Introduction
In process control design, there are countless “standard solutions” widely accepted in textbooks and control system guidelines. However, real-world applications often present unique constraints, legacy configurations, and operator habits that challenge theoretical ideals. A mature and pragmatic control engineer must learn not to rigidly impose theoretical models but instead respect existing operational knowledge—especially when current solutions, although seemingly unreasonable, are functioning effectively.
1. Respect Existing Control Logic Before Making Changes
When arriving at a new site, it’s tempting for engineers to immediately “fix” control strategies that seem flawed based on theoretical knowledge or prior experience. However, this instinct—while well-intentioned—can be immature if not guided by a deep understanding of the existing system and its operational history.
One must always consider the technical cost of changing control logic, especially within a DCS (Distributed Control System). Even small modifications can introduce instability or create unintended consequences. Therefore, the best approach is to make only the minimal, necessary adjustments and to prioritize the restoration of existing control strategies before considering replacements.
2. Case Study 1: Reboiler Steam Control in Distillation Columns
In one plant, I encountered two adjacent distillation columns. Both used a rare control strategy—steam pressure cascade controlling steam flow—to regulate reboiler heating. This method is quite uncommon, as most setups simply use direct steam flow control.
Interestingly, one column operated with acceptable stability, while the second displayed persistent oscillations. Operators were unsure how to intervene, and the cause wasn’t obvious. Upon deeper analysis, I concluded that the issue stemmed from the steam pressure control loop. By switching to an advanced control approach that directly controlled steam flow, the column’s performance improved significantly—smooth, stable, and operator-friendly.
However, I chose not to modify the first column. Despite sharing the same uncommon configuration, it was operating effectively with no operator complaints. Why “fix” what isn’t broken? This underlines a key principle: just because a control loop is unconventional doesn’t mean it’s wrong. If it works, respect it.
3. Case Study 2: A Creative Take on Liquid Level Control
At another facility, two saponification columns had different outcomes using what appeared to be the same liquid level control strategy. One worked flawlessly. The other struggled—operators constantly complained that the level transmitter was faulty, and despite multiple service attempts, the problem persisted.
Surprisingly, the operators had developed their own workaround. They ignored the level transmitter and instead monitored the suction pressure of the bottom product pump, using it as a proxy for liquid level. This insight had never been formalized into the control system, as it seemed too unorthodox.
Recently, we introduced an advanced control scheme that used either the tank level or the pump inlet pressure as the controlled variable—allowing seamless switching between the two. Operators loved it. The control was stable, intuitive, and finally aligned with their field knowledge.
Interestingly, we made no changes to the second saponification column—it was running fine without intervention. Again, we applied the principle: if it’s working, don’t force a change.
4. Control Strategies: Many Roads to the Same Goal
In another distillation tower I worked on, the pressure at the column bottom was used as a cascade variable to control reboiler steam flow. This setup made little sense theoretically—but in practice, it worked.
This led me to a crucial realization: principles are not absolute; they are part of cognition that can evolve.
There are many acceptable ways to achieve process goals:
Control bottom level using top pressure.
Control bottom level through reboiler steam flow.
Or control it via top product flow.
Operators often create these methods out of necessity, not textbook theory. What may appear “illogical” may actually represent ingrained operational wisdom.
Conclusion: Embrace the Pragmatism of Process Control
Engineering is not about proving theories—it’s about solving problems. In process control, there is rarely a single “correct” approach. Each site is different. Operators are part of the system. Their experience, habits, and local knowledge must be respected.
Effective control doesn’t always look elegant in a PID diagram. Sometimes it looks like a patched-up, semi-logical, field-born workaround. But if it works—and it’s safe—respect it.
The art of process control is about balancing theory with field reality, and knowing when to challenge a strategy and when to leave it alone.