In case you missed my blog from last Monday, I decided to break down various backflow prevention devices a little more for the curious customer. After all, not all backflow prevention devices are created equal. Today, I want to focus on the RPZ or Reduced Pressure Zone Assembly. (Also often called RP and RPBA: reduced pressure backflow assembly.)
How does an RP work?
RPZ’s are intense. They don’t mess around when it comes to water safety. With both an inlet and outlet shutoff valve, 2 independent operating check valves, and a spring-loaded differential pressure relief valve, they’re uber safe and effective in preventing backflow and backsiphonage. They are, in my opinion, second only to air gap in most settings, but as mentioned last week…air gaps aren’t always possible.
How an RPBA works is that the intended water flow enters several checkpoints–so to speak– before it enters into your water source. The water flows in to the first section, and the pressure allows the spring-loaded valve to push forward…when this next section is filled, the next spring-loaded valve pushes forward, allowing the water to flow through your pipes. If the water pressure reverses (backflow) the valves close due to a pressure difference. If there is something blocking the first one from closing, the second one will close independently. And if for some strange reason they can’t close? That’s where the relief valve comes in: all the water from the backflow will start leaking physically out of the device. (Yes, purposely.)
The video below does a great job of illustrating how the Reduced Pressure assembly works.
Because of the fact that the water associated with Reduced Pressure devices is often toxic, there are strict requirements on how they are installed–particularly when they need to be installed underground. Leaching toxic chemicals into the ground could wreck havoc on the soil and groundwater, so there are stringent ways to install trenches, and they vary by locality. When installed above ground, there is a required drain for the RP to prevent water damage since they do periodically drip minimal amounts of water, and when a backflow occurs the preventer will release a large amount of water.
Why use an RPZ instead of a DC?
How exactly does an RPZ differentiate from an Double Check? Whereas Reduced Pressure devices are acceptable for potentially toxic backflow, DC’s are not appropriate for such. (We’ll talk about Double Check backflow assemblies a little more next week simply because they are used in so many instances.) This is because they lack the relief valve that the Reduced Pressure assembly has.
“Reduced pressure zone assemblies (RPZ) […]is the most complex and expensive backflow preventer.”
Where would you use an RPZ?
Anywhere that’s pretty hazardous, you’d use an RPZ. For example, a soda machine is a place that an RPZ is typically required. States and counties may vary on what they require from an installed RPZ though, particularly locally.
While a DC backflow would typically be installed in a traditional irrigation system, when chemigation is being used—which is the introduction of chemicals into a water source to irrigate such as nitrogen or phosphorus– an RPZ may typically take it’s place for extra insurance against backflow of these contaminants into the public drinking water.
Esentially, an RPZ is used where a DC normally would be placed, but can’t be due to toxic or hazardous water.
Next week, let’s take a glance at Double Check assemblies.