"A Boot or Not a Boot " That be the Question.

Like many of us who find it difficult to get out of bed in the morning, this particular valve had a difficult time getting out of its seat.  In viewing its Valve Signature we see that it appears to require much more actuator force to initialize motion from the seat than was required to move the valve through its first 25% of travel.  Now then, is this due to sticking or is it just due to the phenomenon of which, for lack of a better terminology, we refer to as "boots"?

A boot, or bulge in the pressure reading at the start or reversal of travel is a common occurrence.  It is affected by  the location of the pressure sensor relative to the true actuator pressure.  In this case, the pressure sensor is located within the positioner and not at the actuator.   During the Valve Signature test, the actuator pressure is going from zero to full supply pressure and then back to zero.  At the very start of the test, as the signal is being increased (demanding the valve to move) the positioner is working its heart out pushing as much air as its internal capacity will allow.  Keep in mind, the positioner must fill the volume of the actuator and then build up enough pressure (force) to overcome the initial spring setting (benchset) and packing friction prior to the valve moving.  Because the pressure is being monitored in a relatively small chamber of the positioner near the loading/exhaust ports, and because there are restrictions between the positioner and actuator (i.e. tubing, tubing fittings, positioner ports), the pressure read can be somewhat different from the actual pressure in the actuator.  This pressure is higher than that in the actuator during the initial fill stage and lower during the initial exhaust stage.  Once the pressure (force) in the actuator becomes high enough to start the valve moving, it moves rapidly until it catches up to the ramping signal.  The positioner is now sensing travel feedback and thus slows its output to coincide with the ramping of the input signal.  At this point the pressures are nearly identical within the actuator and positioner.  We can control the size of the boots by controlling the scan time of the test.  The recommended scan times are based on actuator size (volume).  The larger the actuator the longer the scan time.   This keeps in perspective the ramp time to the amount of air that must be moved.

How does one determine whether the bulge at the seat exit is due to the boot phenomenon or the valve sticking in the seat?  What I would do is compare the magnitude of the boot in the open position to the one in the closed position.  They should be comparable in magnitude to the amount of pressure being moved.  Let’s revisit the signature curve.

Closed End	Open End

Notice that on the open end, the boot delta pressure is approximately 10% greater then the extrapolated actuator delta pressure --- (20.3 – 15.2) / (15.2 – 14.7) = 10.2.  I would expect this same relation to hold true on the closed end.  Thus I would expect the effect of the boot to be only 10% of the closed end actuator delta pressure or 0.8 psig (8.1x0.10).  Here we see that the additional pressure (above the required opening pressure of 8.1 psig) required to open is nearly 4 psig.  Much greater then one would attribute solely to boot effect.

By now one would suspect that this guy is sticking in the seat.  To further prove this, let’s take a closer look at the seating profile. We see what appears to be a good seating profile.  The plug makes contact with the seat and then we see a reactive travel of approximately 0.20 inch prior to running out of seating force.  Wait a minute.  Did we say 0.20?   Twenty thousands is too much travel for the relatively small amount of force that’s being applied to this small "E" body with micro-flute trim.   This would suggest seating problems and thus solidify the suspected "sticking in the seat" analysis.

The valve was placed on the workbench and opened up.  It was found that the seating area of the valve plug had been mis-machined.  This caused the plug to wedge into the seat ring causing it to stick.  The valve plug was re-machined and again installed in the valve.   The valve was now in full health and operating to full expectations.  A smooth exit from the seat can now be seen with no evidence of sticking present.

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