Flow control valve diaphragm type is ideal for limiting the flow to a pre-determined maximum (via maintaining a continuous pressure differential across an orifice).
When the pressure differential is less than the set-point, the flow valve opens, allowing flow to meet pre-determined demand. At the desired maximum set-point, the pilot reacts to small changes in sensing pressure and controls the main valve position by modulating the pressure.
When the pressure drop across the orifice exceeds the set-point, the control valve closes slightly, limiting the flow to the pre-set maximum. Adjusting the pilot setting permits the maximum flow to be changed in the field above or below the original point.