What does a float-operated level control primarily do?

A float-operated level control is designed to maintain the water level in a boiler or other system by adjusting the feedwater flow in response to changes in water level. This is crucial because, as the water heats up, it can expand and create a phenomenon known as 'swell.' Conversely, when the water cools, it contracts, leading to 'shrinkage.' The float mechanism detects these changes in water level and automatically corrects the feedwater flow to maintain the desired level, ensuring safe and efficient operation of the boiler.

In essence, option B accurately describes the primary function of a float-operated level control system, highlighting its role in adjusting feedwater flow according to variations caused by swell and shrinkage. This is an essential aspect of boiler operation, as improper water levels can lead to efficiency losses or even operational hazards. Other options do not correctly capture the primary function of float-operated level controls and their specific application in boiler systems.