Automated troubleshooting of pneumatically operated control valves

Many control valve applications are critical to the generation of electric power. This paper presents an approach that will minimize maintenance support required for a given control valve as well as assuring that it performs properly, reducing losses associated with process fluid leakage and trim damage. The first portion of this work presents a free-body diagram of a typical control valve-spring and diaphragm actuator assembly, showing how it operates and how all force elements interact. That is, it shows how increased stem friction can reduce the valve seating load; how minor variations in spring adjustment can produce large variations in seating load, etc. After the basics have been covered, the diagnostic system is presented and explained. The system in practice is connected to a valve-actuator assembly so that such things as diaphragm pressure, stem movement and operating signals can be monitored and the data used as input to a portable personal computer and data logger. The valve actuator assembly is then stroked and the inputs monitored. The computer, having been properly programmed taking into account the valve construction and service conditions, then generates a printed report summarizing the condition of the valve in such areas as: spring adjustment, stroking time, valve travel, stem friction, seat load, and positioner and transducer calibration. This evaluation, which should require one or two hours, can pinpoint operational problems before they become the source of process losses or, in the worst case, unscheduled shutdowns. It should also prove to be an excellent maintenance planning tool, allowing evaluation of valves before an outage, so that proper maintenance scheduling can be done in advance