Q: The PID controller action is based on PV (process variable) – MV (manipulated variable) relationship; i.e., boiler attemperator PID control is a direct action because if PV increases, MV has to be increased. Boiler fuel oil flow PID control is a reverse action because if PV increases, MV has to be decreased. I came across your answer in February's Control magazine ("Deaerator Level Control"), saying that, "In your deaerator level control system, the make-up valve must have an open-failure position, which means that the controller has to be reverse-acting." Does that mean that the PID controller action is based on valve failure position?
A: The valve failure position represents where the valve would move on the loss of the control signal. A valve that fails open is a reverse-acting valve. Coolant valves are normally reverse-acting, and heating valves are normally direct-acting; consequently, most temperature controllers are reverse-acting. In fact, most controllers are reverse-acting.
Q: In one of your articles you discussed the gains, time contants, modeling and controls of non-industrial processes. Are the measurement devices and control models that were developed for industrial applications useful to reliably determine the trends of global processes? Is the knowledge gained in controlling industrial processes useful in modeling global ones, and predicting the future values of the variables involved?
A: Yes, a process is a process, no matter if it's as simple as a heat exchanger or as complex as the weather, the economy or human evolution. The only difference is that global processes tend to have high inertia, require envelope control, and their manipulated variables are not control valves, but such complex variables as human behavior. Therefore, if we want to keep such processes on setpoint (inside a safe envelope), we have to influence (manipulate) human behavior, and that's not as easy as throttling a valve.
Probably the most obvious global process that is out of control today—like the level in a tank where inflow exceeds outflow—is carbon accumulation in the atmosphere. The contribution the process control profession can make to close the debate about the dangerous consequences of this accumulation is first to find a measurement that is undeniably unique, and then reliably determine the time constants, inertia, gain, etc., of this carbon-temperature loop, and predict where the uncontrolled variable (global temperature) will get to if the process stays in the "open loop." Similarly, process control modeling can determine how this loop can be "closed," and after that, how long it will take to reach a particular steady state.
We know the carbon dioxide concentration of the atmosphere today, and we have measured what it was during the past half million years. We know that, in spite of ice ages and asteroid impacts, etc., it has never exceeded 280 ppm. We also know that today it is 390 ppm (Figure 1), and that at the present rate of rise (open loop), it will reach 510 ppm by 2050.
Yet this knowledge is not enough to prove that CO2 emission is the cause of global warming, nor is it enough to calculate how much time we have before the consequences become disastrous. Why not? Because the deniers can still argue that something else is the cause. Yet it is in everybody's interest to close this debate and process control modeling can do that! How?
Well, it is not enought to measure the rising temperature of the oceans or the melting of polar ice. We need a variable that is "CO2-specific" and is affected by nothing else! It is not enough to know that millions of years ago the Permian volcanic eruption caused the extinction of 95% of the planet's species because we do not know for sure if that extinction was caused by catastrophic amounts of carbon which were sent into the air. It is not enough to know that we humans are every bit as good (actually better) at putting carbon dioxide into the atmosphere than are volcanoes. What we need is a reliable "CO2-specific" measurement and an undeniable method to calculate the gains and time constants of this loop. In my view, that reliable variable is the pH and the salt content of the oceans.
During the past century, the pH of the oceans dropped by about 0.3 pH, and the rate of this acidification is rising. Nothing else can cause that but CO2. Based on pH, we (the process control people) can accurately predict where this "open loop" process will take us in a year, a decade or in a century. We can also calculate the size of the "manipulation" needed to reverse this trend and reach a steady state which we can live with.