What does the mercury bulb in a float-operated low water cut off system control?

In a float-operated low water cut-off system, the mercury bulb plays a crucial role in ensuring safe operation by monitoring water levels within a boiler or similar vessel. The mercury switch within the bulb is designed to activate or deactivate based on the position of the float, which moves in response to the water level.

When the water level falls below a predetermined point, the float drops, tilting the mercury bulb and closing the switch. This action interrupts the pump circuit, preventing the pump from running dry, which could lead to serious damage or failure. Simultaneously, the activation of the low water cut-off system typically triggers an alarm, alerting operators to the unsafe condition.

The primary focus of the mercury bulb is hence to maintain the safety of the system by controlling both the pump circuit and activating an alarm when the water level is critically low. This function is essential for preventing overheating and potential hazards associated with low water conditions in boilers. Other aspects like temperature control, steam release, or fuel quality are not the primary function of the low water cut-off system and are therefore not related to the role of the mercury bulb in this context.