The function of high and low level alarms. Low-level alarms will draw attention to low boiler water level and, if required, shut down the boiler. High-level alarms protect plant and processes.
Water Level Alarms
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 regulations covering boilers have built up over the years in response to boiler explosions, damage and loss of life. Whilst boiler explosions are now very rare, damage to boilers which is attributable to low water level still occurs.
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 alarm
The action of the low water level alarms under UK regulations is as follows:
• 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 (often called lockout) - 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.
The rules and regulations covering boiler operation, and the controls required, will vary from country to country, although demands for higher levels of safety, plus a desire to run steam boilers without the permanent presence of a boiler attendant, are tending to drive the regulations in the same direction.
The action of low water alarms outlined above, relates to the regulations governing unattended steam boiler plant in the UK. However, they are similar to the rules which are applied in many European countries and further afield.
High water 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 a 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 the product where it is used directly. Wet steam can increase the water film thickness of the heat transfer surface, lower processing temperatures, perhaps interfering with proper sterilisation of food products or processing of pharmaceuticals, and causing wastage. At best, lower process and production efficiency will increase process time and unit costs.
• Overfilling the boiler can lead to waterhammer in the steam system, risking damage to plant and even injury to personnel.
All of these, taken together, can result in:
• Spoilt product.
• Lower production rates.
• Poor product quality.
• Increased plant and component maintenance.
• Damage to the steam system.
• Risk 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.
The action to be taken largely depends on the individual plant requirements.