Pneumatic Rotary Valves
Rotary feeders pneumatically actuated, definition, design and capacity calculation
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| Section summary |
|---|
| 1. Introduction & Working Principle |
| 2. Design & Pocket Volume Data |
| 3. Performances / Capacity Calculation (Interactive Tool) |
| 4. Function in Process & Plant Integration Rules |
Disambiguation : this page is about rotary valves that are NOT designed to make an airlock. If you are looking for information on airlock rotary valves, or rotary feeders, we have page dedicated to this equipment : just click here Airlock Rotary Valves. The page below is dedicated to designs pneumatically actuated which are actually modified butterfly valves. Please scroll down for more details.
1. Introduction
Rotary valves should not be confused with Airlock Rotary valves. If the former are electrically driven, rotate 360 degrees, and have tight metal / metal clearances to ensure minimum gas is leaking through it, the rotary valves are actually pneumatically driven, rotate 180 degrees back and forth and are tight only in stand-by position.
Rotary valve allow an almost continuous flow and dosing for powders or granules. They can be used in a wide range of industry :
- Specialty Chemicals : feeding of powder
- Food processing : feeding of powder, for dosing equipment or to a filling machine
- Pharmaceuticals : feeding of granules, tablets
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Figure 1 : Dosing principle of a rotary valve
Rotary valves are used for dosing solids. This dosing can be rough or quite fine.
2. Design
Rotary valves are made of the following equipment :
- Pneumatic drive
- Rotor with alveoles
- Housing
- Gasket fit to the housing
Some specific design for fine dosing have actually a second rotor placed in the main one. When running a coarse dosing, the main rotor is activated, then when running fine dosing, the small one is activated. It is a volumetric device, the throughput that can be achieved depends on the size of the valves (pockets) and the frequency at which it is turned. The filling coefficient is usually assumed to be 100% since the system is rotating quite slowly compared to a star valve and because there is no pressure differences between the inlet and outlet of the valve.
The volume dosed for 1 cycle (180 degree in one direction + 180 degree on the other) is given below - this may change slightly from 1 supplier to another.
Table 1 : Dosing volume of rotary valves per complete cycle (order of magnitude)
| Valve diameter mm | Volume 1 cycle in liters |
|---|---|
| 80 | 0.09 |
| 100 | 0.26 |
| 150 | 0.88 |
| 200 | 2.09 |
| 250 | 4.09 |
The sizing can be controlled according to the number of cycles / min that are needed to achieve a given throughput. The calculation should not exceed 60 cycle / min.
Roots blowers are volumetric, they will always take a volume of air
The cycle frequency \(n\) (in complete cycles per minute) required to deliver a specified mass flowrate \(Q\) with bulk density \(\rho_{bulk}\) is calculated using the pocket displacement volume \(V\) per cycle:
Alternatively, in terms of volumetric throughput \(Q_v\) (in \(\text{L/h}\)):
Equation 1 : Estimation of rotary valve cycle frequency for a given throughput
With \(V\) the volume of 1 complete cycle of the valve in liters, \(Q\) the throughput in \(\text{kg/h}\), \(\rho_{bulk}\) in \(\text{kg/L}\), and \(\eta\) the volumetric filling efficiency factor (typically 0.85 to 1.0).
3. Performances / capacity calculation
The valve is controlled by playing on the time span between 2 rotation of the valves. It should however not be too close in order to avoid mechanical issues with the actuator of the gasket.
Valve Sizing & Frequency Analysis
Plant Engineering Best Practices: Pneumatic Rotary Dosing Valves
- Actuator Cycle Limit: Never operate a pneumatic oscillating valve above 50 to 60 cycles/min. High cycling rates cause premature failure of rack-and-pinion actuators, solenoid valve burnout, and thermal fatigue of polyurethane or silicone seals.
- Fill Factor & Powder De-aeration: Unlike star airlock feeders running at 15-30 RPM, oscillating rotary valves rely on gravity filling during dwell periods. Free-flowing powders achieve 90-100% fill factor; fluidizable fine powders (Geldart group A) or cohesive cohesive bulk solids require de-aeration dwell times or vibration pads.
- Differential Pressure Sensitivity: Oscillating rotary valves are not differential pressure seals. They must operate under gravity discharge into atmospheric or equalized pressure downstream (pressure difference \(\Delta P < 0.05 \text{ bar}\)). For positive pressure pneumatic conveying, use a star airlock rotary valve.
- Dual-Rotor Dosing Strategy: For high-accuracy batch dosing (e.g., weighing reactors, FIBC filling), specify a concentric dual-rotor unit: coarse dosing is performed with both rotors at ~40 cycles/min, followed by fine dribble dosing via the micro-rotor to achieve batch accuracies of \(\pm 5\text{ g}\).
4. Function in process
Rotary valves can be used as feeders to replace dosing equipment like screw conveyor, vibrating tube or even star valve, if the dosing expected is rough or fine, not too high in flowrate and it does not require an air sealing between 2 parts of the process. The layout should allow such equipment also - next process step located right below the valve.
Key process use for rotary valve :
- Rough control of flow towards a vibrating sifter
- Rough control of the flow towards a pneumatic vacuum transport line
- Fine dosing to a weighing hopper
- Rough control of the flow towards a filling machine hopper
Frequently Asked Questions (FAQ)
What is the core difference between a pneumatic rotary valve and an airlock rotary valve?
An airlock rotary valve (rotary feeder) features a multi-vane star rotor continuously turning at 10-35 RPM via an electric gear motor, designed with tight 0.1-0.2 mm clearances to resist conveying pressure up to 1.5 bar. A pneumatic rotary valve (oscillating valve) uses a pneumatic actuator to turn 180° back and forth, providing airtight elastomer sealing only in standby/closed mode, intended strictly for gravity dosing without positive counter-pressure.
Why is the maximum operating speed restricted to 60 cycles per minute?
Oscillating valves require the pneumatic piston to accelerate, stop, reverse, and seat against elastomer seals twice per complete cycle. Above 60 cycles/min, pneumatic shock loads destroy internal mechanical end-stops, solenoid valve pilot response lags, and powder has insufficient dwell time to settle into the pocket cavities.
Can pneumatic rotary valves handle abrasive powders?
Standard silicone or EPDM seal gaskets suffer rapid abrasive wear from hard bulk minerals (silica sand, alumina). For moderately abrasive powders, ceramic-coated rotors or hard chromium plating with PTFE lip seals are required. For severe abrasive duties, heavy-duty rotary feeders with replaceable hardened liners are preferred.