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Valve positioners are utilized to have greater control over a process. Specific set points are indicated, and a signal is sent to the positioner by the operator, specifying exactly how much flow should be passed through the valve. But sometimes valve positioners have a difficult time finding where they’re supposed to be. This erratic motion is called “hunting”, and it could be caused by a number of things.
Generally, the first reaction is to pull the valve. It’s got to be the valve right? Not necessarily. It could be the valve, but it could also be the actuator, or the positioner itself. Before the valve is pulled, check for this simple fix first.
An Electro-Pneumatic Positioner works like this. Your control device will send a 4-20mADC Control Signal to the Valve Positioner I/P Converter which will in turn convert to a linear 3-15 PSI Output to the Positioner. The 4-20 indicates the milliamperage for the positioner, 4 milliamps indicates a closed valve, where 20 indicates fully open. The PSI is directly proportional to the milliamperage required to operate the positioner. For example, when the positioner milliamperage reads 12, the actuator is supplied with 9 PSI, opening the valve to 50%.
Inside the base pneumatic valve positioner is a piece called a spool valve. It is a cam characterized, forced balanced instrument that is simple and user-friendly for calibration and maintenance. It is also critical to the operation of the positioner. The spool should move freely inside the block, allowing air pressure to dictate its position. The spool valve is gold plated, and over time, that plating can wear. This can be caused by dirty air, or just everyday use. As this happens, the spool piece tends to stick inside the block, which can cause air pressure to build, then overshoot the set point. The position of the valve becomes unpredictable, and control over the valve is lost.
To clean the spool valve, blow it out with air. If air is not available, wipe it down with a cloth. The inside of the spool block should be cleaned with a soft material, like a pipe cleaner. Do not, however, use WD40, or another oil based lubricant to clean the spool piece.
Pulling and replacing the valve could take hours of your day. Simply cleaning, or if necessary, replacing the spool valve can take less than 5 minutes.
We set up a demonstration of this to give you a visualization of what occurs with a hunting valve.