Control valves can be fickle pieces of equipment. If we take a step back and say, “What are we asking the control valve to do?” The answer is: we are asking this piece of mechanical equipment to provide the same type of precise movement time and time again.
There are many factors working against this goal including tight tolerances in the valve assembly, mechanical wear, system issues, mis-sized valves/actuators and even installation/operational errors. Over time, many of these issues compound and plants go through a “trial and error” process trying to get the valve assembly to achieve its goal. Lots of times, we see the positioners being blamed as the problem and become one of the first major “trial and error” casualties.
So, what can you do? How can you determine whether the positioner really is the problem?
Proven time and time again, a positioner equipped with diagnostics is the “eyes and ears” into the valve assembly. The positioner is the only piece of equipment on the valve assembly that will give you feedback about the valve – good or bad. When encountering valve assembly issues, you should look to the positioner and ask, “Why is it performing the way it is?”
Is the positioner performing poorly, or is it responding to the system and/or changes that have occurred in the system and valve assembly?
More often than not, during the troubleshooting stage, we find the root of the cause lies with the valve and/or actuator. We are able to find this “root cause” using specific maintenance and diagnostic information either accessed by the positioner or stored in it. For example, maintenance information will provide you with the number of valve strokes, fault messages, alarms, operating hours, stroke times, air leakage, and temperature. It will allow you to monitor trends for control deviation, feedback and histograms. Diagnostic data typically provides you with more in-depth performance related information. As an example, a valve performance test will provide you with data points that have been defined by IEC specifications. These data points include the hysteresis, repeatability and linearity of your control valve. Other valve performance testing will help you determine your overshoot, break out time and settling times. Pulling this data together provides you with the information needed to assess a control valve assembly.
You may only need to operate a leakage test to identify that the issue is a leaking actuator but for more complex system issues, using a positioner with diagnostic capabilities is critical. Positioners with diagnostic capabilities can be used to determine why the valve assembly isn’t performing to your standard and can help predict the next time your valve assembly needs service.
In summary, the positioner is a vital tool in not only positioning the control valve but also diagnosing issues that occur in the assembly. Use the positioner as your eyes into the valve and listen to what it is communicating.
Read more about the importance of positioner diagnostics.
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