What causes the large-scale, regular fluctuations in the valve during startup?
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When we started working a few days ago, right from the beginning the flow valve was set to 25% open, and then the valve kept fluctuating regularly between 0 and 40, going back and forth every 2 to 3 seconds, in a manner similar to that of a sewing machine. The valve positioner is from Siemens and is of the single-pass type; it was tuned only before operation. The valve is a single-seat valve that operates due to a Koso fault. The medium is 75 kilograms of high-pressure water at 150 degrees Celsius, which is sent to the boiler. Due to the back-and-forth movement of the valve, the pipelines vibrated severely, and the supports were almost damaged. Eventually, the manual valves were closed before and after the process started; the valve continued to fluctuate at that time, but once it was fully closed, there were no more problems, and manual movement up and down also worked without any issues. We have analyzed and determined that the sensory valves and positioning systems cannot cause such faults; moreover, discussions with the process team showed that it is not a problem related to their operations. Could anyone help us understand how to conduct this analysis?
Has it been set to automatic mode? It shouldn’t fluctuate; check the automatic parameters
Sometimes, after self-tuning, the required performance cannot be achieved, so the PID parameters must be set manually
It seems that the wrong valve was selected; the pressure difference before and after is too large, exceeding the rated pressure difference of this valve, which leads to such a situation!
Have you tried adjusting the valve opening manually, instead of using the automatic mode?
The locator and PID parameters are being reconfirmed
This post was last edited by cqdfwy on 2016-4-12 at 14:41 to check if it was caused by automatic submission. That is, if automatic operation causes significant vibration, check whether the parameter settings are appropriate and whether the transmitter and control valve have been properly calibrated ； If significant vibration is caused manually, factors such as excessive pressure difference and resonance with the piping system may be considered, that is, an inappropriate selection of the control valve. I have encountered a similar situation before, where resonance occurred with the piping system; the problem was resolved by replacing the original single-seat valve with a cage valve.
1. The retaining spring of the positioner feedback rod is not secured, or there is looseness at the connection point. 2. The pressure difference before and after the valve is too large, exceeding the valve’s rated pressure difference. 3. When the circuit is set to automatic mode, the measured value deviates too much from the set value. Or the main circuit is activated before the secondary circuit has stabilized!
Your locator has been adjusted, but the PID values for the control loop may still need to be adjusted again.
1. What is the normal pressure of a steam drum? 2. What is the approximate operating pressure range of the drum when the valve fluctuates back and forth? 3. Does the drum pressure also fluctuate once it returns to normal?
After the central control system detected it, manual control was used; when I arrived at the site, I found that the set value remained at 25, the valve kept fluctuating back and forth, and the set value of the positioner did not change