How should the PID parameters be designed in this control system?
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I’m facing this problem now: the liquid chlorine system uses hot water to heat the liquid chlorine and cause it to vaporize, with the temperature of the hot water being maintained through additional heat supplied by steam. The issue is that my temperature measurement point is located at the inlet of the vaporizer; however, the pipeline between the vaporizer and the hot water tank is about 10 meters long and has an inner diameter of 150 mm. By the time the temperature at the measurement point reaches the set value, the actual temperature in the hot water tank has already exceeded that set value. How can I eliminate the interference caused by this distance? Urgently need a solution, thank you!
Just add a temperature measurement point to the hot water tank, and the problem will be solved. If only one point is measured and one wants to rely on lead time to predict the temperature of the hot water tank, this is only feasible if the process conditions are very stable. If one wants to control the temperature of the hot water tank, it is likely that there will be requirements regarding the vaporization rate of liquid chlorine. Otherwise, there is a requirement regarding the lower limit of the vaporization temperature; in that case, it doesn’t matter even if the temperature of the hot water tank has exceeded the set value.
A delay of 10 meters is very short, just a few seconds, and it does not affect control. There seems to be an issue with the control scheme or parameter tuning.
The key control parameters are vaporization temperature and pressure. It’s not the water temperature of yours; if the set value for the water temperature is too high, either there is an issue with the set value or there are errors in the process design calculations.
Thank you all for your answers. I have a question: is it possible to set two temperature measurement points? By comparing the values measured at these two points with the set values, adjustments can be made. Is that feasible?
The hot water flow rate is controlled using the pressure at the vaporizer outlet in order to maintain the pressure of chlorine gas. A temperature sensing point is installed in the hot water tank, along with a hot water temperature control circuit, to regulate the hot water temperature. Generally, the vaporization pressure of liquid chlorine vaporizers is an important parameter.
I don’t know what controller you’re using; if it’s a DCS, it can be set up in a cascade configuration. You can refer to some books for guidance!
Your main problem is that when the temperature reaches the set value, the steam valve closes, but due to the long length of the pipes, the steam inside them continues to raise the water temperature, causing it to rise further above the set value. You can lower your set temperature and observe each cycle until the range of fluctuations is within the acceptable limits to your satisfaction.