Regarding the issue of the outlet temperature of atmospheric-pressure furnaces, yet another common place with strange phenomena

Please answer my question: when using automatic interlocking and cascade control, which aspects should be considered?

Often, setting the value at +5 or -5 degrees is done to avoid exceeding the limits set by superiors, thereby reducing potential problems during evaluations. However, if there are such large fluctuations in actual production, it indicates poor operational skills, inadequate management, and unstable production processes.

The PID settings are incorrect; quickly go to the instrument to adjust the PID parameters

First, remove both the automatic and cascade controls to see if stability can be achieved. If stability is possible in manual mode, then adjust the PID parameters once the conditions are stable, and subsequently enable automatic control; after that, introduce cascade control.

Adjust the PID parameters! Also, as mentioned above, try it manually first; I think if the device continues to fluctuate in this way, it’s because the PID control isn’t adjusted properly, which results in a delay in the temperature response! Adjusting PID parameters is the best approach!

There are many factors that cause large fluctuations in the furnace outlet temperature. 1. Fluctuations in the fuel composition, especially those in the gas composition, have the greatest impact on the furnace outlet temperature. In such cases, using automatic control will only make the situation worse; it is better to switch to manual control temporarily until the composition stabilizes before reintroducing automatic control； 2. Fluctuations in the heat exchange network have a significant impact on the feed temperature of the heating furnace, which in turn affects the outlet temperature of the furnace. To stabilize the inlet temperature of the heating furnace, it is necessary to first stabilize the flow rates in various processes; especially for those processes controlled by liquid levels, it is important to maintain stable flow rates ； 3. Changes in the feed rate also have a significant impact on the outlet temperature of the heating furnace; if the feed rate to the furnace fluctuates, it is difficult to maintain a stable outlet temperature or to minimize any fluctuations ； 4. Changes in the properties of crude oil have a significant impact on the outlet temperature of the heating furnace. Variations in the water content of crude oil, as well as changes in the amount of light components in it, can also greatly affect the final temperature of heat exchange, thereby causing fluctuations in the furnace’s outlet temperature ； 5. Additionally, factors such as changes in ambient temperature, furnace negative pressure, air temperature entering the furnace, and even changes in atmospheric pressure can all affect the temperature at the furnace outlet

If the fluctuations in manual control are reduced, it indicates that there is an issue with the PID parameter settings.

Is the liquid level in your initial distillation tower stable? Will the final temperature be stable after the replacement? If the liquid level in the initial distillation tower is unstable, check the electrodeionization pressure and whether the feed rate is stable. The instability of the final temperature after the change indicates that the operation of your tower is also unstable.

Back then, the technicians in our area used two-type instruments, and their skill in operating the furnaces was excellent – the temperature variations were usually within 1°C! But perhaps the precision at that time wasn’t as high as it is now!