As stated in Part 1, PID loops are a central component of a modern boiler control system. In this section, we will examine some of the more common applications of PID loops in boiler control systems. We will look at four types of PID loops: Reverse Acting, Direct Acting, Cascading, and Override.
Reverse Acting PIDs
The most common PIDs found in boiler control systems are Reverse Acting PIDs. In this setup, the Control Output has a reverse response to changes in the Process Variable. When the PV increases above setpoint, the Output will decrease to maintain the setpoint. The below example shows a boiler steam pressure control loop.
Here, the error is calculated using SP-PV which results in a greater output as the Process Variable drops below the Setpoint. In this specific example, the Track Override input will set the “I” term equal to the Track Signal – “P” which disables the Integral for bumpless transfer and to prevent windup when the controller has been place in Manual, or if the burner is not in the Modulating state.
There are many other examples of reverse acting PIDs used in boiler control systems, such as:
- Drum Level
- Feedwater Pressure
- Full-Metered Fuel Flow
- Full-Metered Air Flow
- Oxygen Trim
Direct Acting PIDs
Direct Acting or Forward Acting PIDs work in the opposite direction of Reverse Acting PIDs. In a Direct Acting PID, the Control Output Increases as the Process Variable increases. The most common uses of the Direct Acting PID in boiler controls are for Draft Control and Feedwater Pressure when the Control Output is a feedwater circulation valve. Below is an example of a boiler stack Draft Control PID loop.
In this case, the Error is calculated by using PV-SP which results in the draft damper output increasing when the draft pressure signal increases. This can also be achieved by reversing the 4-20mA transmitter signal to 20-4 mA, which causes a safe condition of a wide-open draft damper when the draft transmitter fails or if there is an open circuit.
This example of Draft Control loop also includes the addition of a Feedforward input that is added together with the “P” and “I” terms. This input is an F(x) curve of the draft damper position at various firing rates that is setup when the controller is commissioned and tuned. For this loop, bumpless Auto/Manual transfer and windup prevention is achieved by forcing the Integral Term to be equal to the Track Signal – “P” – “FF”.
A Cascading or Nested PID loop involves taking the output of one PID and making it the setpoint input of another PID that then sets the Control Output. In cascading PID control loops it is necessary for the Integral time constant to be set significantly longer in the Primary Loop than in the Secondary Loop to reduce loop interaction and increase stability. A common example of a Cascading PID loop is 3-Element Drum Level Control as shown below.
In this example, the Primary Loop compares the current drum level reading to the expected drum level setpoint and adjusts the feedwater flow demand output. This output is then used by the Secondary Loop as the feedwater flow setpoint being compared to the Process Variable coming from the feedwater flow transmitter. The Secondary Loop then adjusts the feedwater control valve output to maintain the feedwater flow setpoint.
The Primary Loop also includes a Feedforward input from the steam flow transmitter that indicates when more feedwater will be needed to replace the increased steam flow and vice versa. Finally, the Track Mode Override input for both loops is controlled by the Auto/Manual state and a 1-Element fallback that occurs when the steam flow or feedwater flow transmitter signals are low.
One last form of the PID loop that we will be looking at is the Overriding PID loop. In an Overriding PID configuration, two PID algorithms work in parallel with two separate sets of Proportional and Integral settings. The choice of which PID output is passed along to the Control Output is determined by the process overriding conditions. There are a few variations of how an Overriding scheme can be set up Below is an example of and one to reduce the chance of nuisance high pressure trips on a boiler from overshooting the setpoint.
The Cutback PID loop will be set with a higher gain to cause it to react more quickly to setpoint errors. If the Primary PID loop is overshooting the setpoint and not dropping the Firing Rate output quickly enough, the higher gain Cutback PID will take over and reduce the firing rate faster. This application can be used in plants that can experience sudden load swings.
Next up in Part 4 we will look at PID loop tuning methods and rules of thumb.