Abstract: Safety valve, characterized in that it principally consists of a housing (1), provided with an inlet (2) and and outlet (3); a main cut off valve (4) installed in the housing (1); and means working together with the flow of liquid which with the removal of liquid and after a lapse of time shut the main cut off valve (4), which are reset in their starting position each time an equal or almost equal pressure in the liquid is created at the inlet (2) and outlet (3) of the safety valve and which when the main cut off valve (4) is shut, hold it shut as long as a well defined drop of pressure remains in existence over the main cut off valve (4), whereby these means are driven by means of a by-pass, such by means of a channel (6) that is situated apart from the main passage (5).