Pete Chapman, general manager at Conbraco, says that the four most common backflow prevention systems are atmospheric vacuum breakers (AVBs), pressure vacuum breakers (PVBs), reduced pressure principle assemblies (RPs) and double check valve assemblies (DCs). Each system has its own strengths and weaknesses.
AVBs are designed to prevent backsiphonage and not backpressure. They rely on standard air-gap technology to break up the vacuum that pulls polluted water back through a cross-connection into the potable water supply.
The standard AVB set-up includes a disc-float assembly and an atmospheric vent area (the air gap) that automatically seals off a cross-connection when pressure conditions likely to cause backsiphonage are present. Gravity causes the disc float to fall, plugging the atmospheric vent area and injecting air into the line. ABVs must be installed at a six inch elevation relative to downstream piping and outlets attached to the suspect irrigation system, otherwise the disc float won’t act as intended.
PVBs, just like AVBs, are only effective at preventing backsiphonage backflow. These are mechanical systems that consist of a spring-loaded check valve and a spring-loaded air inlet valve. The air inlet valve opens when the internal pressure rises above atmospheric pressure. This action prevents polluted and contaminated water from an application site being siphoned back into a public well. The springs in a PVB replace the gravitational force used by AVBs.
RPs, according to the ABPA, “consist of two independently acting, spring-loaded check valves with a hydraulically operating, mechanically independent, spring-loaded pressure differential relief valve between the check valves and below the first check valve.”
When a cross-connection protected by an RP is acting normally, the pressure between the two check valves is lower than the supply pressure. When the pressure in this RP protection zone begins to approach the supply pressure, the relief valve kicks in to prevent both backflow and backsiphonage. Because RPs are uniformly effective at preventing all forms of backflow, they are the recommended devices for irrigation sites that expose the potable water supply to fertilizer and chemical contamination.
Last, but not least, the DC is a backflow prevention device that also relies on spring-loaded check valves. Elevation is not as important for DC installation, but many contractors report that DCs installed belowgrade have proven difficult to maintain and drain for freeze protection.
Just like the RP, the DC is effective against backpressure and backsiphonage, but it is not recommended for use on cross-connections between potable water and irrigation systems. This is because the DC only protects against pollutants and not contaminants.
For almost every irrigation site that you are going to be asked to maintain, an RP is the ideal choice for backflow prevention. But selecting a device is only the tip of the iceberg; your biggest challenge after installation is going to be responding to changes in the physical condition of the site, and maintaining a regular inspection schedule. Unless this function has been delegated to a plumber, it’s the responsibility of the irrigation contractor to inspect all backflow prevention devices after installation, and run annual maintenance to make sure the system remains in proper working order.
Backflow preventers are complicated machines, with an abundance of moving parts that operate in a dynamic, pressurized environment. A unit’s internal seals and springs are at a high risk for wear and fatigue. Annual inspection is the bare minimum necessary to ensure that a specific unit is functioning as intended.
For simple air gap systems, a cursory visual inspection is sufficient to confirm that the size and elevation of the gap are substantial enough to meet best practice standards. You should invest in properly calibrated gauge equipment for inspecting the more complex, mechanical systems.
Before installation, it is recommended that you identify suitable sites for backflow devices that are easily accessible for inspection. There is a high probability that you will be the one performing regular inspection and maintenance, so installing the system in an accessible location will only make your job easier down the road.