Here is the answer from Jack Osborn’s latest “Ask the Expert” article in Powder & Bulk Engineering News. PBE Article
Your situation and question covers an extremely important topic to anyone who has a dust collection system with manual slide gates (also called blast gates). Balancing the airflow is a far more difficult problem and laborious task in a dust collection system with manual slide gates than you’d initially surmise given the device’s simplicity.
NPFA 652: Standard on the Fundamentals of Combustible Dust sections 22.214.171.124.3, 126.96.36.199.1, and 188.8.131.52.2 directly apply to this situation. Basically, these sections require that the dust collection system’s design and operation ensure sufficient airflow velocity is maintained in all ducts to prevent accumulations; that each pickup point or system pickup point has sufficient airflow to capture, control, and convey dust emissions; and that each pickup point must have a documented minimum airflow volume. Frankly, these are requirements that should be applied to all dust collection systems, whether or not combustible dusts are involved.
Using manual slide gates to achieve these combustible dust compliance requirements is far more difficult compared to the simpler and more effective balanced-by-design method. The main reason using slide gates is more challenging is the fact that adjusting one slide gate effectively changes the airflow for every other system pickup point, making the task a continuous adjustment process until the system’s flowrate variations fall within an acceptable range.