What are the methods of detecting water level in steam boilers?

On a steam raising boiler there are three clear applications for level monitoring devices:

1) Level control - To ensure that the right amount of water is added to the boiler at the right time.

2) Low water alarm - For safe boiler operation, the low water alarm ensures that the combustion of fuel does not continue if the water level in the boiler has dropped to, or below a predetermined level. For automatically controlled steam boilers, national standards usually call for two independent low-level alarms, to ensure safety. In the UK, the lower of the two alarms will “lockout” the burner, and manual resetting is required to bring the boiler back online.

3) High water alarm - The alarm operates if the water level rises too high, informing the boiler operator to shut off the feedwater supply. Although not usually mandatory, the use of high-level alarms is sensible as they reduce the chance of water carryover and water hammer in the steam distribution system.