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.

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.