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What is an Air-Vac (Air Vacuum Valve)?

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An air-vacuum valve, often called an air-vac, is a device installed on pressurized, aka active, water pipelines to control the movement of air within the system, and they play a critical role in protecting pressurized water systems. It automatically releases trapped air when the pipeline is being filled and allows air to enter when the pipeline is being drained. This prevents issues such as air pockets, pressure surges, or vacuum conditions that can damage pipes or disrupt flow.

The valve is typically installed at high points along the pipeline where the pipe grade rises, and where air naturally accumulates, and at other strategic locations such as long pipeline runs, near changes in elevation, or at points where the line terminates. No dips or low spots should occur upstream, as these can trap air before it reaches the valve.

During filling, the valve opens to expel large volumes of air; once water reaches the valve, it closes to maintain system pressure. When the line is emptied or pressure drops, the valve reopens to let air in, preventing the formation of a vacuum that could collapse the pipe.

Each valve assembly includes the valve body, inlet and outlet connections, and an enclosure or protective box set on a concrete slab at ground level which provides easy access for maintenance. The materials are corrosion-resistant and designed to handle the operating pressure of the system, usually around 150 psi or higher. Smaller valves, such as one- or two-inch sizes, are used on distribution mains, while larger installations may use four- or six-inch valves.

They are often visible to the public as small metal or fiberglass boxes or green, blue or brown utility enclosures, aka boxes, located beside roads, on or next to the sidewalk, on hillsides, or along open utility easements. These enclosures usually sit on concrete pads and may be marked with identification tags or vents. By automatically venting and admitting air as needed, air-vacs maintain steady flow, prevent damage, and extend the life of the pipeline network.