"The Phantom Travel"

At times, one may experience a diagnostic signature in which the travel characteristic seems to have defied the laws of physics.  The travel appears to have done just the opposite as to what one would expect with the forces applied (i.e. an increase in travel with a decrease in force and visa versa).  In many cases, it appears that the valve has actually traveled beyond its physical stops (seat and upper actuator stop).  This anomaly can occur within the data collected by either of the diagnostic tools (ValveLink™ or FlowScanner™ Software).   The following signatures give examples of this anomaly.

Here we have a ValveLink signature.  Notice that although the force (actuator pressure) is decreasing, the travel had actually increased past what would normally be defined as the upper actuator stop.  We also see chatter.  This was attributed to metal to metal contact between the feedback arm and the digital valve controller’s mounting bracket.

Here we have a FlowScanner trace.  Notice that the travel at both ends appears as though it is going opposite to what the force dictates.  The travel is increasing with a decrease of force and decreasing with an increase in force.

In both of these signatures, the anomalies were similar in appearance and the root cause of the problem was the same.   During both tests, the travel feedback arm was making contact with a foreign object.  In the case of the ValveLink signature, the locator pin was not used during the installation of the feedback arm to the digital valve controller.  Thus, the relationship between this arm and potentiometer rotation was in error.  The feedback arm was set too high.  This allowed for the feedback arm to contact the mounting bracket in the "up" position.  In the FlowScanner test, the feedback arm of the positioner was the wrong length for the valve travel.  This allowed the arm to contact the indicator tabs on the side of the Type 3582 positioner case.

In both cases the feedback mechanism made contact with an unauthorized stop prior to the valve stops being reached.  This allowed for the feedback arm to bend slightly and was picked up as travel reversal by both the potentiometer and the Celesco travel transducer.

The moral of the story is that anytime you see a reversal of travel in relationship to the actuator forces, and there are no other known forces present (i.e. process pressure), suspect interference within the travel feedback path.  In FlowScanner testing, more times than not, this interference was created via the operator’s setup.  Typically, a feedback extension is attached to the valve to facilitate connecting to the travel transducer.  This extension is prone to contacting objects attached or adjacent to the valve.  It is always good practice to stroke the valve prior to testing to verify a clear path for this feedback mechanism.

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