The development of novel control valve diagnostic software based on the Visual Basic programming language

Abstract

A number of commercially available control valve fault diagnostic systems were considered by the authors and it was established that they produce test results that require specialist knowledge to interpret them fully and, therefore, to diagnose faults accurately in control valves. This provided the incentive to develop a graphical user-interface diagnostic program, as reported in this paper, which can be used easily by field technicians and operators to diagnose faults in control valves and actuators. A commercially available diagnostic system had been used to generate a large number of test results for various faults in a typical control valve and actuator. These results were incorporated into the newly developed control valve diagnostic program as its database. Then, when tests are conducted and available information is entered into the program, a diagnostic statement is produced. The statement is based on the comparison between the entered value for given valve performance parameters and those which are in the established database. In addition, an option was included which enables the user to import certain types of test data directly from the commercially available diagnostic system so that the condition of the control valve can be established with a higher degree of accuracy and without any additional hardware components. The diagnostic program has three options. They are as follows: determine condition, estimate condition and step response analysis. The determine condition option enables the types of fault to be determined with absolute certainty. However, this option can only be used if all the 12 valve performance parameters are known and are entered into the diagnostic program. On the other hand, the estimate condition option was developed in such a way that faults can be predicted when at least one of the valve's performance parameters is entered into the program. This is because, when the valve's performance parameters are entered and the estimate condition option is selected, the diagnostic result is produced in the form of a distribution table showing the conditions that are satisfied by the input variables (the performance parameters) and the extent to which each variable has satisfied the conditions. The step response analysis option enables the condition of a control valve and its actuator to be determined, either by importing test data that are produced when the control valve is cycled and the commercially available diagnostic system is used to monitor the valve stem position or by monitoring the pressure that is developed in the actuator's upper diaphragm casing, using a pressure transducer and a data acquisition system. The reliability and the accuracy of determining a number of faults within a control valve were established. Faults considered included the gradual blockage of the actuator's vent hole, damaged valve stem and damaged valve stem packing rings, and the accuracy of determining the type of fault was dependent on the selected diagnostic option. The paper concludes by providing result screens for a number of conditions using all the available options and outlines the limitations of the current version of the developed program.