Every conveying line is different, which is why a customized rotary valve goes a long way toward boosting efficiency in your process. The features and accessories you choose will impact a range of factors including speed, product capacity, reduced product degradation, uptime and the longevity of your equipment.
The same goes for rotors. Valve rotors come in various shapes and materials of construction, all of which have a different effect based on the size, density and consistency of the materials being conveyed.
Once you’ve chosen a rotor, consider different materials to match your application — for example, stainless steel for food processing or Teflon for sticky or abrasive construction products. You may also have a choice of rotor tip type for your job, whether it’s slicing through wood chips or scraping sticky sugar off the valve interiors.
When it comes to choosing the right rotor for your process, there are many different factors at play. It’s important to understand the nuances before you decide.
Choose your rotor
For all-around reliability: Open-end rotor
Specialty: Dust collection and pneumatic conveying
As the most popular rotor type available, the open-end rotor handles most powdered materials with ease. It’s considered an industry standard because of its open, accessible pocket shape, which makes it faster to clean and maintain. Beveled rotor tips and sides also aid in material flow. That, in turn, prevents material build-up and extends the longevity of the valve.
The open-end rotor minimizes leakage with lighter material and has plenty of longevity with flour, limestone and other dry bulk material. It’s best suited for most powder and granular product applications. The most common is an 8-vane rotor, which ensures at least two rotor blades are always in contact with each side of the valve housing — a requirement for National Fire Protection Association (NFPA) compliance. An 8-vane rotor is the most efficient rotor style for reducing air leakage.
Best suited for dust collection, gravity-fed or light pneumatic conveying applications, an open-end rotor with 10 or 12 vanes reduces air leakage and provides a more uniform feed in higher-pressure pneumatic conveying applications.
Rotors that tackle stubborn materials
For heavy-duty jobs: Closed-end rotor
Specialty: Gravity-fed applications
This tough rotor type excels at handling abrasive materials, such as sand and cement. The closed-end rotor features a disc on either end to keep material away from the valve’s endplates and shaft seals, where the most leakage and wear tends to happen.
Note that closed-end rotors require more frequent cleaning and preventative maintenance to ensure they stay functional and NFPA-compliant. The rotor shroud clearances need to be checked regularly to make sure they stay under 0.0079 inches.
To avoid having materials slip into the space between the endplates and rotor discs, it’s best to steer clear of light materials with this rotor. An additional feature, called a rotor pocket air purge assembly, applies clean dry air to help keep the area between the endplate shroud and rotor clean.
For longevity: Adjustable tip rotor
Specialty: Abrasive material
Harsh, abrasive materials wear down rotor tips more quickly. To avoid replacing the entire rotor on a more frequent basis, get an adjustable-tip rotor. Adjust the tips by sliding them up as they wear, keeping them compliant and close to their starting clearances.
Adjustable tips come in various materials, from hardened steel to EPDM to Teflon. Steel adjustable tips are the only NFPA-compliant option, but if the application requires a different kind of tip, there are other ways to achieve NFPA compliance. It’s best to ask a rotary valve specialist for guidance in cases like these.
That being said, sometimes this feature is not used to the fullest and the tips are not adjusted often enough. That creates uneven wearing on the rotor tips, increased valve clearances and increased air leakage, and the valve will not work as efficiently. An alternative to an adjustable rotor tip is a fixed-tip style rotor constructed from hardened steel, which is great for abrasive product applications without the need for rotor tip adjustment.
For flexibility: Flex-tip rotor
Specialty: Hard-to-move products and gravity-fed applications
Fibrous materials like sawdust, plastic pellets, plastic flake and wood pellets can be difficult to move down a conveying line, particularly when they cause jams and slowdowns. The flex-tip rotor’s tips bend out of the way as material passes through, ensuring a tight seal.
A word of warning, though: these rotors are not NFPA-compliant. With rubber or plastic tips, a flex-tip rotor won’t stop the passage of flames if the valve is used under a dust collector. Again, there are ways to cover your bases with fire prevention — with fire suppression and spark monitoring systems, for example — but ask a conveying expert for specific advice.
For jam prevention: Helical rotor
Specialty: Wood chips
The helical rotor is another option for hard-to-move materials, but unlike the bendable flex-tip, it slices up jammed material to keep things moving. The slanted edges act as a knife to take care of jams as they happen.
This particular rotor is generally used for wood chips and wood dust, but it may come in handy in similar applications.
Rotors that shine in sanitary applications
For easy cleanup: Radius pocket rotor
Specialty: Food-grade valves and sanitary applications
This is an open-end rotor with a sanitary twist. The radius pocket rotor features U-shaped pockets (as opposed to the classic V shape), which prevents sticky materials from compacting in the grooves and piling up. The shape is also quicker and easier to clean, particularly if the valve is built on a rail system for easy tool-less access.
Because of its fast-cleaning format, the radius pocket rotor is ideal in food-grade and sanitary valves.
For less sticking: Scraper rotor
Specialty: Chemical and food applications
Sticky granulated materials build up in the valve’s interior as they are processed, eventually coating it and compromising performance. To combat that issue, this rotor comes with two rotor tips 180 degrees apart. With closer tolerances than the other blades, the special tips act like scrapers and keep the interior housing and endplates clean as it rotates.
Aside from the two close-tolerance rotor blades — which are counter-aligned to provide full coverage — the other vanes are standard open-end. The rotor excels in sanitary applications where it reduces the required cleaning and maintenance time, especially with sugar and after-roaster applications.
Rotors for precise, calculated flow
For finesse: Reduced volume rotor
When working with certain materials and equipment, a faster rate of material flow can clog the conveying line beyond the valve. A reduced volume rotor lowers the cubic foot per revolution (CFR) inside the valve, making it easier to control the flow.
To get a close feed rate, don’t settle for using a smaller valve at reduced speeds; you’ll still get material compacting at a faster rate than you want. This rotor makes metering easy with a large flange opening and flat pockets that prevent sticking.
For uniform flow: Staggered pocket rotor
Specialty: Metering and gravity-fed applications
Sometimes a more uniform flow is needed, like when working with a mixer — or any application where a calculated flow is needed for optimal output and efficiency. Be sure not to overflow the conveying line or gravity-fed equipment beyond the valve. A staggered pocket rotor provides a consistent feed rate and a smooth flow of material into the system.
In the case of regular rotors, there are short breaks between the points where material flows out and the rotor moves on its axis. The 16 offset pockets on a staggered rotor allow for continuous movement.
For tight control: Metering rotor
Specialty: Metering powders
When conveying smaller amounts of material or making test batches, this rotor will meter them out with precision. Unlike a standard rotor with vanes, the metering style typically has about 30 smaller pockets, or radius grooves, machined into a solid steel rotor. The number and depth of the grooves may vary based on the exact material flow rate desired.
The metering rotor is designed mainly to convey free-flowing material, such as powders, at a highly controlled rate. It’s used in small valves measuring 8 inches or less, mainly in sampler and dosing sanitary applications like chemistry, food and pharmaceuticals.
Which rotor fits best?
Your rotor of choice will have an impact on factors like conveying speed, precision and even your cleaning schedule. The right rotor can save money and downtime. If you’re not sure which rotor best fits your process and application, get in touch with a rotary valve expert who understands pneumatic conveying from start to finish. They’ll help you make a decision that meets your needs.
Megan Thompson is president and chief operating officer of ACS Valves, which designs, engineers and manufactures rotary valves for metering, feeding and air in bulk material processing and pneumatic conveying applications. Thompson is an NFPA expert with 26 years of experience in the industry helping plants prevent explosions and keep workers safe.