AIR FLOW AND OXYGEN CONTROL

AIR FLOW AND OXYGEN CONTROL


This story concerns air flow and oxygen trim control. I also comment on the kind of things that can happen when years pass before a tuner is brought on site.

…Several years had passed since my last visit to this site. During the course of my work I discovered that the fuel master was no longer interlocked to manual when the FD fan maser was in manual. There is a hierarchy in the control loops that make up the combustion controls. It is as follows:

1. In order to place the air flow controls in automatic mode and keep them there the ID fan master (furnace pressure controls) must be in automatic mode.
2. In order to place the fuel master in automatic mode and keep them there the air flow controls must be in automatic mode.
3. In order to place the boiler maser in automatic mode and keep it there the fuel master must be in automatic mode.

This is a matter of safety. We want the furnace pressure and the air flow controls to be able to respond to fuel flow changes. If not, one can foresee a tired operator forgetting that his controls are in manual, increasing load beyond what the air flow can support, and if he panics, very bad things can happen.

The reason that this had been done became obvious rather quickly. There had been a long standing problem with the air flow control, especially at lower loads. The air flow characterization curve was a mess, the relationship between the air and fuel demands were hard for the operators to see, and at low loads the burner management system would inhibit firing or trip the unit if the air dropped too low. By the way, the BMS was acting correctly, the combustion control needed help.

Over the course of years, and lost in the shroud of antiquity by the time I returned to site, people unnamed had tried to address the situation. I am sure, that after trying their best, in exasperation, it was decided to disable the manual interlock to the fuel master that required that the air flow remain in auto.

Of course the proper thing to do at that point was to let the plant know the situation. Of course they knew that they were able to operate with the air in manual. They agreed that this was not how they really wanted to operate. But before I re-instituted the manual interlocks on the fuel master we had other fish to fry. The air flow and O2 trim problems had to be addressed, and the operators confidence in the controls improved, or else at the first instance of trouble, and with me gone from the plant, the controls would be put right back the way they had been.
The first thing to do was to address the air flow characterization curve. In the figure above the original curve and the curve as it came to be are displayed.

As you can see there is a discontinuity, or knee, in the original curve and it is very non-linear at lower loads. This is just where the problems that caused the manual interlocks to be ripped out occurred. This curve was changed by going to a few load demands, ranging from about 30% load to full load, and modifying the characterization curve, with the O2 trim in manual and at an output of 50%. This corresponds to a trim factor of 1.0, or no trim at all. As you can see, the resulting curve is much more linear, and therefore more believable. Then with a good air flow to operate on, the air flow demand was addressed. It turned out that the minimum air flow setting was at the exact value that triggered a furnace firing inhibit in the burner management system. So if the air flow dropped just a smidgeon below this value and then returned to the minimum, or slightly higher, the operator was not allowed to start any burners for five minutes. This was probably the straw that broke the camel’s back and had forced the manual interlocks to be removed from the fuel master, but we will never know for sure. Regardless, the low limit for the air demand curve was raised so that there was a cushion there for normal control deviation.
After this the O2 trim control loop was addressed. Because O2 trim control is an integral only controller, it does not have the dynamic capabilities of most controllers. As a result there are times when the controller should not be allowed the full range of control. At low loads, less than 30 to 35 %, the output from the O2 trim controller should not be allowed to go below 50%. At these low loads the air flow demand is at some minimum setting. The O2 trim controller should not be allowed to decrease air flow below this amount. As this control loop is intended to trim the air flow at steady state, it is a good idea to block any decrease in the controller output on any load change. Why is that? The question really should be; what is it that we want the boiler oxygen content to do? The answer to this is that on an increase in load we want the O2 to spike up and then return to set point. On a decrease in load we want the O2… to spike up and then return to set point. Blocking the decrease in the controller output on any change in load accomplishes this. Think of it this way, on a load decrease what will naturally occur? Due to the lag function in the cross-limited air demand the air will lag behind, that is the air will remain elevated for a period of time as the load, and the fuels, decrease. As a result the oxygen in the flue gas will spike up. If the O2 trim controller is not stopped from decreasing, the controls would see the O2 higher than set point and start cranking down. Then when the load gets where the operators have sent it, and the fuels are no longer decreasing, the air flow demand will catch up with the boiler demand and the O2 will, quickly, begin to fall. Now the O2 controller is not, nor should it be, equipped with proportional control. It will see the O2 falling and begin to crank the other way, but it cannot easily or quickly respond to the falling oxygen level, and then the O2 goes low. This tends to make operators nervous and unhappy. And if they get too unhappy, the controls are often placed in manual. So these proper limits and other normal bells and whistles were added or turned on. Finally, after the air flow and O2 trim control loops were placed in auto, and the operators saw, and more importantly believed that the controls were not going to bite them, the manual interlocks on the fuel master were reinstituted.