Abstract: A diaphragm valve mechanism comprises a diaphragm valve support structure having passages extending therethrough. Projecting normal to a grid are a number of locating pins which are adapted to extend through holes in the diaphragm valve. The diaphragm valve is formed of flexible material and is provided with a pair of cross slits, each corresponding with the grid bridge element. The central portion of the diaphragm valve is located within a major flow passage whereas the outer peripheral edge portion of the diaphragm valve is located within an exit passage.