Valve designed to connect two vessels one in stand-by to the other. It is useful with high operating pressures and it offers the security to block the wrong put in service of the stand-by vessel if the pressure between the vessels is not balanced assuring the filling of the stand-by vessel. In case of fire the valve keeps the seal. Seal is guaranteed by the in line pressure that acts positively on the shutters. The put in service of the stand-by vessel is without interruption or reduction of flow rate. The valve, when the pressure is balanced through the by-pass, it’s very easy to handle. The valve has no sliding action because the seal is head to head type (not male-female seal) so to guarantee the possibility of treat a dirty flow with no risk of seizure.