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In viewing the Valve Signature curve no major anomalies are seen. Mechanical stops were reached at both ends as designated by the two pressure spikes. The upstroke and down stroke lines are uniform and parallel. The spacing between the lines (friction) indicates this valve most likely contains a graphite type packing. All in all we see no internal problems with this valve. But, keep in mind the pressure peaked out at 18 psig in the open position.
In viewing the Overall Valve plot, we see a drooping of the travel near the open position as the input signal is being increased. This represents a slowing in output change in relationship to the change in input. On decreasing signal, the relationship is quite linear. This linear relationship is what one would expect for both signal directions. This droop is characteristic of "pressure starvation" within the positioner. Keep in mind that for optimum performance, a minimum of five (5) psid pressure must be maintained across a positioner (supply pressure should be set at 5 psig above the pressure requirement to stroke the valve). As the pressure differential drops below the 5 psid minimum, the efficiency of the positioner deteriorates thus causing the sluggishness or slowing down of the output. Now, what could cause this pressure starvation? (1) The supply pressure regulator being set too low (premeditated) or (2) a leak within the actuator and/or instrument assemblies (accidental).
In viewing the supply pressure, we see that the regulator was set just below 22 psig. In revisiting the valve signature, we see that the required pressure to open the valve (operating range) was 17 psig. This shows that the supply pressure was set at approximately 5 psig above the operating range. Though marginal, there is sufficient pressure above the operating range to provide for efficient operation of the positioner. One can rule out that the starvation was do to a low supply regulator setting. That brings one to the conclusion that the sluggish performance is do to a leak. To verify this, one must look at the plot of Supply Pressure vs. Time (top line, labeled "s") and Actuator Pressure vs. Time (bottom line, labeled "as"). Here we see that the output of the positioner (actuator pressure) does not reach full supply pressure (about 21 psig). We also see that there is a pressure demand on the supply regulator as the positioner is exhausting the actor pressure. There should be no draw on the supply source during exhaust unless the regulator is simultaneously feeding a leak.
The conclusion of this case study is that if you see a droop (change in slope) in the "Total Valve" curve (FlowScanner) or "Dynamic Error Band" curve (ValveLink), suspect a starvation of supply pressure. This starvation is either do to a low supply setting or a system leak. In this case it was a leaking o’ring seal within the actuator. |
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