Steam Boilers' Control Automatic

By Lars Josefsson

Is an automatic controlled boiler an explosion risk?

An easy way to find out if your boiler control system lights up the first burner safely.
Safely shut off the fuel supply to the burner before the test.

1. Power up the boiler control system.
2. Start the burner.
3. Start a stopwatch when the combustion air damper has reached its maximum position.
4. Stop the stopwatch when the combustion air damper begins to close again, and note the purge time. The air in the furnace should be changed at least five times during the prepurge period.
   Find the furnace volume and the fan capacity from the documentation’s and calculate the purge time needed. The time must never be less than fifteen seconds even if your calculation says so.
5. Start the stopwatch again when the automatic fuel oil shutoff valves open.
6. Stop the stopwatch when the fuel oil shutoff valves close and you get a flame failure or misfiring alarm, and note the trail-for-ignition time. If the time you get is more than fifteen seconds, then you must not ignite the burner ever, until the time has been adjusted. Five seconds is a relevant trail-for-ignition time, but different classification societies specify different maximum time. Get the correct maximum time from the rules of the actual classification society.
   N.B. This shut off delay is only allowed during trail-for-ignition. When you got at flame failure during normal firing the fuel oil valves must shut off instantly.

Some further checks to improve the safety

1. The fuel oil flow during light-up must not exceed 20% of the full load flow, but in burners with limited turndown ratio the burners' minimum load has to be accepted.
2. A corrupt flow transmitter signal may cause sever problems therefore:
   • When purging the furnace with air prior to light-up, the position of the combustion air damper should be confirmed by means of a limit switch rather than relying only on the air flow transmitter's signal.
   • At burner light-up the position of the fuel oil control valve and the combustion air damper should be confirmed by means of limit switches rather than relying only on the flow transmitters' signals. You should of course use the transmitters' signals, but they ought to be confirmed to be reasonable by means of limit switches.
3. Direct the light from a flashlight onto the flame scanner sensors, when the burner is off, to confirm that the auto-check-function works correctly and you get an alarm. If you get any other action, such as opening of the fuel valves, then your system needs a thorough improvement.
4. Using the igniter during the post-purge of the last burner's lance (or a single burner's lance) has some disadvantages. Upon reset of the system, after a flame failure, the igniter will start firing before the furnace has been properly purged with air, which will cause impending risk of furnace explosions. Consider the following:
   • A well-tried method to purge the fuel line and the burner-lance slowly is to let the fuel continue to burn, without igniter support, until the lance is empty.
   • Not purging the burner-lance at all is another method, but it requires a stand-by heating of the tubing and the lance to keep the fuel sufficiently heated to be floating.
   • What ever you do, secure that the igniter not under any circumstances starts before the furnace has been properly purged with air.