A rotary valve meters the flow of particulate material from a supply source into a pneumatic conveying system and includes a central rotor rotatable within a cylindrical housing. The interior surface of the cylindrical housing is formed with grooves extending from pinch points at the inlet opening to facilitate the passage of particulate material past the pinch points without being trapped between the rotor vanes and the housing. The grooves are preferably formed in a configuration that has an increasing depth from the proximal wall to the distal wall, and preferably is configured with a spiraled surface shaped to increase the depth of the groove channel to a rounded end chamber at the distal wall. The groove channels can be oriented in alignment with a triangular ridge, in a circumferential direction perpendicular to the direction of movement of the rotor, or perpendicular to the angle of the triangular ridge.