What is a Pressure Vacuum Breaker (PVB)? Protecting Water from Backflow

Backflow is the unwanted reversal of water flow in a plumbing system. One type of backflow is backsiphonage, which occurs when a drop in supply pressure creates a vacuum, potentially sucking contaminated water from sources like sprinkler systems or hoses back into the clean (potable) water supply.

This poses a significant health risk.Introducing the Pressure Vacuum Breaker (PVB) A Pressure Vacuum Breaker (PVB) is a mechanical backflow prevention assembly specifically designed to protect against backsiphonage. It's a step up in complexity and application compared to its simpler cousin, the Atmospheric Vacuum Breaker (AVB).

How Does a PVB Work?

A PVB consists of several key components working together:

1. A Check Valve: This is typically spring-loaded and opens to allow water to flow through in the normal direction.

2. An Air Inlet Valve (Vent): This valve, also spring-loaded, is located on top. Under normal flow, water pressure keeps this vent closed.

3. Shutoff Valves: PVBs include inlet and outlet shutoff valves for testing and maintenance.

4. Test Cocks: Small ports used by certified testers to verify the device is functioning correctly.

Here’s the process:

• Normal Flow: Water pressure pushes the check valve open and keeps the air inlet valve sealed shut, allowing water to flow downstream.

• Backsiphonage Condition: If the upstream water pressure drops significantly (creating a vacuum), the check valve closes due to the spring and backpressure. Simultaneously, the loss of pressure allows the air inlet valve to pop open.

• Breaking the Vacuum: Air rushes into the PVB through the open vent, breaking the siphon effect and preventing any potentially contaminated downstream water from being pulled back into the potable water system.

The Key Difference: Handling Continuous Pressure

The most significant advantage of a PVB over an AVB is its ability to be subjected to continuous water pressure upstream.

This means it can be installed on the main water line before downstream shutoff valves (like individual sprinkler zone valves). AVBs, in contrast, cannot handle continuous pressure and must be installed after the last shutoff valve.

Common Applications for PVBs

Due to their ability to handle continuous pressure, PVBs are frequently used in:

• Mainline Irrigation Systems: Installed on the main supply line before the zone valves.

• Commercial Buildings: Used in various plumbing applications requiring robust backsiphonage protection.

• Fire Sprinkler Systems: Sometimes used where backsiphonage is the primary concern.

Installation Requirements

Like AVBs, PVBs must be installed vertically and positioned above the highest point in the downstream piping (typically at least 12 inches higher). They also require periodic testing by certified professionals to ensure they are functioning correctly, which is why the test cocks and shutoff valves are essential features.

The Pressure Vacuum Breaker (PVB) is a vital component in safeguarding our water supply. By effectively preventing backsiphonage in systems under continuous pressure, it plays a critical role in maintaining potable water safety and complying with plumbing codes.