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Steam Boiler Safety Interlock

By: Thomas Yoon

A boiler could be potential bomb if you do not take care of it sufficiently! Because it is a fired pressure vessel, it has the potential to be pressurized beyond its designed strength if something goes wrong with the operating equipment.

Water treatment has to be done carefully to prevent corrosion and scaling that can lead to deformation, overheating, and carry over of the water.

That is on the waterside of the boiler. What about the fire side? A strong fire is burning inside the boiler. The combustion equipment of a boiler must be maintained in a tip-top condition all the time.

Bad combustion will give rise not only to reduction in combustion efficiency, but it will also result in deposits of soot. The soot tends to cover the pipes, and the steel plates. The heat transfer will be reduced, or worst still, the soot may catch fire inside the boiler.

This is especially a concern in oil fired boiler. If the atomization of the fuel is not good, or the nozzle itself leaks, an imperfect combustion will be the result. Very often, oil may drip inside the combustion chamber or may be sprayed on the boiler tubes.

Automatic boiler burners are often started using an electric spark over a fine mist of light fuel oil. If the flame does not start, the oil must not be allowed to continue to spray inside the combustion chamber, otherwise a dangerous explosive mixture of fuel and air will develop inside.

So one of the boiler safety interlocks is to cut out the boiler burner when the flame fails to ignite.

Another interlock is low burner air pressure. If the forced draft fan does not develop enough pressure, combustible gases will not be able to be driven away. This is dangerous.

The above two safety interlocks work during the starting up of the boiler. How about the times when the boiler is already in operation?

The water level interlock is one of the most important safety features. If the water level in the boiler becomes too low, perhaps due to feed water pump failure, or other reasons, some parts of the tubes, or shell that are exposed to the heat from the flame will not be cooled by the water. The temperature will rise up and the steel parts will eventually lose their strength due to the intense heat from the flame of the burner.

The safety interlocks will shut off the burners of the boiler and prevent further damage.

The steam pressure will continue to rise as long as the burners continue to burn. How do boilers maintain the steam pressure?

Think Safety!

Until next time ...

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