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This feature's valve has a severe case of hiccups. After performing the routine examination (i.e. valve signature test, dynamic error band test, and step test), it was confirmed that the valve displayed sporadic travel readings between 30% and 60% travel. Following are the graphs from that examination:
As we examine the results, we see that there is only one data point per travel spike. The tests were run at a collection rate of 150 ms or in other words, one data point every 0.150 seconds. We also see that the spikes in travel appear to be exceeding 125% of the designated rated valve travel. The designated travel for this guy is 1.00 inch. So, if one were to believe the data, the valve would be moving from 0.300" travel to better then 1.250" in 0.150 seconds. It is physically impossible for this standard valve assembly to stroke that fast without some outside force being present, of which there was none. Also because of the physical restraints (stops) within the valve and actuator, that amount of excess travel would be very difficult to achieve. In reality, what we are seeing is the result of a worn resistant element within the instrument’s feedback element. These elements are designed to withstand millions of cycles. However, it is possible for the above problem to occur if the valve is throttling at one position for a lengthy period of time and there is high vibration present. The diagnostic performance traces shown above indicate that the travel feedback element must be replaced. |
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