Image Number 5 for United States Patent #7464721.
A Safety Instrumented System for use with a process control system receives pressure of process fluid in the process piping. A valve positioner positions a valve which controls flow of process fluid through the process piping. The valve positioner is caused to perform a partial stroke of the valve or otherwise introduce a perturbation into the process. A resulting change in sensed pressure due to the perturbation is used to diagnose operation of the process.