This article gives an overview on how safety critical shutdown valves can be monitored and condition evaluated. These types of valves are the last barrier in maintaining the safety of personnel and environment, hence they must always function as required. Safety critical valves can be found throughout the whole petro-chemical industry, offshore topside, sub-sea and onshore, in a wide variety of sizes. To detect internal leaks in a closed valve, the operator can choose between remote monitoring, do intrusive manual inspection or non-intrusive inspection. Some methods can even be done without shutting down the system, and there are several other advantages and disadvantages that must be considered. The person in charge of the plant valves needs to have an overview to help make sure the reliability of the valves is as good as possible, but without inflicting too much down-time and inspection cost related to maintenance. Furthermore, methods for valve internal leak quantification estimation are reviewed. And finally, the presented maintenance methods are compared and discussed so that recommendations for the best possible scheme can be made.

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