Problems with valve fluctuation and failure to close properly
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While adjusting the valves on site today, I noticed that one of the valves would fluctuate up and down slightly before settling at its set position when moving over a large range – I’m not sure what the reason for this is! Another issue is that when the central control sends a signal to close the valve, the actual valve position stays around 20%; the valve will only close completely after the solenoid valve is turned off. What are the reasons for these two phenomena, and how can they be resolved? (Note: The valve is a single-acting spring-return diaphragm valve with a straight-stroke distance of 51 mm. ）
1. There is a leak in the valve’s air supply; check the connector behind the actuator using soapy water. 2. Check whether the air supply pressure is within the range specified on the nameplate. Adjust the valve stroke again. I don’t know if your locator is smart or mechanical
What the original poster is referring to is likely a mechanical positioner. What you’re describing – where the valve reaches 60% of its position before returning to 50% – can be caused by several factors; for example, excessive air flow or an incorrect installation of the positioner’s feedback rod. As for the claim that the upstream pressure is outside the range specified by the positioner, that’s completely nonsense. The instrument air pressure in your plant is at most around 8 kilograms. To put it bluntly, even if you increase the pressure to 8 kilograms, as long as the diaphragm isn’t damaged or leaking and the positioner can handle it, the adjustment will still be accurate. The original poster should check their valve: if, when set to 50% position, the feedback rod isn’t in a horizontal position with a significant deviation, then it needs to be recalibrated or reinstalled. Moreover, this issue is closely related to inaccurate adjustments. The situation you described – where the control system sets the value to 0 but the on-site indicator shows 20%, and the valve reaches its proper position after the solenoid valve is powered off – clearly indicates that the calibration isn’t accurate. For ordinary positioners, when the valve position is set to 0, the backpressure gauge should indicate a value at least 2 scales below the minimum mark, which is what is commonly referred to as below 0.2. Keep an eye on that
The first issue is likely due to the high sensitivity of the locator; lowering it should resolve the problem. The second question is to determine whether the valve is air-open or air-close It’s also possible that the zero point has shifted upward; re-check and adjust it.
This post was last edited by dellkey on 2012-6-15 at 12:58. While testing valves on site today, I noticed that when a valve moved over a large distance and reached its set position, it would fluctuate up and down before settling down. I’m not sure what the reason for this is! Another issue is that when the central control sends a signal to close the valve, the actual valve position stays around 20%; the valve will only close completely after the solenoid valve is turned off. What are the reasons for these two phenomena, and how can they be resolved? (Note: The valve is a single-acting spring-return diaphragm valve with a straight-stroke distance of 51 mm. ) It’s caused by a too-large air supply pipe, resulting in excessive exhaust volume; the fluctuations occur because it wasn’t adjusted properly: First, the zero point of the positioner isn’t set correctly (it is recommended to readjust the zero point range) ； If it still doesn’t work, check the control room; there might be a speed limit in place. WELLKEY, Wick Automatic Control
Valve fluctuation is due to the actuator having slightly high sensitivity; reducing the sensitivity will solve it； It could also be caused by excessive friction between the packing of the control valve and the valve stem; apply lubricant to the area where the valve stem meets the packing, and operate the control valve back and forth several times to ensure thorough lubrication of that area. If the valve cannot be closed, it is likely that the zero point of the actuator is too high; lowering the zero point of the actuator will resolve this issue.
It uses the intelligent Fisher DVC6000 series! I’ve checked all the issues you mentioned – there are no problems!
1. Try reducing the throttle valve； 2. Check if the shutdown sequence is correct
The valve closes fine; what is a throttle valve?
It’s about installing a throttle orifice plate on the inlet pipe; it should be mentioned in the instructions, so take a look. We have one that was set up in this way; the reason is that the cylinder is small and the intake volume is too fast and too large. You can also try reducing the pressure first
When adjusting the valve, first confirm: 1. The valve can move through its full stroke. Try it by connecting directly to the air source. According to the poster, there is no problem with the valve. 2. Unplug the power cable. Use 832 and 375, isolated from the central control. It is possible that, as mentioned on floor 5, it was limited by the central control system. Reinstall the verification; note that the stroke value of the valve must match the value of the feedback rod. Selection between gas open and gas closed. 3. During verification, check whether it is level at 50% progress. 4. The self-calibration of the valve is complete. Try adding a signal. If that doesn’t work, try replacing the locator. It’s possible that there is dirt in the air. Take it apart, clean it.