On-site instrument failure analysis (one question per week) – Question 4! (Reveal answers)
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This post was last edited by Yiding on 2009-10-17 at 23:49. There is a water supply control valve on one boiler (a double-acting cylinder valve). The process operators reported that the control valve did not respond to signals sent from the control room. When the instrumentation technician went to the site, a 4–20mA signal was detected; the output of the positioner was also normal, yet the control valve still would not operate. What could be the reasons for this? Answer: The operator switched the control valve to bypass control. After the instrumentation technician went to the site, he used a multimeter to measure the signals coming from the DCS; the 4–20 mA values were normal. The output of the positioner was also checked – moving the damper closer to or farther from the nozzle showed that it could produce either the maximum or minimum output, which was normal as well. However, the control valve did not move at all. The O-rings in the cylinder were inspected, and there were no signs of air leakage. At this point, the actuator was separated from the valve body, and the valve stem was turned using a wrench; it was found that the valve core moved freely, with no signs of sticking or obstruction. It became necessary to open the cylinder body for further inspection. Upon opening it, it was discovered that the gear connected to the central shaft was broken into pieces. As a result, when the piston in the cylinder moved, it could not drive the connected gear to rotate; without a functional gear, it was impossible for the gear to drive the central shaft and thus cause the valve core to move. A post-analysis of the cause of gear failure revealed that the product quality was substandard; gears cannot be made of cast aluminum or cast steel, as this will lead to wear and failure after prolonged use. Note: Thank you all for your active participation in this event! Although there is only one correct answer for each question, everyone’s understanding of control valves (or the instruments involved in other types of failures) as well as their approach to analyzing such failures was generally correct, which shows that there are many experts among the users. I hope everyone can benefit from each fault analysis, achieving the goal of learning from one another. Your participation is the driving force behind our successful organization of this event. Wishing HaiChuan healthy development and continued success! May Haiyou enjoy good health and a happy family!
Is the valve stuck inside or is the air supply pressure insufficient?
First, check whether the pressure of the air supply is appropriate; second, see if there is any jamming inside the valve. Another possibility is that the piston inside the double-acting cylinder is leaking air. If the valve has power-off protection, it is also necessary to check whether the solenoid valve is powered.
Another possibility is that the valve’s exhaust port is blocked.
If the valve positioner’s output signal is normal but the valve does not move, it is possible that the valve stem is stuck.
If all the above issues can be ruled out, it is possible that the instrument air supply valve is not open, or if there is a handwheel, it may be in the manual mode; in such cases, it is still considered to be a problem related to on-site operation.
If the air supply is normal, it is usually due to a stuck valve core
Check if there is a common gas supply available. First, see if you can operate it manually on site; if so, check whether your valve is part of a interlock system. In such a system, there is a five-way three-port valve that must be reset before it can function. Thirdly, check whether there is any leakage in the valve’s cylinder. If there is no leakage, then check whether the valve is stuck
1. The control valve is stuck; the signal pressure generated by the positioner is not sufficient to make it rotate； 2. The signal pressure output by the positioner is too low to drive the actuator ； 3. The seal inside the cylinder of the actuator is damaged, resulting in air leakage.
What I said before was rather simple and didn’t draw much attention. Let me analyze it for you again. I’m not sure if everyone will agree! The valve positioner of a double-acting cylinder has two outputs, which go to each of the two cylinders respectively. Since the output of the positioner was found to be normal, unless the valve is stuck, the most likely explanation is that the balance valve used to balance the two cylinders is in the open position. This balance valve is mainly used to facilitate operation via the valve handwheel; that is, when the valve is operated using the handwheel, the balance valve opens to equalize the air pressure in the two cylinders. When the valve is operated remotely, the balance valve closes.