How to tune combustion control systems
In the previous post, we covered the basics of how gas/oil burners work. This post covers how to tune the combustion control systems to make it as safe and efficient as possible.
In the previous post we covered the basics of how gas/oil burners work. By what procedures should we tune them? There are three aspects to tuning a gas/oil burner combustion control system (CCS):
- Setting the floor area ratio (FAR) curve
- Setting curves for output devices
- Tuning proportional-integral-derivative (PID) loops.
Don't shortcut safety
We controls people are able to defeat burner management system (BMS) interlocks to run a burner when the BMS would not otherwise let it run. But do NOT do this.
Fix the problem. Calibrate or replace the part. Don't be in a hurry. Don't shorten purge timers. Push back against anyone who prioritizes schedule over safety. Most boiler explosions happen when people who know what they are doing are in a hurry and bypass safety equipment.
This is a serious, life-and-death issue.
Prerequisites for tuning a gas/oil CCS
Before starting to tune a gas/oil CCS:
- Make sure the instrumentation and outputs are calibrated and working properly. The accuracy of the O2 sensor is critical-if it is out of calibration, the FAR curve and almost everything to do with air will be wrong.
- The BMS and its instrumentation need to be in good working order. It needs to be able to light off.
- If possible, the CCS human-machine interface (HMI) should have a screen listing all the parameters you will be recording, in order. This will help with recording consistency and save time hunting for all your variables. A nice additional feature here is to display smoothed values, run through a longer filter than you would use for control or normal displays.
- If you will be recording any values from gauges that do not run to the CCS, identify their locations, tag names, and ranges before beginning.
- Ideally, the HMI will also have a trending package you can use to help tune loops and to go back and verify suspicious or outlying recorded data.
- I recommend tuning the drum level control first, and tweak it as you go if necessary, so that you can focus on combustion tuning without tripping the unit on drum level.
- Be prepared to produce a lot of steam, and to possibly trip a boiler at high load. If your plant is already in use, the ideal situation is for two boilers to be running in automatic, and for the facility to be consuming at least 20% to 40% more steam than the boiler you will be tuning can produce. If it is not consuming that much, be prepared to open a sky valve or to have another method to waste steam. Tuning a boiler without running it all the way to its maximum is not a good plan. As the boiler runs from minimum to maximum, the other two boilers will automatically back off. If you trip the boiler you are tuning, it's easier for two other boilers to quickly take the lost load than just one, which might also trip on such a rapidly changing demand.
Setting piecewise characterizations
Setting piecewise characterization (PWC) pairs is a significant portion of tuning a gas/oil CCS. The general idea is to run the boiler from minimum to maximum load, recording all the key parameters, including anything related to the input or the output of these PWC blocks. Once that data has been collected, it can be plugged into the PWC table.
Tips on plugging recorded data into PWC tables:
- After changing load, wait a few minutes to allow the system to stabilize at the new level.
- Values jump around (especially airflow), and you want to record the average value for the steady-state condition. If you have a system with heavily smoothed values on a special tuning screen, you can just run down the list and write down the numbers. Otherwise, watch each value for a moment, or look at it in a trend to get a sense of the average value it is jumping around, and write that down.
- PWCs should start with a pair at (0,0) and end with a pair at or above (100%,100%). For example, the furnace may need more than 100% nominal airflow depending on conditions, so if the 100% load nominal airflow is 70 k#/H, the last X on your curve should extend to perhaps 90 k#/H. Output device curves should end at (100%,100%), not above.
- You should have a look at your curve before plugging it into a PWC. Put the values in a spreadsheet and produce a chart (in Excel you want "XY scatter"). You will probably notice that your line is a bit "bumpy." You should adjust your curve numbers to smooth out these bumps, and enter those numbers into the PWC. This will also help you pick reasonable values that are beyond the maximum the boiler could be run at the time of tuning.
Some actuators, particularly air dampers, have a lot of hysteresis. They produce a very different flow when the command is raised to a position than when the command is lowered to the same position. This can make it difficult to make the fine adjustments required to get the O2 close to its setpoint (SP).
To overcome this, make small adjustments by raising the command by around 10% for a second or two, then bring it back down to the new position. It will take a bit longer to settle out, but that may enable you to get closer to your SP.
You may also want to quantify the hysteresis. For the air damper, this is easiest to do with the burner off: Run the damper from minimum to maximum in small increments, recording the damper position command and resulting airflow. Then run it back down to minimum on the same increments and record them again. The difference in flow at each position tells you the amount of hysteresis.