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.