A variable orifice assembly includes a housing defining a flow orifice therethrough and configured to be connected in fluid communication between two pipes of a particle conveyance system. The assembly includes a plurality of blades pivotally mounted to the housing to pivot between a retracted position and an extended position. Each blade includes a gas path edge. In the retracted position, the gas path edges of the blades conform to the flow orifice. In the extended position, the blades extend inward from the flow orifice.