The check valve units are made up of a clapet valve, available in the DN125, DN150 and DN250 versions, which are fitted with a manifold and a flange for connection to the suction hose. The check valve units are made up of a disk, constrained at one end to a guide, which allows the passage of fluid flow in one direction, while it prevents backflow in the opposite direction. The check valve units are predisposed to typically be installed in correspondence of the suction line of vacuum pumps: in this configuration, when there is no longer the vacuum condition created by the decompressor, backflow is prevented along the line, thus maintaining depression.