Knowledge of the flows and the accumulation of material in a unit operation are fundamental to the understanding and analysis of process and equipment performance. Flows are the primary way of affecting the process. Root cause analysis requires sensitive and repeatable flow measurements. I have seen costly expert systems fail to deliver benefits because of missing or inaccurate flows ("Drowning in Data, Starving for Information - 1").

The process gains of the more important process variables (e.g. composition, pH, and temperature) are best quantified and visualized in a plot versus a ratio of flows (e.g. coolant/feed, reactant A/reactant B, reagent/feed, reflux/feed, and steam/feed). If you are still into differential equations, you can checkout my Advanced Application Note 4 to see how process gains are dependent upon the ratios of flows.

The importance of flow ratios for affecting the process is seen in the prevalence of flow ratio control as detailed in my entries "What Have I Learned? - Flow Ratio Control" on this website.

The amount of time material spends in a unit operation is critical for crystallization and reaction. For continuous operation of well mixed volumes, the amount of time is the residence that is the fluid volume divided by the total throughput flow. Conversion is maximized by increasing volume or decreasing feed flows. For batch processes, the amount of time is the cycle time. Conversion is maximized by charging the feeds as fast as possible (increasing feed flows), to leave more of the batch cycle time for conversion.

In the direct material balance control scheme where the distillate flow is manipulated for overhead receiver level control, the sensitivity of the temperature and hence the composition control requires an exceptionally sensitive level measurement, low noise, and a high controller gain. Changes in distillate flow do not affect the column until there is a corresponding change in the reflux flow that maintains the material balance.

Then of course, there is the need to minimize the amount of storage of materials in the process. Ideally, storage tanks would be almost empty with just enough raw materials and intermediates to continually meet the flow demand of downstream operations and just enough products to continually meet the flow demand of customers.

For more information on how advances in flow and level measurements can improve material balance control, residence time control, inventory control, and process analysis and modeling, checkout "Advances in Flow and Level Measurements Enhance Process Knowledge, Control"

This piece could have been titled "Exceptional Failures in Process Control." Despite my 25 years of explaining the importance of using middle signal selection, I don't see much evidence of what I have said has taken root outside of Monsanto and its spin-off Solutia, where it became a part of the culture and best practices to use middle signal of three pH electrodes for all important pH control loops. We ended up taking a view that all pH loops are important because if they are unimportant why should we go through the maintenance headaches and the risk of control system failure by installing a loop dependent upon the integrity of a single electrode. I think the main hurtle besides hardware and installation cost is the feeling that if one electrode requires so much effort, why should I add more? If the electrode life expectancy is too short, the feeling is right. We should not add more of a bad application or installation. Instead, we need to find a better design, technology, implementation, and location verified in testing via wireless pH or an alternate measurement (e.g. conductivity for concentrated acids or bases).

As an important side note, the use of three transmitters and middle signal selection on all of the important measurements (e.g. flow pressure, temperature, and level besides pH) used in the control system and safety system for a large intermediates plant has consistently eliminated false trips saving several million dollars per year.

What boggles my mind is that the risk of poor product quality and an environmental violation do not provide a wakeup call that the lifecycle cost of the measurement itself is insignificant in comparison to the risk. A simple quick ball park benefit versus cost analysis would show the absurdity of when the dollars of events likely to occur each year is more than 1000 times the dollars of the additional automation to prevent them. Unfortunately, we tend to get too focused on short term costs. Consider bioreactor batches worth millions of dollars each of a sold out pharmaceutical that are dependent pH upon control to within 0.02 pH. A second electrode is added but I am not sure it helps or just adds to the confusion. I find it almost bizarre how favorite electrodes are picked for the loop's PV and the stories that ensue about which one is best. It is a "fact of life" that electrodes will not agree in the short term due to non-ideal effects too numerous to get into here (check out my ISA book Advanced pH Measurement and Control for more info and previous blogs by searching for "pH"). The continual disagreement between two electrodes often leads to calibration adjustments chasing calibration adjustments. If left alone, the electrode that reads high today may in a couple of hours or at least by tomorrow read low. Electrodes can fail anywhere on the scale (including the most insidious failure of all type where the bad electrode signal is stuck at the pH set point). I maintain that the correct use of middle signal selection will actually reduce the long term maintenance cost by simple observation and the use of more intelligent practices eliminating unnecessary calibration and removal of electrodes.

A middle signal selection inherently ignores a single failure of any type and avoids the slowest electrode (e.g. coated electrode). This selection reduces noise and eliminates spikes without any addition of a signal lag like what you get from signal filtering. Middle signal selection also ignores an electrode with lower efficiency (shorter span) or that is drifting. Theoretically, electrodes of different "in service" time should be used to reduce the occurrence of concurrent failures. The middle electrode is the best signal on the average, but please don't use the average. I have seen some very smart attempts of computing average signals with built in intelligence on signal rejection that were out foxed by a single electrode failure scenario. You would think you could devise something smarter than the simple middle signal selection when in fact inherently it is impossible for a single failure. There can be additional intelligence for more than three electrodes or for protection against multiple concurrent failures.

To summarize, middle signal selection can improve process quality and on-stream time, reduce maintenance, and prevent environment violations by adding understanding and ignoring spurious signals, inaccurate measurements, and failures.

A prolonged deviation from the middle should be alarmed because if you don't fix the first failure or sustained error, middle signal section has a fifty-fifty chance of preventing the next failure or electrode inaccuracy. I could go on and on but I suspect you are pressed to move on. Before I go let's be frank with closing remarks in recognition of an engineer named Frank who was particularly astute at telling it the way it is.

There is an opportunity to use a statistical or first principle model based on titration curves to generate a third signal. Even if the model is wrong, it will be ignored by middle signal selection. There appears to be here mostly an upside where you will at least learn more about your process by developing a model.

I have no illusions as to whether this blog will change one person's mind enough to install middle signal selection even though it is a feature of a standard function block. I also have no expectations that enough users will see the need to take advantage of wireless measurements to eliminate the wiring installation and maintenance costs of going to three measurements. Even more unlikely is that users will end up using middle signal selection enough that it will be offered in a smart transmitter that inputs three electrodes even though electrodes are the weak link in regards to accuracy and reliability.

Startups, grade transitions, and abnormal conditions are the most difficult, operator intensive, hazardous, and inefficient periods of plant operation. Operators often believe these conditions require operator evaluation and action. The conditions are thought to be too special and the response too situation dependent to automate. The operators are right in saying these periods of operation require the best in operator expertise. However, case histories show that the power of the PID can be used to automate the best operator responses and build on them to provide faster, safer, and more efficient plant operation during these difficult process conditions. For some specific examples dealing with compressors and reactors check out the two chapters "Wally and the Beave Automate Reactor Startups" and "Wally and the Beave Return to Automate Another Reactor Startup" in my E-book on this website A Funny Thing Happened on the Way to the Control Room. For impressive examples for chemical, mining, and pulp and paper operations, check out the Control Talk columns "Show Me the Money - Part 1" (November 2009) and "Show Me the Money - Part 2" (December 2009) in Control magazine.

An extensive interview of the operators and process engineers is necessary to capture the best responses for a preliminary functional description of the control system. There are often a lot of surprises hidden by the diversity of actions that are inevitable from human responses. Free will implies these decisions are basically unpredictable. The operator actions consistent with first principles and process knowledge offer a good starting point but not the final strategy. During the commissioning of the control system, the plant response must be carefully observed and the best operator actions verified and improved by the use of the many options built into a PID loop to deal with rampant problems as the plant goes from zero to full rate, or vice versa. For example, output tracking, dynamic reset limiting, set point ramping, PID structure, gain scheduling, adaptive control, and override control can be used to deal with the problems at low rates such as noisy or inaccurate flow signals, excessive valve stick-slip near the closed position, larger transportation delays, and unrepresentative measurements. One of the common solutions is to head start (initialize) the controller output via output tracking to the best valve position for startup, transition, or abnormal situation. The initial position can be a "Full Throttle" position for fastest set point response. When the set point approaches the set point, the controller output can be momentarily set to a resting value based on experience or average position captured from a representative operating point from the last run. For fast loops such as flow and pressure, the resting value can be used as the "head start". One of the common mistakes is for process engineers to get carried away with trying to sequence the PID controller output too much or hold the controller output in the track mode for too long. For shutdown, the output must normally be held but otherwise the PID controller should be returned to automatic as soon as possible to deal with disturbances, unknown process effects, raw material variability, and nonlinearities. The process is not known or measured well enough to sequence flows without feedback control. It is particularly important to return pressure loops to automatic as fast as possible. Smart techniques for startup, transitions, and abnormal situations that take full advantage of the flexibility of the PID controller have been the source of the most impressive benefits in process control improvement. In general, these were also "quick hits" in that they were implemented in a matter of a couple of weeks by just configuration changes and controller tuning.