Backflow Prevention Assembly Types
RPZ, DCVA, PVB, SVB, AVB, air gaps, and DCDA - components, applications, and selection criteria.
- Identify the components and function of RPZ, DCVA, PVB, SVB, and AVB assemblies
- Explain the role of the relief valve in an RPZ assembly
- Distinguish between assemblies that protect against backpressure and those that protect backsiphonage only
- Define air gap requirements and DCDA applications
- Select the correct assembly type for a given hazard scenario
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Reduced Pressure Zone (RPZ) Assemblies
RPZ Overview
The Reduced Pressure Zone (RPZ) assembly is the highest level of testable backflow protection available. It is required for high hazard cross-connections where backflow could introduce substances that are dangerous to health. The RPZ is the only testable assembly approved for high hazard backpressure and high hazard backsiphonage conditions.
An RPZ assembly contains three critical components arranged in series:
- First check valve (Check #1) - the upstream check valve that provides the primary barrier against backflow
- Relief valve - a hydraulically operated valve located in the zone between the two check valves
- Second check valve (Check #2) - the downstream check valve that provides a secondary barrier
How the RPZ Works
Under normal flow, water passes through Check #1, through the reduced pressure zone, and through Check #2. The reduced pressure zone is the space between the two check valves. This zone is maintained at a pressure lower than the supply pressure by the differential created by Check #1.
The relief valve is the defining feature of the RPZ. It is designed to open and discharge water to atmosphere whenever the pressure in the zone rises to within 2 psi of the supply side pressure. This means that if Check #1 fails, the relief valve opens and dumps water rather than allowing contaminated water to flow backward into the supply.
Critical RPZ Principle
The RPZ provides fail-safe protection. If both check valves fail simultaneously, the relief valve opens and discharges to atmosphere. The RPZ will dump water on the ground rather than allow any backflow into the supply. This is why RPZ assemblies require adequate drainage at the installation location.
RPZ Test Cocks
Every RPZ assembly has four test cocks used during field testing:
| Test Cock | Location | Purpose |
|---|---|---|
| #1 | Upstream of Check #1 | Reads supply-side pressure |
| #2 | In the reduced pressure zone | Reads zone pressure |
| #3 | Downstream of Check #2 | Reads downstream pressure |
| #4 | On the relief valve | Used for relief valve opening point test |
The RPZ is the highest level of testable protection, containing two check valves and a relief valve that opens when zone pressure rises within 2 psi of supply pressure. It is the only testable assembly approved for high hazard backpressure conditions.