My conveying line handles dusty materials like cement or sand. How effective are rotary valves at minimizing dust leakage?

Rotary valves can be very effective at minimizing leakage when properly configured by an expert.

As a general recommendation to prevent dust leakage, valves should be equipped with ACST-4 shaft seals and an outboard bearing design. You should also check rotor-to-housing tolerances often, as enlarged tolerances can also cause leaks.