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Rotary Valves Vs Double Flap Valves - Air Leakage 

Rotary Valves require tolerances between the tip and body to be able to operate; the tip to body clearances allows air to pass from one environment to another, be it a negative pressure environment drawing in air from below or positive pressure environment forcing air up past the Rotary Valve. 
Double Flap Valves only leakage is the volume of high pressure air trapped between the two flaps after the bottom flap has opened and closed. The gas between the two flaps can be purged if necessary before actuation. 
Rotary Valve Versus Double Flap Valve Air Leakage
Rotary Airlock Air Leakage 
The problem with rotary valves: 
A common component of most pneumatic conveying systems is the rotary valve often and mistakenly referred to as a rotary airlock. Rotaries are used primarily as a feeder or metering device 
Rotaries require precision tolerances to maintain an adequate pressure/vacuum seal between the vanes and housing. With many materials, abrasion and wear quickly and constantly degrade the seal. Therefore, the valve cannot function as an airlock unless a dense head of material above the valve is sufficient to create a seal. 
Rotary valve are generally the highest maintenance item in the system, often requiring frequent removal for replacement or service to maintain process efficiencies. 
Rotary Valve Air Leakage
Types of Rotary Valve Air Leakage 
The following text and diagrams are taken directly from a paper written at the Centre for Bulk Solids and Particulate Technologies, University of Wollongong, NSW, Australia. 
Carry-over leakage occurs when compressed air is “carried” up into the feed hopper through the returning empty pocked of the rotary valve. This type of leakage can be reduced by venting air from the returning pockets before coming in contact with feed material. However venting also will tend to increase the total amount of air leakage for a given valve and operating condition. 
Rotary Valve Pocket Leakage
Rotary Valve Tolerance Leakage
Clearance Leakage occurs when air leaks through the gaps between the moving rotor and the housing. 
Double Flap Valves 
Electric Double Flap Valve
Pneumatic Double Flap Valve with Pipeloader
Rotary pneumatic Double Flap Valve
Metal to Metal Seal 
No wiping or rotating action 
Limited and consistent air leakage throughout the life of the valve. This air can be purged. 
Serviceable in place without need to remove from process. 
Short and Tall series valves, openings 150mm up to 600mm, custom sizes too. 
Range of Applications 
Temperatures up to 1000°C 
Handles the most abrasive materials 
Differential pressures to 102 Kpa where positive below 
Materials of Construction 
Valve bodies made in Cast Iron, Cast High Nickel Alloy, Cast Aluminium, Fabricated Carbon and Stainless Steels. 
Seats and Flaps made in Cast Hi-Chome, Stainless Steel, Abrasion resistant materials. 
Actuation Options 
Linear and Rotary pneumatic 
Electric motors with Cams or Push Rods 
Hydraulic actuation 
Gravity or Manual 
Optional Equipment 
Mini-PLC for sequence control 
Failsafe for slam-shut upon power or air supply failure 
Heat shields, shaft air cooling for high temperature applications 
Purged high efficiency shaft seals 
Designed and created by it'seeze
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