System for shutting-off and relieving the pressure in a liquid supply line

A shut-off and pressure-relief device for controlling a supply of foam precursors into a foam mixing and dispensing means, said device comprising a first and a second supply unit, each having a first input for inputting one of said precursors into said supply unit, and a first output connected to a mixing chamber, each supply unit further comprises a piston type pressure pump, having an expansion and a pressure chamber, and a piston arranged to be moved within said pressure and expansion chambers, said pressure chamber being connected to said first input by means of a first duct and to said mixing chamber by means of a second duct, a first head of said piston being provided to be driven into said pressure chamber by means of a driving power supplied on said piston within said expansion chamber so that when said driving power is supplied, said first head is moved within said pressure chamber so as to enable the latter to be filled with one of said foam precursors through said first duct until said first head reaches a position in which said first head opens a passage for said foam precursor from said pressure chamber to said mixing chamber through said second duct.