Abstract: A method and devices for applying the generally known principle of cascade flow in control of self-adjusting fluid handling systems, which devices are capable of maintaining predetermined flow rate/pressure difference ratios, over a wide range of variations.The capability of maintaining predetermined flow rate/pressure difference ratios is based on a capability of maintaining predetermined displacement/pressure difference ratios of axially displaceable means of control in application of the principle of cascade flow.One embodiment is a draught control device, which is capable of maintaining substantially constant draught, e.g., in central heating boilers of solid fuel, over a wide range of variations in flue pressure.Another embodiment is a constant flow valve, capable of maintaining constant rate of volume flows in house ventilation.A third embodiment of the present invention is a constant pressure valve, capable of maintaining constant pressure of fluid upstream the valve.