Best Practices in Backflow Prevention and Protection

Back Pressure

Back pressure occurs when downstream pressure on the private side exceeds the pressure of water being delivered through the distribution lines on the public side. Events that can trigger back pressure include severe or widespread power outages that disable electric pumps and draining down a building to make plumbing repairs.

Back Siphonage

Back siphonage is less common. It occurs when insufficient pressure reaches the public distribution lines, causing contaminated water to siphon backward into the drinking water supply. Triggers include broken water mains or distribution pipes and the sudden use of large quantities of water, such as during firefighting operations.

Jurisdictions across the country increasingly acknowledge that without proper prevention for both types of backflow, they cannot comply with the Clean Water Act (CWA), the Cross-Connection Control Manual (CCCM) or the AWWA's standards. In response, water utilities have mobilized efforts to approve and standardize requirements in accordance with the American Society of Sanitary Engineers (ASSE) and the Foundation for Cross-Connection Control and Hydraulic Research (FCCCHR) at the University of Southern California.

These efforts focus on two approaches:

1. Isolation at the point of use involves installing a backflow preventer assembly (BPA) just before the point of use to protect building occupants from contamination. The control valve under a sink is a basic example. This valve is typically left open so users can control flow with the faucet, but a plumber can manually shut off the isolation valve during maintenance to stop water before it reaches the fixture.

2. Containment at the point of supply involves installing a BPA at the building's main water service inlet to prevent contaminated water from backflowing into the public supply, where it could travel to other buildings. The main water shutoff valve between where water enters the building and the utility meter serves this function.

1. Air Gap

An air gap BPA uses a physical separation between the water distribution system (such as a faucet) and the sewer system (such as a drain). Water flows from the faucet to the drain, but it cannot reverse. Air gaps provide the highest level of backflow protection available. However, they are not always practical, especially in appliances with internal water systems such as dishwashers, dialysis machines, dental hygiene instruments or soft drink carbonation systems.

Approved for: back pressure, back siphonage, low-hazard applications, high-hazard applications.

Key considerations: Spatial and design constraints of the application, the ratio between the gap size and the inside diameter of the inlet pipe, and the proximity of the gap to the overflow level.

2. Double-Check Valve Assembly (DC)

A DC valve assembly uses two independently operating spring-loaded check valves. When downstream pressure exceeds public-side pressure, the first check valve closes to stop backward flow. If that valve malfunctions due to debris or mechanical issues, the second check valve closes as a backup. DC assemblies can be installed above grade or below grade in subterranean vaults and are commonly found in fire protection systems.

One important limitation: there is no way to detect a check valve malfunction without conducting a full-scale test. For this reason, DC assemblies are not recommended for high-hazard applications.

Approved for: continuous pressure operation, back pressure, back siphonage, above-grade and below-grade installation, low-hazard applications only.

Key considerations: Horizontal installation unless approved for vertical by the local water authority, safe access for inspections and testing, adequate clearance for maintenance, and the risk of undetected valve malfunction leading to cross-connection. Installing this assembly in a concealed location such as an underground utility vault compounds the risk, since malfunctions produce no visible indication.

3. Reduced Pressure Zone Valve Assembly (RPZ)

An RPZ valve assembly is similar to a DC but features a fail-safe design suited for every cross-connection situation, including high-hazard applications. In addition to two independently operating spring-loaded valves, RPZ assemblies include a hydraulic differential relief valve located between the two check valves. If a backflow event occurs and both check valves fail, the relief valve opens to create an air gap and dispose of the contaminated water.

Unlike DC assemblies, an RPZ's design inherently signals malfunction. If water dumps from the relief valve, the primary valves have a problem. If no water discharges, the primary valves are working properly.

Approved for: continuous pressure operation, back pressure, back siphonage, all hazard levels, above-grade installation only.

Key considerations: Horizontal installation unless approved for vertical, safe access for inspections and testing, adequate clearance for maintenance, proper drainage capacity that meets minimum air gap requirements, and placement inside a commercial building is not recommended.

Even if double-check valves were once considered acceptable for a particular installation, it is important to confirm whether local standards have changed. Most municipalities now require RPZ valve assemblies due to their superior reliability.

For the most current list of approved backflow prevention assemblies from the FCCCHR, visit the USC FCCCHR approved list.

Despite numerous risks, subterranean utility vaults remain among the most common housing options for backflow preventer assemblies. Underground vaults are constructed of reinforced concrete or brick. Their subterranean location protects equipment from the elements and vandalism while minimizing the appearance of utility boxes on commercial properties.

However, their location frequently requires variances that diverge from recommended standards. In 2016, the FCCCHR updated its stance, urging water utilities to avoid installing assemblies in subterranean vaults.

According to OSHA, vaults and other confined spaces contribute to nearly 100 deaths and more than 11,000 injuries every year.

Additional drawbacks of underground vaults include: higher cost compared to other installation methods, longer lead times for construction and installation, safety concerns related to air quality and slips and falls, poor accessibility due to location and limited internal space, flooding that can damage equipment and create additional hazards, cross-connection risk caused by flooding, regulatory restrictions that prohibit RPZ assemblies below grade, and deterioration that can require costly vault rehabilitation.

Vaults must also comply with additional OSHA standards for confined work environments, remain watertight with adequate drainage, and include protective measures for falls, ventilation and gas detection.

Designers and contractors should also consider building ownership changes. If a building's use changes, the hazard threshold can shift from low to high, putting the owner at risk for noncompliance unless they retrofit with an RPZ valve assembly — which cannot be installed below grade.

Inside Buildings

Another common option is installing BPAs inside commercial buildings, such as in a mechanical room or basement. Indoor installation provides easy access, climate control and protection from vandalism. However, if the system is high-hazard and requires an RPZ assembly, building owners face serious flood risk.

Depending on the assembly and pressure combination, an RPZ malfunction can dump approximately 375 gallons of water per minute, resulting in millions of dollars in damages.

Additional drawbacks of indoor installation include the fact that BPAs are often larger than building owners expect, creating spatial constraints, the need for greater liability coverage due to elevated flood risk, and the potential for cross-connection caused by flooding.

Indoor installations require an adequate drainage system with capacity to prevent catastrophic flooding, compliance with building codes for proper clearance around assemblies, and awareness that it is never advisable to install an RPZ assembly with a pipe size of 2-1/2 inches or larger indoors.

Additional advantages of above-ground enclosures:

• Enclosures can be fully customized to meet the exact needs of an application, including custom enclosure designs for nonstandard projects.
• Costs are significantly lower due to the absence of heightened compliance and liability risks. The four primary cost factors are the physical equipment and enclosure, a concrete pad for support and stabilization, running an electrical line for heated enclosures, and labor for design, installation, testing and maintenance.
• Easier access allows technicians to maintain equipment health and extend the enclosure's lifespan. Safe-T-Cover enclosures typically last more than 30 years.

Important installation factors for above-ground enclosures:

• BPAs should be installed above the 100-year flood level to prevent external flood damage.
• The enclosure must meet OSHA 29 CFR 1910.145 and ASSE 1060 Class 1.
• The enclosure must house all required piping and meet clearance requirements for maintenance.
• Designs must include a tamper-resistant locking mechanism for vandalism protection. Safe-T-Cover's LOK-360 and MUNI-LOK systems are engineered specifically for this purpose.
• If heated, enclosures must include a thermostatically controlled electric heater that meets Section 49 of UL 943 NEMA 3R and maintains a constant temperature of 40°F. Learn more about Safe-T-Cover's slab-mounted heaters.

Misconception: Above-Ground Enclosures Are Too Large

Traditional enclosures were often oversized. Today, enclosure designs are up to 70% smaller, making them far easier to integrate into a site plan. Many jurisdictions also combine backflow preventers and water meters in the same above-ground enclosure, further reducing the equipment footprint.

Use the Safe-T-Cover sizing guide to find the right enclosure for your project.

Misconception: Above-Ground Enclosures Are Aesthetically Displeasing

Contractors have many options to help enclosures blend in with building facades or landscaping:

• Color: Enclosures are available in a full spectrum of weather-resistant colors and finishes, applied using prefinished sheet metal coating rather than paint to prevent chipping, discoloration and weathering.
• Vinyl wraps: Some building owners choose high-resolution vinyl graphic wraps to showcase a company logo, advertise a product or feature art. Wraps also protect enclosures from UV and precipitation exposure.
• Landscaping: Creative placement of trees, shrubs and bushes can conceal enclosures of any size. Certain species of privet grow up to 12 feet tall and can be hedged for year-round concealment.
  
  

Misconception: Above-Ground Enclosures Invite Vandalism

Backflow cages can result in theft and vandalism, but ASSE 1060-certified enclosures are designed to protect and secure equipment. These enclosures are constructed of marine-grade aluminum and include a tamper-resistant locking mechanism on the access panel that has been tested and approved for safety and security.