What happens when the line pressure drops to zero in an atmospheric vacuum breaker?

When line pressure drops to zero in an atmospheric vacuum breaker, the float allows air to enter the system. This is a crucial function of the atmospheric vacuum breaker, which is designed to prevent backflow and protect the potable water supply.

In the absence of line pressure, the float mechanism that is part of the vacuum breaker responds to the change in pressure by lifting, which opens the vent. This opening allows air to enter the system, thereby breaking any potential vacuum that could cause back siphonage. Allowing air to enter prevents a negative pressure situation that could result in contaminated water being drawn back into the potable water supply.

The other scenarios presented do not accurately describe the function of an atmospheric vacuum breaker when faced with a drop in line pressure. For instance, while a disc-float could seal the vent area if the pressure is stable or positive, this is not the operational response during a pressure drop. Creating a siphon is counter to the intended design of the breaker, which specifically aims to prevent the conditions that lead to siphoning. Similarly, water flowing back into the system would signify backflow, which the atmospheric vacuum breaker is designed to prevent. Thus, option B correctly identifies the operational response of the atmospheric vacuum breaker in a scenario of zero line