Which valves are responsible for allowing air into the pipeline when the pressure drops below atmospheric pressure?

The correct answer is that air release valves (ARV) and vacuum valves serve the function of allowing air into the pipeline when the pressure drops below atmospheric levels. These valves are critical in maintaining proper hydraulic conditions in water distribution systems.

When the pressure in a pipeline decreases—possibly due to water being drawn from the system or other changes—vacuum valves come into play. They automatically open to allow air to enter the pipeline, preventing the formation of a vacuum that could lead to pipe collapse or other operational issues. Air release valves also help to release trapped air that can accumulate in air pockets during water flow, thereby ensuring smooth and efficient water transportation.

In contrast, other types of valves mentioned in the options serve different purposes. Check valves prevent backflow, stop valves are used to shut off flow, float valves regulate water levels based on buoyancy, and pressure relief valves help to relieve excess pressure, but do not assist in introducing air into the system under low pressure conditions.