Where boilers are operated without constant supervision (which includes the majority of industrial boilers) low water level alarms are required to shut down the boiler in the event of a lack of water in the boiler. Low level may be caused by:
• A feedwater shortage in the feedtank.
• Failure of a feedpump.
• Accidental isolation of the feedwater line.
• Failure of the level control system.
The effect of low water level in a boiler is that the heated tubes or the furnace tube(s) become uncovered and are no longer cooled by the boiler water. The metal temperature rapidly increases, its strength is reduced and collapse or rupture follows.
Low Water Level Alarm
• 1st low level alarm - Shuts down the burner at the alarm level, but allows it to re-fire if the level recovers.
• 2nd low level alarm - Also shuts down the burner at the alarm level, but the burner controls remain 'locked out' even if the water level recovers and any faults have been rectified. The lockout has to be manually reset to allow the burner to re-fire.
High Water Level Alarm
With the exception of one or two operating standards, the risks from a water level too high are treated very lightly, if not ignored altogether.
The dangers of an excessively high water level in steam boiler include:
• Increased carryover of water into the steam will result in poor operation and/or malfunction of the steam system components, due to dirt.
• Wet and dirty steam can contaminate or spoil product where it is used directly.
• Overfilling the boiler can lead to waterhammer in the steam system, risking damage to plant and even injury to personnel.
As can be seen, the dangers of an excessively high water level are too serious to ignore, and deserve equal consideration to that given to low water level conditions.
A high water condition could:
• Simply sound an alarm if the boiler house is manned.
• Shut-down the feedpump.
• Lockout the burner.
• Close the feedwater valve.