Cla-Val 40-01 and 640-01 Rate of Flow Control Valves prevent excessive flow by limiting flow to a preselected maximum rate, regardless of changing line pressure. They are hydraulically operated, pilot controlled, diaphragm valves. The pilot control responds to the differential pressure produced across an orifice plate installed downstream of the valve. Accurate control is assured as very small changes in the controlling differential pressure produce immediate corrective action of the main valve. Flow rate adjustments are made by turning an adjusting screw on the pilot control.