This investigation covers the diagnosis of the cause of splitting of aluminum bronze bonnets on valves for cylinders for handling corrosive gases and the remedial steps taken to minimize the failures.
The splits in the bonnets were found to be due to stress corrosion cracking which was induced by excessive torque applied to the bonnet, by a design weakness in bonnet itself and by the presence of longitudinal stringers of eutectoid in the material.
Laboratory tests showed that design changes could be made to strengthen the bonnets and that the stringers could be eliminated by heat treatment. The combination of heat treated bonnets, along with controlling torque applied to the bonnet nuts, eliminated bonnet failures in the valves. Improvements in bonnet design were incorporated in a new design for valves.
