Given a good measurement and final control element selection, location, and installation, we move on to designing the control scheme and achieving the best loop performance. The control system should minimize process interactions and optimize process quality, efficiency, and production rate. The control loops should minimize disturbances and effectively achieve new operating points. Here is a concise overview of how to succeed.
The control engineer should confer up front with the process and mechanical engineers to ensure their design minimizes process delays, disturbances, noise, and nonlinearity as discussed in the blogs on Mixing, UA, Pumps and Piping, and Equipment.
The control system first must manage inventories of gases, liquids, and solids. Equipment pressure loops control gas inventories. Pipe pressure loops control gas, liquid, and melt inventory. Equipment level loops control liquid and solids inventory. Properly designed inventory loops can inherently provide endpoint control and hence product composition control as noted in the blog Batch vs Continuous Control and Optimization – Part 3. The inventory controlled is a mass for purposes of material balance control but there are often constraints in terms of volume (e.g. high level). When starting up a process model or the plant, the inventory loops are some of the first loops to be commissioned to prevent levels or pressures from triggering Safety Instrumentation System (SIS) actions, opening relief devices, or sucking in equipment.
The control system must have a production rate controller to meet the dynamic demands of a flexible and efficient plant where inventories are minimized, fluctuating market demands are met real time, and advantage is taken of varying energy source costs as noted in the blog Flexible Manufacturing. For continuous operations, the production rate could be simply set by a flow loop that is the leader for the other flow loops. For batch and fed-batch operations, the production rate would be a batch cycle time controller that manipulates the scheduling of resources and feed rates. Valve position controllers can be quickly added to push production rates and minimize utility and raw material costs as discussed in the November Control article “Don’t Over Look PID in APC”.
The control system must be able to optimize product quality, minimizing the margin between supply and demand. Reducing recycle and waste particularly prevalent in startup and transitions is important in terms of environmental impact and waste treatment cost but also process efficiency and capacity. Temperature is often used as an inference of composition. The use of more online and at-line analyzers would enable better batch and continuous process quality control as noted in my December Control Talk column Process Analyzers. Analyze this!
The key to achieving these objectives is to choose the best pairing of manipulated and controlled variables per relative gain analysis, using cascade control to isolate nonlinearities (valve and process) and disturbances by a fast secondary loop, and using feedforward control for coordinating loops and rejecting load disturbances. Secondary flow loops can compensate for pressure upsets and installed valve characteristic nonlinearities. Secondary cooling and heating temperature loops can make the vessel temperature loop nearly linear. Secondary flow loops and plant wide flow feedforward control can enable a plant to reach and maintain optimum operating points as discussed in the InTech article “Feedforward control enables flexible, sustainable manufacturing.” Some key considerations are discussed in the answer to an ISA Mentor program question posted on ISA Interchange website How do you know when feed-forward control is needed?
So we have great loops. Are we done? Tune in next week.