In this study, non-destructive technologies that can be applied to evaluate the integrity of valve materials, safety against internal pressure caused by corrosion, and the blocking function of large-diameter water valves during operation without requiring specimen collection or manpower entering the inside of the valve were tested to assess the reliability of the technologies and their suitability for field application. The results showed that the condition of the graphite structure inside the valve body can be evaluated directly through the optical microscope in the field without specimen collection for large-diameter water butterfly valves, and the depth of corrosion inside the valve body can be determined by array ultrasound and the tensile strength can be measured by instrumented indentation test. The reliability of each of these non-destructive techniques is high, and they can be widely used to evaluate the condition of steel or cast iron pipes that are significantly smaller in thickness than valves. Evaluation of blocking function of the valves with mixed gas showed that it can be detected even when a very low flow rate of mixed gas passes through the disk along with the water flow. Finally, as a result of evaluating the field applicability of non-destructive technologies for three old butterfly valves installed in the US industrial water pipeline, it was found that it is possible to check the material and determine the suitability of large-diameter water valves without taking samples, and to determine the corrosion state and mechanical strength. In addition, it was possible to evaluate safety through the measurement results, and it is judged that the evaluation of the blocking function using mixed gas will help strengthen preventive response in the event of an accident.

In this study, we collect water control valves that have had accidents due to existing cracks, etc. are collected, and propose investigation items for strengthening the valve structural safety evaluation through a series of analyzes from valve specifications to physicochemical properties are proposed. The results of this study are as follows. First, there was a large variation in the thickness of the body or flange of the valves to be investigated, which is considered to be very important factor, because it may affect the safety of the valve body against internal pressure and the flange connected with the bolt nut. Second, 60% of the valves under investigation had many voids in the valve body and flange, etc. and the decrease in thickness due to corrosion was relatively large on the inner surface in contact with water rather than the outer surface. It is judged that the investigation of depth included voids is very important factor. Third, all valves to be investigated are made of gray cast iron foam, and therefore it is judged that there is no major problem in chemical composition. It is judged that the chemical composition should be investigated. Fourth, as a physical investigation item, the analysis of metal morphology structure seems to be a very important factor for nodular cast iron from rather than a gray cast iron foam water valve with a flake structure. As it was found to be 46.7~68.8% of the standard recommended by KS, it could have a direct effect on damage such as cracks, and therefore it is judged that the evaluation of tensile strength is very important in evaluating the safety of the valve.