About 80% of boiler failures can be attributed to the either low water condition or operator error. Low water level being more controllable of the two, for safe boiler operations it must be ensured that boiler is not operated at low water level.
Low-water level conditions in the boiler occur when the water in the boiler drum falls below the low water level mark. Such 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.
When water level in the boiler drum approaches the low level, usually the feed pumps are signaled to start pumping in feedwater to raise the level. Due to the critical need for water, most boilers are equipped with automatic low-water trip switches which in the event of low water, will trip the burner (or fuel flow for solid fuel boilers) and shut down the forced draft fan. This shuts down the heat input.
Despite the boilers being equipped with low-water trips, the cause of several boiler related accidents still lies in low water level. This is primarily due to two reasons:
- By passing the trip circuits - A typical scenario involves disabling the trips to eliminate nuisance trips due to improperly tuned controls, etc.
- Inoperative trip switches – The trip switches are installed in dead legs where no circulation occurs and thus sludge gradually accumulates in the leg and plugs the piping or the switch itself. The trip switches should be blown down regularly to remove sludge and keep it in operating condition.
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. Recurring trips should not be a concern with a properly tuned boiler with proper drum internals, so this is not a valid reason to disable low-water trips. Additionally dysfunctional low-water trips should be a "no go" item and should be corrected before the boiler is fired.