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Rotary airlock valves are critical components in many material handling systems.From dust collectors to pneumatic conveying systems, rotary airlocks not only control feed rates, but also minimize air leakage between equipment above and below the valve with varying pressure or vacuum levels.In dust extraction systems, rotary airlocks allow continuous discharge of product without the need to turn off the vacuum to dump accumulated material.Rotary airlocks are critical in pneumatic conveying systems, allowing material to be introduced into a pressure or vacuum flow while minimizing air loss.
Design and Construction The rotary airlock valve has a heavy duty cast housing consisting of a body and two end caps, an internal rotary rotor and a drive assembly.The valve body has an inlet and outlet to allow material to enter and exit the valve.The rotor has a shaft with a number of blades, the ends of which extend through the housing to external bearings supported by end caps.Inside the casing, the vanes radiate outward from the shaft to the casing.The space between the rotor blades forms a bag of material that picks up at the inlet and, as the rotor rotates, delivers the bag of material to the outlet.The empty pocket is then rotated back into the inlet to pick up more material.The drive assembly consists of a gearbox and an electric motor, connected to the rotor shaft by a chain drive, or directly.The rotor spins relatively slowly, typically at 22 rpm or less.
So how does a rotary valve provide an airlock?While the valve will not produce a 100% seal, if properly designed, the leak rate can be greatly reduced.The valve body and end caps must be heavy duty designed to withstand differential pressure and precision machined to very tight tolerances.Raised surfaces on the inside of the end cap fit snugly and accurately into the valve body bore to ensure a good seal and proper alignment between the rotor and housing.Rotors require a large shaft to prevent deflection and typically require eight or more blades.The rotor is also precision machined to fit snugly inside the housing.The typical gap between the rotor and housing is 0.004 to 0.006 inches, or about the thickness of a thick human hair.These tight tolerances minimize air leakage as they allow the rotor to rotate with minimal play.The tighter the gap, the less air will leak.The geometry of the body design is also important, as the more blades and pockets that are included in the shell and not exposed at the throat – the greater the seal.
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Rotor Types and Choices Rotary airlock manufacturers will have several different valve body styles, each best suited for certain applications.In this article, we will continue to discuss the different types of rotors and how to choose the best design for the application.Proper selection of rotors is a critical part of specifying the valve best suited for the intended service, which has a major impact on valve life and overall performance.
The first thing to consider is whether to use an open-end rotor or a closed-end (shrouded) rotor.See everyone’s photos.When looking at an open rotor from the shaft end, you will see multiple blades radiating outward to form a V- or U-shape, or grooves.The ends of the blades remain open.If you pour water into the top pocket, it will only flow out the sides.
Enclosed or hood rotors have circular end discs welded to the ends of the shaft and vanes.Looking at the rotor from the end of the shaft, you won’t see a “V” groove, but a disc.The ends of the leaves are closed.Unlike open rotors, the water poured into the top pocket is retained by the end disc.
Which is better, open or closed rotor?As mentioned above, the rotary airlock provides an airtight seal due to the tight tolerances between the rotor and housing.This “seal” is maximized by the surface area between the rotor blades and the casing.The larger the surface area, the better the seal.Since the open rotor blades extend all the way to the end caps, it has a larger surface area and provides a better air seal than closed rotors.
However, open rotors have their drawbacks because the material is in contact with the end caps.For abrasive products, this results in increased wear and possibly shortened valve life.In higher temperature applications, this increases tolerances that require careful machining.Delicate materials tend to leak through the shaft seal if not maintained properly.Other materials, such as flakes, can get caught between the sides of the rotor blades and the end cap faces, which can cause the valve to squeal or even stop.
Enclosed rotors are primarily used for gravity flow applications or low differential pressure systems, but can also be used for high differential pressure if the rotary airlock is properly designed.Closed-end rotors are ideal for grinding and high temperature applications where possible.In abrasive applications, the product is kept away from the end caps where most wear occurs.At high temperatures, the gap between the end cap and rotor eliminates any expansion issues, leaving only the diameter to be machined.The product also does not get wedged between the sides of the rotor blades and the end cap faces, as the grooves are closed by the end discs.
Rotor Blade Design Rotor blades can be solid fixed blades welded to the shaft or shorter blades with adjustable tip bolts.Fixed blades are generally thicker and stronger than adjustable blades.Fixed blades are preferred in hygienic applications such as food and pharmaceuticals, as product can get caught under adjustable blades.Some products are also sticky or sluggish, and the fixed blade rotor makes it easier for these products to exit the pocket.
Adjustable tips are used for applications that require special tips.Rotor tips can be rigid or flexible and are available for both open and closed end rotors.Rigid rotor tips should be used in all applications where differential pressure exceeds 2 psi.Rigid rotor tips include mild steel, stainless steel, bronze and wear resistant steel.Bronze tips are often required when dealing with explosive or ignitable materials.The wear-resistant tip is obviously used to grind the material to prolong the life of the valve.
Flexible tips are typically only used in gravity flow applications, but can be used in lower differential pressure situations, such as dust collection systems.At higher differential pressures, the flexible tip can pull away from the housing and cause excessive leakage.The flexible tip can be made from almost any rubber.They are typically used for larger grain or fibrous products that would normally clog solid blade rotors.If a particle gets trapped between the flexible tip and the shell, the tip either gives way to fall into the next pocket, or drags it to the exit to drop it.The rubber used for the flexible tip should be selected based on the maximum temperature and the type of material passing through the valve.
The most common blade treatment for the rotor blade option is the beveled edge.Some products tend to smear or build up on the inside of the case.This tailing can cause additional drag, squeal, and possibly even valve seizure.By chamfering (or lightening) the trailing edges of the blades, drag is reduced by reducing the surface area of the rotor.The beveled vanes also allow the product to escape more easily if wedged between the rotor and housing.For open rotors, the grooves should be on the radial edges and sides of the blades.In overspray applications, use a specially designed scraper or blade to remove product buildup.
Rotor Slot Options Standard rotor slots have a “V” shape.While this works for most applications, it can become cumbersome for sticky products.Sector pocket rotors are where the bottom of the rotor pocket is rounded into a “U” shape and ground or even polished.This is desirable in both hygienic and viscous applications, making it easier for the material to drain from the pocket.
The reduced volume serves to reduce the capacity of the rotor.Smaller rotating airlocks may cause material to bridge over the entrance.The reduced volume rotor allows the use of larger valves to eliminate bridging and reduce discharge rates when you do not want to overfeed downstream equipment.Reduced volumes can also be used when feeding accurately at a more consistent rate.Material tends to exit the rotary valve in clumps comparable to the number of pockets.The reduced volume rotor allows for a more even flow, consistent flow.
The staggered rotor grooves consist of a disk that passes through the center of the rotor, and the opposing grooves on each side of the disk are off-center.This actually takes an 8 hole rotor and turns it into a 16 hole rotor.This helps provide a more consistent flow for more accurate discharge.
Rotor coatings and finish coatings are used to help prevent wear and corrosion, or to achieve better product release.Coatings are usually only applied to fixed blade rotors.Nickel has some wear resistance but is mainly used as a less expensive alternative to stainless steel to help prevent corrosion.Teflon is used for sticky materials that tend to accumulate in pockets.Stellite is a highly wear resistant material that is welded to the outer edge of the rotor and then machined to the required tolerances.The rotors can also be chrome plated for improved wear resistance.There are also various plasma coatings offering abrasion resistance and a low coefficient of friction for better product release.Stainless steel rotors can be polished to almost any surface.
The valve manufacturer’s application experts will help you specify the correct rotary airlock for your system.Information critical to choosing the best valve includes: material being processed, bulk density, particle size, material properties, moisture content, temperature range, equipment above and below the valve, pressure or vacuum level, and desired discharge rate.They should have extensive experience handling all types of products in various types of systems.Selecting the correct rotary airlock valve, rotor and design features is critical to ensuring successful operation with minimal air leakage and downtime and maximum valve life.
Paul Golden is a Manager at Carolina Conveying, specializing in powder handling components and material handling systems.The company manufactures a wide range of products including rotary air gate valves, diverter valves, gate valves, silo dischargers, screw feeders, bulk bag handling systems and portable bulk containers.For more information, call 828-235-1005 or visit carolinaconveying.com.
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Post time: May-17-2022
