During the calibration of a pneumatic control valve with a positioner, a technician notices that the valve does not respond linearly to the input signal. When the input signal is increased from 4 mA to 20 mA, the valve position does not change proportionally and exhibits a hysteresis effect. What is the most likely cause of this issue?
It sounds like you’re dealing with some mechanical issues in your valve setup. The non-linear response and hysteresis you’re seeing are classic signs of mechanical friction or stiction.
Here’s what I think might be going on:
- Valve Stem Packing: If the packing around the valve stem is too tight, it can cause a lot of friction. This makes the stem hard to move until enough force builds up, causing it to jump suddenly and creating that non-linear behavior.
- Positioner Linkage: The linkage between the valve stem and the positioner might be out of alignment or in need of lubrication. Over time, these parts can wear out or get dirty, which increases friction.
- Actuator Issues: The actuator might have internal friction or stiction problems. This can happen due to wear and tear or lack of lubrication inside the actuator itself.
- Gaskets and Seals: Worn or damaged gaskets and seals can also add friction or cause binding in the valve mechanism.
- Pneumatic Lines: Sometimes, the pneumatic lines can get dirty or contaminated, which can cause sticking and delay in response.
To fix this, you’ll need to:
- Check and lubricate the valve stem and packing.
- Inspect the positioner linkage for proper alignment and smooth operation, and lubricate if necessary.
- Look at the actuator for any signs of wear or lubrication needs.
- Check and possibly replace gaskets and seals if they’re worn out.
- Clean out any dirty pneumatic lines.
Addressing these mechanical issues should help smooth out the valve’s response and eliminate the hysteresis.