Studies on the Problem of Three-Quarter Inch Cylinder Valve Bonnet Nut Seizure/Thread Deformation

摘要

Due to a recent increase in three-quarter inch cylinder valve changeouts as a result of deformed bonnet threads, a project was initiated to study the problem and determine ways of improving the service life of the valves by recutting the threads after deformation. Valve bodies and assemblies were acquired from Stores, assembled, and the bonnet nuts were torqued to 110 ft-lb (the maximum allowable torque limit) repeatedly to cause bonnet thread deformation. The cycles to deformation requiring repair were recorded. It was found that the valves from lot number 8204 achieved bonnet thread deformation within 7, 110 ft-lb cycles. In contrast, valves from lot number 7908 took 50 cycles at 110 ft-lb without deformation before testing was discontinued. Measurements of valves from each lot number by the Gauge Laboratory showed that the bonnet threads of valves from lot number 8204 failed the Go/No-Go gauges and had major diameters greater than the thread specification allows. At least three factors must be considered for preventing premature valve failures. It appears that if the bonnet threads of three-quarter inch valves meet specification, sufficient bonnet thread damage is absent, and the torque used on the bonnet nut is kept at or below 110 ft-lb, the three-quarter inch valves can be expected to give an adequate service life. (ERA citation 12:041601)