The valve operates when the pressure in the pipeline sinks to under a predetermined value. A piston or membrane type positioner (6) is arranged in the valve chamber (5). It is activated by a valve above the outlet (3) which is connected to the shutter (9). Pressure activation on the positioner towards the shutter is achieved by an introduction of a medium into the chamber.An emptying pipe (15) connects the inlet (2) to the pre-valve chamber (16) and is also connected to the chamber. The positioner is activated by a spring (11) in the opening direction of the valve which is a throttle which works when the chamber pressure rises due to refilling of the pipeline.
