Why do boiler furnace explosions occur?

I believe that everyone of you have heard about or, unfortunately, witnessed at some point in your career a boiler explosion due different reasons. Unfortunately, despite all safety features, checklists and procedures in place this kind of accidents occur and they are mainly caused by human errors.

Example of a clean boiler burner

Explosions in boiler furnaces mainly happen when unburned fuel collected in the furnace or air box, vaporizes and mixes with other elements to create an explosive mixture. If this mixture of flammable vapours comes in contact with a source of heat, an explosion will happen.

Example of boiler fire due unburnt fuel collected into the air box

In a way similar to crankcase explosions, there is sometimes a primary explosion that breaks some part of the structure, followed by a much bigger secondary explosion as more air and fuel are vaporized by the first explosion.

So, where does the unburned fuel come from that causes a boiler furnace to explode?
Unburned fuel can build up in the furnace or air box if:

    • the boiler can’t be started, maybe because of a problem with the igniter or because the fuel isn’t mixed well enough;
    • fuel leaks past broken fuel oil shut-off valves while the boiler is off;
    • fuel spills into the furnace during maintenance;
    • the boiler doesn’t burn the fuel well enough while it’s running, causing unburned fuel to build up in cooler parts of the boiler. This could happen if someone tries to burn fuels in the boiler that it wasn’t made to burn, like sludge or very low-grade heavy fuel oils.

Now that we know where the fuel for an explosion can come from, let’s look at where the heat can come from.

    • When you try to light a boiler, you add a source of heat, which can cause an explosion if there is flammable vapour in the furnace;
    • If the boiler was just fired up, this could definitely be the source of the heat. This is because the refractory on the walls of the furnace will still be hot enough to both turn unburned fuel into vapor and start a fire;
    • When hot boiler furnaces were opened up for maintenance and fresh air rushed into the furnace, this can cause an explosion if unburnt fuel is present inside.

What are the conditions for a boiler to work correctly?

Let’s assume that everything is fine on the water and steam sides and that all the devices we need are working and set up correctly. For good combustion, we will need:

    • a good supply of fuel at the right pressure and temperature for atomization;
    • any other service needed to get atomization and this could be done by atomizing steam or by atomizing air from a supply fan.
    • a way to fire the boiler, which is usually a spark made by electricity from a pair of electrodes or something similar. In some situations, a pilot burner that uses distillate fuel is used to light the main burner.

Example of boiler ignition electrodes

Auxiliary boilers have a number of safety features that are meant to stop explosions in furnaces. These are:

    • The boiler purging sequence – before every attempt to light the boiler, an automated boiler will run through a “purge” sequence. This means that the combustion fan blows air through the furnace to remove any explosive present vapours. During the purge sequence, up to seven times the volume of air in the furnace is removed in order to ensure that any explosive vapours are pushed out of furnace.
  • Example of boiler modulation device

    • Double shut-off valves on the fuel oil system – these should prevent leakage of fuel oil into the boiler when the boiler is not meant to be firing.
  • View of double fuel oil shut-off valves

    • A low pressure trip for boiler fuel oil supply – this should stop the boiler in the event of low fuel oil pressure and help prevent poor atomization and combustion due to low fuel oil pressure.
  • Example of fuel line pressure switch

    • A low combustion air pressure trip – this should prevent poor combustion or flame failure in the boiler if the combustion air fan is damaged or not running. For boilers that use steam or air to assist atomisation, an alarm and trip will be fitted to prevent poor combustion or flame failure if the atomising medium is not present.
  • Example of primary and combustion air pressure switches

    • A flame detector – the flame detector may  have a number of roles. Its main purpose is to make sure that there is a flame in the furnace and to stop and cut out the boiler in the event of flame failure. This prevents fuel entering the furnace if the flame is extinguished for any reason. The flame detector  will also prevent burner start up if there is a flame already present in the furnace. This is partly a self check, to ensure that the “no-flame” condition can be identified by the detector. If there is a flame in the furnace before burner start up, this would indicate that there is unburned fuel, and therefore possibly an explosive fuel vapour mix. When flame failure occurs in the boiler, the safety shut-down solenoid valves shut off the fuel supply to the burner within few seconds and the control locks out the burner, requiring a manual reset.

Example of boiler flame detector

Automation is there to help prevent boiler furnace explosions, but when boiler is operated in manual or emergency mode, the operator should follow the procedure and operate the boiler with caution in order to keep everyone safe. In order to do so:

    • Make sure that there us enough time for purging the furnace, with the air dampers fully open to ensure that any dangerous vapours are pushed out.
    • Never try to reduce the purging time or bypass the purge sequence.
    • Carry out the correct boiler combustion equipment planned maintenance.
    • Only use fuel that the boiler is designed to use, and beware of fuels which have high wax points when in cold temperatures.
    • Make sure that the air fuel ration is correct and is giving complete combustion. Too much air, although inefficient, is safer than not enough.
    • Keep any heat exchangers or economizers fitted to the boiler clean.
    • Soot blow or water wash as required to avoid back pressure in the furnace.
    • Do not repeatedly reset the boiler after a flame failure. Find and fix the fault.

If you have any questions regarding above, please feel free to use our existing forum Seafarer’s World and will try to answer to all your queries.

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