A majority of the boiler related accidents occur because of water level falling below the low level. For safe boiler operations it should be ensured that the boiler low water level alarm is functional at all times.
Most industrial boilers are ‘natural circulation’ boilers i.e. they do not utilize pumps to circulate water through the boiler tubes. These boilers rely on the differential density between hot and cold water to provide the circulation. As the water travels through the tubes it removes heat from the tubes, the water temperature increases and it rises to the boiler steam drum. Eventually, sufficient heat is transferred and steam is generated. Colder feedwater in the steam drum replaces the water that rises, thus creating a natural circulation of water.
Low-water level conditions have the potential to severely damage the boiler. The furnace temperatures usually exceed 1,800°F (980°C) and the only thing that allows a boiler to withstand these furnace temperatures is the presence of water in all tubes of the furnace at all times that a fire is present. The strength of steel drops sharply at temperatures above 800°F (480°C) and so a Low-water condition has the potential to melt steel boiler tubes.
Due to the critical need for water, most boilers today are equipped with automatic low-water trip switches. In the event of low water, the low-water trips will trip the burner (or fuel flow for solid fuel boilers) and shut down the forced draft fan. This shuts down the heat input.
The damage caused by low-water level can lead to repairs that range from re-tubing to total destruction of the boiler if the drum overheats. Not to mention the safety hazard created. Thus it is recommended to ensure the low level alarms are kept operational at all times.
Method to Test low level alarm:
- Put the boiler in manual operation mode
- Drain the mobery below the low level limit
- When the water level crosses the low water limit the visible and audible alarm should be raised.
- If this happens the alarm is functional.
- Put the boiler back in auto mode.