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Packing is typically the main source of friction in a control valve that is in good operating condition. The total valve friction can be determined by measuring the difference in actuator pressure between the up stroke and down stroke at a given point in travel. This delta pressure, times the effective actuator area divided by 2 equates to the force required to overcome friction (Friction Force = dP * Actuator Area * 1/2). Keep in mind that friction, solely due to packing drag, typically remains constant throughout the valve travel. Thus, the plotted lines on the "Valve Signature" curve should be parallel and uniform throughout the full signature trace. The curve below illustrates what a Valve Signature should look like when the friction is associated to packing alone.
This next illustration depicts an atypical friction plot. The valve friction is being derived from a source other than packing. The source of the friction could either reside in the valve body or within the actuator. Although the test does not differentiate between the two, the most likely candidate is the valve body.
This illustration shows a valve signature trace of a 2" cage guided unbalanced valve that contains PTFE packing. With this type of packing, one would expect very minimal friction. However, in addition to atypically high friction, we see a huge increase in friction as the valve travels towards the open position. Because of this, it was recommended that the valve be opened and the parts inspected. Galling was found between the plug and cage. Further investigation found the root cause of the problem. The valve bonnet had been mis-machined causing side loading of the trim parts. If it had not been for the diagnostic test, this problem would have gone undetected and the valve would have eventually seized up in service. |
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