Summary
Technical discussions about
powder rotary valves can quickly become confusing if both parties are not speaking the same language. Is "tip clearance" the same as "radial clearance"? What exactly is a "fill factor"? This glossary defines 20 essential terms used in the specification, operation, and maintenance of rotary airlock feeders, helping engineers, purchasers, and maintenance teams communicate with precision.
Abrasiveness
The ability of a material to wear away the metal surfaces of the valve. Measured by the Mohs scale or K-factor. High abrasiveness (e.g., fly ash, silica sand) requires hard-faced rotor tips and wear liners.
Airlock
The primary function of a rotary valve in pneumatic conveying. It refers to the valve's ability to maintain a pressure differential between the inlet (hopper) and outlet (conveying line) by minimizing air leakage through the rotor clearances.
ATEX
A European Union directive (2014/34/EU) covering equipment intended for use in potentially explosive atmospheres. An ATEX certified rotary valve is designed to prevent ignition of combustible dust clouds.
Blow-Through Valve
A configuration where the pneumatic conveying line passes through the lower housing of the valve. Air sweeps material directly from the rotor pockets. Ideal for low headroom and sticky powders.
Closed-End Rotor
A rotor design where the ends of the pockets are closed off by discs. This provides the best pressure sealing (airlock) but is harder to clean than an open-end rotor.
The most common configuration. Material enters the top and exits the bottom by gravity. Used for feeding into dilute or dense phase conveying lines via a transition shoe.
Explosion Pressure Shock (Pstat)
The maximum internal pressure a valve can withstand during a dust explosion without rupturing. Heavy-duty valves are designed with thicker castings to handle specific Pstat ratings.
Fill Factor (FF)
The percentage of the rotor pocket volume actually filled with material. Rarely 100%. Typical values: Pellets (0.8-0.95), Free-flowing powder (0.6-0.75), Cohesive powder (0.4-0.6).
Flame Quenching
The phenomenon where a narrow gap (annular clearance) between the rotor tip and housing absorbs enough heat to extinguish a flame, preventing an explosion from propagating upstream.
Hard Facing
The application of a wear-resistant alloy (e.g., Stellite, Tungsten Carbide) to the rotor tips using welding or thermal spraying. Increases service life in abrasive applications by 5-10x.
Kst Value
A measure of dust explosion severity (explosion violence). A higher Kst value indicates a more explosive dust. Used to size explosion vents and specify pressure shock ratings for valves.
Open-End Rotor
A rotor design where the pocket ends are open. Easier to clean and inspect, but provides less effective pressure sealing than a closed-end rotor.
Purge Air
Clean, dry compressed air or nitrogen injected into the seal housing (lantern ring) to create positive pressure, preventing abrasive or moist product from entering the bearing cavity.
Radial Clearance
The gap between the rotor tip and the housing bore. Critical for airlock performance. Standard: 0.15-0.25 mm. Abrasive service: 0.25-0.40 mm.
Rotor
The rotating element of the valve, consisting of a shaft and vanes (pockets) that scoop and carry material. Also called the impeller.
Service Factor (SF)
A multiplier applied to the calculated horsepower/torque to account for operating conditions. A SF of 1.5 is recommended for 24/7 dusty environments.
Stall Torque
The torque produced by a motor when its shaft is prevented from rotating. Must be higher than the starting torque required to break a jammed rotor free.
TIR (Total Indicator Reading)
A measurement of how much a rotor "wobbles" when rotated. Low TIR (e.g., 0.05 mm) indicates a precisely machined and balanced rotor, which reduces vibration and wear.
Tip Clearance
Synonymous with Radial Clearance. The distance between the outermost edge of the rotor vane and the inner surface of the housing.
Volumetric Efficiency
The actual discharge rate divided by the theoretical discharge rate. Affected by fill factor, rotor speed, and air leakage. High efficiency means accurate metering.
FAQ
Why is there no "perfect seal" in a rotary valve?
Because the rotor must spin, there must be a physical gap (clearance) between the rotor and housing. The goal is to minimize this gap to reduce air leakage while preventing mechanical contact.
Is a higher fill factor always better?
No. While it increases capacity, forcing a higher fill factor by speeding up the rotor can cause product degradation, increased wear, and centrifugal ejection of material.
What is the difference between a "Rotary Valve" and a "Rotary Feeder"?
A Rotary Valve is the physical device. A Rotary Feeder emphasizes its function as a metering device. In practice, they are the same machine, but "Feeder" implies a focus on controlled dosing.
Conclusion
Understanding these 20 terms empowers you to read datasheets accurately, challenge supplier claims, and specify the correct powder rotary valve for your application. Precision in language leads to precision in engineering.
Need help interpreting your valve specs?
Contact Doebritz Shanghai Co., Ltd. Our engineers speak fluent "Rotary Valve" and are ready to assist with your technical queries or RFQ requirements.
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