Water Safety and Backflow Prevention

Protecting drinking water, conserving water resources, and providing resilience to all buildings
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Sponsored by WATTS Water Technologies, Inc.
By Celeste Allen Novak, FAIA, LEED AP

Testable Backflow Preventers

Testable backflow prevention valves include the following common types.

Pressure vacuum breakers are commonly used on residential sprinkler systems.

Pressure Vacuum Breaker (PVB) ASSE 1020

Like an atmospheric vacuum breaker, these are for backsiphonage only and appropriate for health hazard applications. They are designed for applications that have continuous pressure and therefore can have shutoffs downstream. These are most commonly specified for residential sprinkler systems since there can be zone valves downstream. These devices will occasionally discharge water. They must be installed a minimum of 12 inches above the flood level rim or highest outlet of the downstream piping, such as the highest sprinkler head in an irrigation system.

Primarily installed indoors, a spill-resistant vacuum breaker prevents the hazardous release of chemicals.

Spill-Resistant Vacuum Breaker (SVB) ASSE 1056

A spill-resistant vacuum breaker operates largely the same way as the PVB and is installed when there is a health hazard present. The SVB has a spill-resistant feature that closes the vent prior to the check opening. These can be used indoors and prevent the occasional discharge of water. They are often used in chemical dispensers and must also be mounted above downstream outlets.

Double check valve assemblies are designed for use in non-health hazard applications.

Double Check Valve Assembly (DC) ASSE 1015

Double check valve assemblies are larger devices typically designed for 12-inch to 10-inch outlet connections. They are used for containment or isolation of non-health hazard applications or pollutants. They are able to handle backpressure, backsiphonage, and continuous pressure situations. They can be installed horizontally or vertically with the flow upward. These devices are rated assemblies with shutoffs on either side. These shutoffs are part of the assembly and should not be removed unless they are replaced with another manufacturer-approved shutoff. Code regulates the orientation of the assembly and assembly components.

These assemblies are designed for use in non-health hazard cross-connections and continuous pressure applications subject to backpressure or backsiphonage incidents, such as lawn sprinklers, fire sprinkler lines, commercial pools, tanks and vats, and food cookers. Testing will verify that the assembly is working but does not provide a mechanism for early notification of failure.

Reduced pressure zone assemblies (RPZAs) provide the highest possible degree of protection from hazards caused by cross-connection failures. In catastrophic events, an RPZA will discharge a lot of water and require a design location that will accommodate this potential for water damage.

Reduced Pressure Zone Assembly (RPZA) ASSE 1013

Reduced pressure zone assemblies (RPZAs) provide the highest possible degree of protection from hazards caused by cross-connection failures. This is a more complex assembly and will occasionally discharge water. An RPZA must be installed a minimum of 12 inches above grade and typically cannot be installed vertically. In catastrophic events, an RPZ will discharge a large amount of water and require a location that will accommodate this potential for flooding.

Double check detector assemblies provide a bypass on an unmetered sprinkler system and are used with non-hazard conditions.

Double Check Detector Assemblies (DCDAs) ASSE1048

Double check detector assemblies (DCDAs) are similar to double checks but have a bypass that monitors unauthorized water usage or leaks. These are typically installed on unmetered fire sprinkler system connections. They detect the unauthorized diversion of water. A bypass around the device is designed to identify the first 2 gallons per minute of flow to alert the water authority to a problem. The bypass must contain a backflow preventer with the same level of protection as the device it is bypassing, typically another double check assembly. They are used with non-hazard conditions.

Reduced pressure detector assemblies are similar to double check detector assemblies. They are installed in applications with potential health hazards.

Reduced Pressure Detector Assembly (RPDA) ASSE1047

A reduced pressure detector assembly is an RPZ assembly that also monitors unauthorized water usage or leaks. They are used in high-hazard fire-protection applications and function in much the same way that a DCDA does, but instead of a double check in the bypass, they are constructed with a reduced pressure zone assembly. They are installed in applications with potential health hazards.

Detecting a Discharge

Reduced pressure assemblies are designed with a unique relief valve that responds to differential pressure changes. The normal operation of an RPZ is shown in this short online video example. The second video demonstrates what occurs when debris clogs the first check, causing a backflow event.

Shown is a normal RPZA flow example.

Shown is an RPZA backflow example demonstrating a fouled first check example.

In this example, debris is stuck in the first check when the water is flowing. A fouled second check is less common but can also potentially cause the relief valve to open if there is backpressure in the system. The opening of the relief valve releases water to drain potentially contaminated water rather than allow it to enter the potable water supply. The size of the debris will dictate the size of the discharge.

Something as small as a grain of sand, water sediment, or pieces of tape, or construction dust can cause an RPZ relief valve to open. A large rock can cause high flow and even the release of a constant stream of water. The discharge provides a visual and physical indication of failure and prevents contamination of the drinking water by breaking the cross-connection.

RPZAs are designed to release water when the lines are clogged or when a valve is frozen due to line pressure changes and weather incidents causing extreme flooding. Particularly in southern climates, these systems may be installed outside. With changing climate conditions, more of these systems may need to be protected from freezing.

 

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Originally published in Architectural Record
Originally published in April 2020

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