Summary
Reading a
rotary valve datasheet can feel like deciphering a foreign language. Terms like "Fill Factor," "Bulk Density," and "Airlock Ratio" are tossed around, but do you truly understand their impact on valve performance? Misinterpreting these terms leads to undersized valves, premature wear, and system inefficiencies. This glossary defines the 20 most critical terms every engineer, maintenance manager, and procurement specialist must know. We go beyond dictionary definitions to explain
whyeach term matters in the real world, helping you specify, operate, and maintain your rotary airlock feeder with confidence.
1. Bulk Density (ρ)
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Definition: The mass of a powder per unit volume, including the void spaces between particles. Units: kg/m³ or lb/ft³.
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Why It Matters: This is the single most important variable for sizing. A valve sized for "Sugar" (700 kg/m³) will starve if used for "Titanium Dioxide" (1100 kg/m³). Always use the compacted bulk density from your process, not the "poured" density from an MSDS.
2. Fill Factor (FF)
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Definition: The percentage of a rotor pocket's geometric volume that is actually filled with powder during operation. Expressed as a decimal (e.g., 0.75) or percentage (75%).
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Why It Matters: It bridges theory and reality. A valve with 100% fill factor doesn't exist. Free-flowing granules might achieve 0.75, while cohesive powders might only reach 0.50. Overestimating the fill factor is the #1 cause of undersized valves.
3. Rotor Speed (N)
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Definition: The rotational speed of the rotor, measured in Revolutions Per Minute (RPM).
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Why It Matters: Speed directly controls feed rate (TPH = Volume x FF x ρ x N). Too fast, and centrifugal force throws powder out of pockets (reducing FF). Too slow, and the valve cannot meet capacity. Optimal range for powders: 10–40 RPM.
4. Rotor Tip Clearance
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Definition: The radial gap between the rotor tip and the housing bore. Measured in millimeters (mm) or mils (thousandths of an inch).
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Why It Matters: This is the heart of the airlock. Tighter clearance (0.08–0.15mm) minimizes air leakage but increases wear risk. Wider clearance (0.20–0.40mm) suits abrasive service but allows more air bypass. It must be checked and adjusted periodically.
5. Airlock Ratio (Seal Ratio)
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Definition: The ratio of the pressure drop across the valve to the pressure drop across the rotor tip clearance. A higher ratio indicates better sealing capability.
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Why It Matters: In pneumatic conveying, if the system pressure drop exceeds the valve's airlock ratio, air will blow back through the inlet. A well-designed valve aims for an airlock ratio > 10:1.
6. Volumetric Capacity (Q)
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Definition: The theoretical volume of material a valve can discharge per unit time, assuming 100% fill factor. Units: m³/h or ft³/h.
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Why It Matters: This is the "ideal" number manufacturers quote. Your actual capacity is always Q x FF. Never specify a valve based solely on volumetric capacity without discussing fill factor.
7. Gravimetric Capacity (Mass Flow)
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Definition: The actual mass of material discharged per unit time. Units: kg/h or TPH (Tons Per Hour).
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Why It Matters: This is what your process cares about. It accounts for bulk density and fill factor (Mass Flow = Q x FF x ρ). Always specify your requirement in TPH, not just "valve size."
8. Differential Pressure (ΔP)
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Definition: The difference in pressure between the inlet and outlet of the valve. Units: bar(g) or psi.
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Why It Matters: A rotary valve is not just a feeder; it's a pressure barrier. The valve housing, shaft seals, and drive must be rated for the ΔP. Ignoring ΔP leads to air leaks, rotor seizure, or housing rupture.
9. Pocket Volume (Vp)
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Definition: The internal volume of a single rotor pocket. Units: cm³ or in³.
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Why It Matters: This is the building block of capacity. Total displacement per revolution is Number of Pockets x Vp. Engineers use this to calculate required RPM for a given feed rate.
10. Shaft Seal
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Definition: The component that prevents powder from escaping along the rotor shaft. Common types: Lip Seals, Packing Glands, Mechanical Seals.
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Why It Matters: This is the #1 maintenance item. Lip seals are cheap but wear fast. Packing requires adjustment. Mechanical seals are expensive but offer superior containment for toxic or abrasive powders. The wrong seal choice guarantees frequent leaks.
11. Housing Bore
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Definition: The precision-machined inner cylinder of the valve body where the rotor spins.
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Why It Matters: This surface must remain round and smooth. Wear here (from abrasion or corrosion) increases tip clearance, destroying the airlock. High-end valves use replaceable wear sleeves to protect the bore.
12. Rotor Types
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Closed-End Rotor: Rotor vanes are welded to solid end discs. Provides maximum pressure containment. Used for pneumatic conveying.
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Open-End Rotor: Rotor vanes are cantilevered or attached to a central hub, allowing powder to pass through the ends. Used for gravity discharge or non-pressurized applications.
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Why It Matters: Using an open-end rotor in a pressure system is a critical error. It allows air to bypass the rotor entirely.
13. Adjustable Tips
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Definition: Rotor tips that can be moved radially outward via set screws or wedges to compensate for wear.
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Why It Matters: This feature turns a consumable item into a serviceable one. Instead of replacing the entire rotor, you spend 15 minutes adjusting tips, restoring clearance and extending service life by years.
14. Shear Plane
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Definition: The theoretical plane at the inlet where powder is sheared off as the rotor pockets move past.
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Why It Matters: If the powder is sticky or the inlet design is poor, powder can build up above this plane, causing bridging. Understanding the shear plane helps in troubleshooting feed inconsistencies.
15. Starting Torque
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Definition: The torque required to start the rotor from a standstill, especially when the pockets are full of material.
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Why It Matters: Starting torque can be 2–3 times higher than running torque. If the drive motor isn't sized for this, it will trip on overload every time you restart under load (e.g., after a weekend shutdown).
16. Runout (TIR - Total Indicator Reading)
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Definition: The amount of wobble or eccentricity in the rotor as it spins, measured with a dial indicator.
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Why It Matters: Excessive runout indicates a bent shaft or poor machining. It causes cyclical rubbing against the housing bore, leading to rapid wear and vibration. Acceptable runout is typically < 0.05mm.
17. Hard-Facing
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Definition: Applying a wear-resistant alloy (e.g., tungsten carbide, stellite) to the rotor tips via welding or brazing.
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Why It Matters: This is the primary defense against abrasive powders. Hard-faced tips can last 5–10 times longer than uncoated steel tips in services like fly ash or cement. It's a must-have for abrasive applications.
18. Purge Air / Nitrogen
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Definition: Clean, dry air or inert gas injected into the seal chamber or bearing housing to prevent powder ingress.
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Why It Matters: This is your first line of defense for bearing protection. A continuous purge creates positive pressure that keeps abrasive powder out of the bearings, dramatically extending their life. Essential for toxic or explosive dusts.
19. ATEX / IECEx Rating
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Definition: Certification indicating the valve is safe for use in explosive atmospheres (e.g., Zone 21/22 for dust).
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Why It Matters: Using a non-certified valve in a combustible dust environment is illegal and dangerous. Certified valves have flame-quenching clearances, grounded components, and temperature-limited surfaces to prevent ignition.
20. L10 Bearing Life
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Definition: The life expectancy of a bearing, defined as the number of operating hours at a given speed that 90% of a group of identical bearings will endure before the first signs of fatigue.
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Why It Matters: This helps you predict maintenance. A bearing with an L10 life of 20,000 hours should last about 2.3 years in continuous service. It guides your spare parts inventory and preventive maintenance scheduling.
Putting It All Together: A Practical Example
Let’s say you need a valve for 5 TPH of Calcium Carbonate (ρ = 900 kg/m³) in a +0.5 bar pneumatic line.
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Calculate Required Volumetric Flow: Q = Mass Flow / (ρ x FF). Assuming a conservative FF of 0.65, Q = 5000 / (900 x 0.65) = 8.55 m³/h.
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Select Valve Size: A DN200 valve might have a pocket volume of 1.2 liters/pocket and 8 pockets. At 20 RPM, its volumetric capacity is 1.2 x 8 x 20 x 60 = 11.52 m³/h.
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Check Pressure: Ensure the valve housing and shaft seals are rated for +0.5 bar.
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Specify Features: Request Adjustable Tips (for maintenance), Hard-Facing (for abrasion), and a Purge Air Connection (for bearing protection).
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Confirm Safety: Ensure the valve has the correct ATEX Rating for your dust hazard.
FAQ
Q: What's the difference between "Poured" and "Compacted" Bulk Density?
A: Poured density is measured by gently filling a container. Compacted density is measured after vibration or tapping. Powders in a hopper compact under their own weight, so always use compacted density for sizing.
Q: How often should I check Rotor Tip Clearance?
A: For abrasive service, check monthly. For non-abrasive service, check quarterly. Always check after any unusual noise or vibration.
Q: Can I use a standard lip seal on a high-pressure valve?
A: No. Lip seals are designed for low-pressure (near atmospheric) service. For pressures above 0.5 bar(g), use Packing Glands or Mechanical Seals.
Q: Where can I find these terms on a Doebritz datasheet?
A: Our technical datasheets list these parameters under "Performance Data," "Materials of Construction," and "Seal Arrangement." If you need clarification, our engineers are always available to walk you through the specs.
Conclusion
Mastering this glossary transforms you from a passive specifier into an active expert. You now understand that capacity isn't just about size—it's about the interplay of Bulk Density, Fill Factor, and Rotor Speed. You know that sealing isn't just about the housing—it's about Tip Clearance, Shaft Seals, and Purge Air. By internalizing these 20 terms, you can ask sharper questions, avoid costly specification errors, and ensure your rotary airlock feeder performs exactly as needed. Knowledge is the best maintenance tool in your arsenal.
Ready to put this knowledge into practice? Contact Doebritz Shanghai Co., Ltd. today. Discuss your application with our engineers using the precise terminology you've learned. We'll ensure your next rotary valve is specified with scientific accuracy and engineered for long-term success.
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