Furnace Pressure Control
Furnace pressure control is a fairly simple loop, but it’s also one that has important safety implications. The National Fire Protection Association (NFPA) codes, such as NFPA 85: Boiler and Combustion Systems Hazards Code, are dedicated to fire and furnace explosion and implosion protection. Before you begin tuning a boiler, you must read and understand the NFPA codes that apply to your boiler.
Balanced draft boilers use induced draft (ID) fans and/or their inlet dampers to control boiler furnace pressure. The typical control system has one controller that compares the difference between the furnace pressure and the furnace pressure set point that uses a feedforward signal usually based on forced draft (FD) fan master output. The output from the controller typically is fed through an ID fan master control station. Smaller units may have a single ID fan, but larger units usually have two or more ID fans. The most I have seen is eight ID fans for a single unit. In this case, the output from the control loop or master is distributed to the individual fan control stations.
The NFPA also requires some additional logic for the furnace pressure control loop to ensure adequate operating safety margins. There should be high and low furnace pressure logic to block the ID fan from increasing or decreasing speed, as is appropriate. For example, because this fan sucks flue gas out of the furnace, on a high furnace pressure signal the fan should be blocked from decreasing speed and on a low furnace pressure signal it should be blocked from increasing speed. On a very negative furnace pressure signal, there should be an override that closes the ID inlet damper or decreases ID fan speed. The settings of these signals are determined by the boiler and fan supplier during the design of the plant.
Also, on a main fuel trip (MFT) there should be MFT kicker logic. An MFT occurs when the burner management system detects a dangerous condition and shuts down the boiler by securing the fuel per NFPA and boiler manufacturer requirements. When fuel is removed, the flame within the furnace collapses violently, which can cause a lot of wear and tear on the boiler and related boiler equipment. It also presents the very real danger of an implosion. The MFT kicker should immediately reduce the control output to the fan(s) proportional to the load being carried at the time of the MFT and then release the device back to normal operation.
I am constantly amazed at how well furnace pressure can be controlled, especially when you consider the amount of fuel and air being injected into a ball of fire many stories tall and the ferocious and chaotic environment inside a boiler. The fact that a well-tuned system can maintain furnace pressure to – 0.5 inches H2O is remarkable.
A typical mistake made by boilers tuners is the use of very fast integral action to the furnace pressure controller. Furnace pressure changes quickly, but not instantaneously, so consider the size of your furnace and the amount of duct work between the furnace and the fans as capacitance in the system, because air is compressible. I recommend restraint when tuning furnace pressure when it comes to adding integral gain. Interestingly, the feedforward for almost every boiler is on the order of 0% to 100% in, and 0% to 80% out.
The trends in the following figures show what you should expect to see from your furnace pressure control. The plant from which these data were taken uses both fan inlet damper position and fan speed to control furnace pressure. Figure 1 illustrates an ID fan tuning trend and the reaction of the ID fans and the furnace pressure to a change in set point.
1. Blowing hot air. Induced draft fans are used to control furnace pressure and primary combustion airflow. In this test, induced draft fan and furnace pressure respond to a step increase in furnace pressure set point. Source: Tim Leopold
Email
Comments
Print
Reuse
