Like capacitors do for electrical circuits, vessel levels provide capacity that can absorb variability within the process. In many cases, a properly tuned level controller can make the variability of the vessel outflow can be much less than the variability vessel inflow. Of course, if it’s not properly tuned, the variability can pass right through the vessel or even be amplified. In fact, an improperly tuned level controller can make the variability of the outflow higher than that of the inflow! Unfortunately, the latter cases are common and directly impact process efficiency and product quality.
I caught up with Emerson’s James Beall, whom you may recall from earlier posts. James is a process control veteran with 27 years of experience including the last seven as a process control consultant. He’s also chairman of the ISA 75.25 committee on control valve dynamic testing.
James stressed that there are often different objectives in tuning level controllers. What’s common is to make sure this tuning is not creating variability on its own. Sometimes you want to hold the level very close to the setpoint at the expense of aggressively moving a manipulated variable. Other times, like in the case described here, you want to use the capacity of the level system to absorb variability in the process and very smoothly move the manipulated variable as little as necessary.
From James’ experience, good level tuning techniques for absorbing process variability are not widely known. When the level controller for a vessel is properly tuned, variability can be reduced by a ratio of a 20:1 or more depending of the nature of the variability and process constraints. For example, a vessel with an input flow which varies plus or minus 20% of the inflow can have the variability reduced to plus or minus 1% of outflow.
Most level processes have an integrating process response. This means the level is an integration (or accumulation) of the difference between the inflows and outflows of the vessel. Absorbing process variability requires that the level control be tuned as slow as possible but still fast enough to hold the level within the allowable deviation for the maximum expected load change.
For an integrating process, Lambda is the period of time from the start of the step change in load until the process variable has stopped changing as a result of the level controller action. The level controller tuning parameters can be calculated to achieve a specific Lambda.
James notes that the required Lambda for maximum variability reduction is a function of the allowable level variation, the product of the integrating process gain and the maximum expected load disturbance. He uses this technique on a number of applications including feed tanks, distillation column bases, intermediate tanks, and reflux accumulators.
He mentioned that in some cases, the level actually needs to be controlled very close to the setpoint. Examples of this case include boiler drum levels, refrigeration evaporator levels, reactor levels, etc. He also has techniques for calculating the level controller tuning for “tight level control” which we’ll explore in future posts.
James has worked with process manufacturers where reoccurring annual benefits from proper loop tuning have yielded savings from several hundred thousand dollars to several million dollars.
If you have the time and inclination to learn more about these concepts, James recommends three courses offered by Emerson Educational Services: Process Dynamics, Control and Tuning Fundamentals; Process Analysis and Minimizing Variability; Modern Loop Tuning.
If you don’t have the time or inclination for these courses but need help in addressing process variability issues like level control, you’ll want to connect with James and the variability management team.