Pressure Vacuum Breaker Assemblies (PVBs) are one of many types of backflow prevention devices designed to protect the public from unsafe conditions caused by cross contamination of the water supply. These devices are used in specific applications and system designs where their unique functionality is required.
The standard governing PVBs is ASSE 1020, Performance Requirements for Pressure Vacuum Breaker Assemblies, which defines the performance and design criteria for these devices.
NOTE: ASSE 1020 was originally published in 1974 under the title Vacuum Breakers, Anti-Siphon, Pressure Type.
WHAT IS A PVB?
A PVB can be described as an advanced version of an atmospheric vacuum breaker, though this is an oversimplification. The assembly includes a check valve that operates independently and is force-loaded to the closed position, along with an air inlet valve (vacuum relief valve) located downstream of the check valve that is force-loaded to the open position. Additionally, the device incorporates two tightly closing shutoff valves — one at the inlet and one at the outlet — and two test cocks positioned immediately upstream and downstream of the check valve. These features make the PVB a testable device, allowing for regular inspection and maintenance.
HOW DOES IT WORK?
When there is a sudden drop in the main water supply pressure, a negative pressure or siphonage effect can occur, potentially drawing contaminants into the potable water system. The PVB prevents this by closing the spring-loaded check valve to stop backflow and opening the air inlet valve to break the siphonage and allow air into the system. Under normal operating conditions, water flows in the intended direction, which keeps the air inlet valve closed.
WHERE IS IT USED?
PVBs are used in systems where the service line pressure must remain continuous and where protection is needed from both high- and low-hazard backsiphonage. However, they do not protect against backpressure. Common applications include fire sprinkler systems and irrigation systems. According to the standard, the maximum design pressure must exceed 150 psi, and the operating temperature range is between 33ÅãF and 180ÅãF. When supply pressure falls between 1.0 psi and 2.0 psi, the air inlet valve opens and the device discharges continuously. Therefore, installation must account for potential water discharge to avoid damage.
INSTALLATION GUIDELINES
Proper installation is critical for performance and compliance. The device must be installed in a vertical position and must be accessible for annual testing. It should be positioned at least 12 inches above downstream piping at the highest point in the supply system. Because water may spill through the air vent, the device should be installed where leakage is not objectionable, or provisions should be made to collect the spillage using an air gap. The device should also be sized to match the supply and discharge piping.
PERFORMANCE REQUIREMENTS
ASSE 1020 outlines the minimum performance criteria to ensure proper function. The standard includes requirements to verify the mechanical operation of the check valve, air inlet valve, and test cocks. Both the check valve and air inlet valve must be accessible for repair. The integrity of the device is confirmed through hydrostatic testing of both the entire assembly and the individual check valve. A water hammer test is conducted to simulate shock waves downstream of the device. The device is also subjected to extreme temperature conditions followed by a 5,000-cycle life test. Additional functionality tests evaluate the air inlet valve operation, the drip-tightness of the check valve, and the overall flow capacity. The most critical test is the backsiphonage test, which ensures that no liquid flows backward through the device even when the check valve seat is fouled and variable vacuums are applied to the inlet.
CONCLUSION
PVBs play a vital role in safeguarding potable water systems from contamination caused by backsiphonage. Their design allows for continuous-pressure operation while providing reliable protection in both high- and low-hazard scenarios. With clearly defined performance requirements in ASSE 1020, these devices are engineered for durability, functionality, and testability. Proper installation and routine maintenance, including annual testing as required by most jurisdictions, are essential to ensure their continued effectiveness. As part of a comprehensive backflow prevention strategy, PVBs contribute significantly to public health and water system integrity.
