Manual adjustment is possible in the control valve chamber; there is slight fluctuation in the valve stem on-site
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Since our reactor requires purified air to control temperature rise, we observed slight fluctuations in air pressure during operations yesterday. The adjustment valve room uses manual control, and it was found that the air pressure trend chart showed significant fluctuations, with flow rates varying from high to low. Upon going to the site, it was observed that the on-site indication of the adjustment valve also fluctuated, and even the valve stem exhibited slight movements. What could be the cause of this?
First, check if there is any issue with the operator’s output! If there are no issues, check the position sending module again!
Since manual operation confirms that the electrical signal received by the control valve is stable, and given that the reactor uses purified air and the air pressure fluctuates, it is suspected that the air supply for the instrument associated with this control valve also comes from this purified air pipeline. The fluctuations in the air supply itself cause large variations in the pressure of the air supplied to the locator, which in turn leads to fluctuations in the air pressure sent from the locator to the control device of the control valve. These fluctuations result in movements of the valve stem, thereby causing changes in the flow rate of the purified air entering the reactor. Therefore, to eliminate faults, it is necessary to ensure stable air pressure for the control valves. With low external temperatures at present, attention must be paid to the issue of water in the air supply used by the instruments.
It is recommended to first check whether there are any fluctuations in the 4-20mA signal received by the valve actuator; if such fluctuations exist, identify the source of the issue, whether it is interference or a fault in the AO module. If the current remains stable, then check whether air is escaping from the amplifier of the positioner when the valve stem fluctuates on site.
I think it’s mainly caused by process fluctuations! 1. Pressure flow rate is related to the amount of purified air used in the process, rather than depending solely on the valve opening degree ； 2. Since fluctuations in flow rate and pressure inevitably lead to unbalanced forces, it is normal for the valve stem indication to fluctuate as well ； 3. It is recommended to use flow or pressure regulation control to ensure system stability.