On my first project as a combustion control engineer, I was responsible for loop checks and for watching the experts tune the system controls. The first loop I tried to tune solo was the drum level control. At that time the trend-tune program defaulted to a 2-minute window, and no one bothered to mention to me that the proper time span to tune drum level control to is 20 to 30 minutes. I also zoomed in on the drum level, which has a normal range of ±15 inches, though my trend range was ±3 inches. Finally, I did not know that drum level can be a very "noisy" signal, so the hours I spent trying to tune out that noise were wasted.
Eventually, I got the bright idea to add a little derivative to the loop control. In the time it took to program 0.01 as the derivative gain and then immediately remove it, the boiler tripped. Thus began my career in boiler tuning.
In the 20-plus years since my inauspicious debut, I’ve had the opportunity to successfully tune hundreds of boilers, new and old, that needed either a control loop tweak or a complete overhaul.
Many inexperienced engineers and technicians approach boiler tuning with a heavy hand and little insight into the inner workings of individual control loops, how highly interconnected they are with other loops in the boiler system, or what change should be expected from the physical equipment the loops are to control. My purpose in writing this article is to explore these fundamentals and share my experiences. I trust these insights will be of value to the power industry and specifically to those who want to tune boilers for rock-solid stability yet agility when responding to process changes.
What Constitutes Good Control?
Every boiler ever built has its own set of peculiarities. Even two boilers built at the same plant at the same time to the same drawings will have unique quirks and special tuning issues. I begin with a description of the various boiler and subsystem control loops before moving to good boiler-tuning practices that are sufficiently robust to accommodate even minute differences between what should be identical boilers.
From a pure controls perspective, the most important goal is to tune for repeatability of a value, not the actual value itself. We do not care that there are exactly 352,576.5 pph of fuel going into the furnace; we only care that, for a given fuel master demand, we get the same amount every time. There will be process variation, of course, but the goal is to tune the controls to keep that variation as small as possible and then tune for accuracy.
Boiler control processes are where I will begin. Additional control functions outside the furnace will be explored in Part II in a future issue of POWER.
Operator Controls
The operator’s window into the control system is referred to as a master or as a hand/auto station, control station, or operator station. The station is the operator interface to a given control loop and is typically a switch located on the control panel in older plants or accessible from the operator’s keyboard in those equipped with all-digital controls. Typically, the control station allows the operator to move between manual and automatic modes of operation. All of the control loops discussed in this article combine to form the set of controls that manage the key boiler operating functions.
When a control loop is placed in manual mode, the operator will have direct control of the output. In automatic mode the output is modulated by the proportional-integral-derivative (PID) controller. In automatic mode the operator usually has some control over the set point or operating point of the process, either directly or through the use of a bias signal. Occasionally, as in primary airflow control, the set point is displayed either on the controller located on the control panel or on the computer screen graphic display. Cascade mode is a subset of the automatic mode in which the operator turns over control of the set point to the master, whose internal logic generates the set point. Usually, there is some digital logic that requires the station to be interlocked to manual, as well as control output tracking and set point tracking.
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