed by either pressure or high temperature. Also, such valves are prone to wear especially when used to work on hard slurries. 

Resilient seated knife edge gate valve 

A resilient seated knife edge gate valve is a type of valve with soft seats. Soft seats are made of soft materials such as plastics and rubber. Resilient seated knife edge gate valve manufacturers recommend these valves for use in applications medium pressure and the fluid temperature not exceeding more than 60 ^oC. If these valves are used in high temperatures the seats deform due to thermal heat. These valves find use in applications such as wastewater, drinking water, and chemicals plants, foods and beverages among others. 

Factors to consider when selecting knife edge gate valve 

Media type 

The type of fluid affects the service life and working of the knife edge gate valve. Valves made of plastic seats such as PTFE and PFA will perform well when used in corrosive media compared to metal seated knife edge gate valves. When considering high-pressure fluids, metal seated valves will perform better than soft seated valves. 

Application condition 

Application conditions include flow parameters such as fluid flow rate, pressure, and operating temperature. An improperly sized knife edge gate valve is likely to produce vibrations and noise as well as breakdown soon after installation. Metallic seated knife edge gate valves will tolerate high pressure and high-temperature applications, unlike soft seated valves. Determine the amount of fluid flow rate, operating pressure, and temperature before purchasing the knife edge gate valve to help in selecting the best valve for that application. 

Use of the knife edge gate valve 

Knife edge gate valve manufacturers design the valves with different considerations for use in specific applications. A knife edge gate valve for use in mining applications can be more expensive relative to the one meant for use in water supply applications since the mining valve will need a stronger gate that can crush solids. So knowing the specific use of that valve will help to select the best valve which will be worth the money and will have a longer service life. 

Method of actuation 

This is the case where the customer has to know how he/she intends to operate the knife edge gate valve that is will the valve be manual or automated? If the knife edge gate valve needs to be automated, then specification of the actuator type is important because different actuators have different working principles and they have their advantages and disadvantages. Such actuators can be pneumatic, hydraulic, or electric actuators. 

Applications of knife edge gate valves 

• Knife edge gate valves are used in mining industries because of their ability to cut through solid particles. 
• They are used in chemical industries. 
• These valves are used in water supply.
• They are used in oil and gas industries. 
• Knife edge gate valves are used in pharmaceutical industries. 
• They are used in the cement industries. 
• Knife edge gate valves are used in the transportation of wastewater. 
• They are used in the paper and pulp industries. 
• They are used in the foods and beverage industries. 

Advantages of knife edge gate valves

• Knife edge gate valves can be used in slurries. 
• They work well in heavy viscous fluids. 
• They are versatile as they can be used in different industrial applications. 
• Knife edge gate valves are durable as they can be repaired when they break down.
• Knife edge gate valves can be uni-directional or bi-directional. 
• These valves have low-pressure drop which enhances pumping efficiency. 
• They are free from fluid leakage. 
• They are easy to do repair and maintenance. 
• They are light in weight.

Disadvantages of knife edge gate valve

• Knife edge gate valves are not suitable for throttling applications. 
• They are open or close slowly unlike ball valve and plug valve. 
• They need high torque to close for cutting through media or zero fluid leakage. 

Troubleshooting knife edge gate valve 

Leakage through the seats

• Worn seats. Replace the seats. 
• Blockage in the valve cavity. Clean materials causing a blockage. 
• Damaged knife gate. Replace the gate. 

Leakage through stem

• Worn out stem packing. Replace the stem packing. 
• Loose stem packing. Tighten the stem packing according to the torque range given by the knife edge gate valve manufacturer. 

Leakage through the valve bonnet 

• Loose connections. Tighten the bolts or screws as recommended by the knife-edge gate valve manufacturer.
• Worn out or damaged gasket. Replace the gasket. 

Leakage through the valve body 

• Impact damage on the body. Contact knife edge gate valve manufacturer for further details. 
• Loose connection between the valve body and the pipeline. Tighten bolts or screws to the torque level recommended by the knife edge gate valve manufacturer. 

Knife edge gate valve is hard to open/close 

• Stem packing has been over tightened. Loosen the packing torque to the level recommended by the knife edge gate valve manufacturer. 
• Solid materials in the valve. Remove any foreign materials clogging the valve. 

Summary 

A knife edge gate valve is a type of gate valve which uses a knife gate. Knife edge gate valves use a knife gate to help cut through slurries and highly viscous fluids. These valves are meant for on/off applications but not throttling. Knife edge gate valves use the knife gate to open and close the fluid flow. The gate works by moving linearly upwards or downwards to allow or block fluid flow respectively. These valves can be uni-directional or bi-directional. Knife edge gate valves are of various types which include through conduit knife edge gate valve, wafer knife edge gate valve, hopper shape knife edge gate valve, rising stem knife edge gate valve, non-rising stem knife edge gate valve, metal seated knife edge gate valve, resilient seated knife edge gate valve among others. Some of the factors to consider when purchasing/selecting a knife edge gate valve include media type, application condition, method of actuation, and use of the valve. Knife edge gate valves are used in various applications such as cement, chemical industries, pharmaceuticals, foods and beverages, mining, oil and gas, and wastewater among others. Advantages of using knife edge gate valves are durability, versatility, zero leakage, bi-directional fluid flow, low-pressure drop, ability to work in viscous fluids, and high strength.