Double Check Valve Assembly Backflow Preventers

Double Check Valve Assembly (DCVA) backflow preventers protect municipal water distribution systems from contamination by preventing reverse flow when downstream pressure exceeds supply pressure. The assembly consists of two independently operating spring-loaded check valves with test cocks between them, creating redundant protection against backflow conditions. DCVAs typically maintain forward flow at pressure drops as low as 2-5 psi and can handle continuous operating pressures up to 175 psi in standard municipal applications. The key limitation is that DCVAs provide only non-health hazard protection and cannot be used for high-hazard cross-connections requiring reduced pressure zone assemblies.

• Service Line Protection: DCVAs protect potable water distribution from cross-connections at commercial/industrial customers within municipal service areas. Installed downstream of water meters on 2"-12" service lines feeding facilities like restaurants, car washes, and small manufacturing. Selected for moderate hazard applications where toxic contamination risk is low.
• Irrigation System Isolation: Municipal parks, golf courses, and landscaping systems require backflow prevention where reclaimed water or fertilizer injection creates contamination potential. DCVAs on 3"-8" lines provide cost-effective protection upstream of irrigation controllers and spray systems.
• Fire Protection Systems: Wet fire sprinkler systems in municipal buildings use DCVAs on 4"-10" supply lines. Protects against stagnant water, corrosion products, and antifreeze additives backflowing into potable supply during system testing or pressure fluctuations.
• Auxiliary Equipment Protection: Booster pump stations, storage tank fill lines, and chemical feed system makeup water connections use DCVAs to prevent contaminated water from entering distribution systems during pump cycling or tank drawdown conditions.

Daily Operations: Operators perform visual inspections during routine rounds, checking for leaks at test cocks and valve connections. No adjustments required during normal operation. Monitor water quality complaints downstream that might indicate backflow events. Record any unusual pressure readings or customer complaints in daily logs.

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Maintenance: Annual testing required per AWWA C510 using differential pressure gauges across check valves. Test procedures require confined space awareness and basic PPE. Valve rebuilds needed every 5-7 years depending on water quality. Maintenance requires journeyman-level skills for disassembly and spring tension adjustment.

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Troubleshooting: Common failures include check valve fouling from debris, causing reduced flow or chattering. Spring fatigue leads to incomplete closure and backflow potential. Warning signs include pressure loss complaints, visible leakage, or failed annual tests. Service life typically 15-20 years with proper maintenance, though aggressive water chemistry can reduce lifespan significantly.

• Check Valves (2): Spring-loaded disc or swing-type valves in series configuration. Bronze or stainless steel construction for 2"-12" municipal applications. First valve opens at 1-2 psi differential, second provides redundant protection. Selection based on flow requirements and pressure loss tolerance.
• Test Cocks (4): Quarter-turn ball valves at inlet, between check valves, and outlet positions. Bronze construction with removable caps. Enable differential pressure testing per AWWA C510 standards. Sized for standard test gauge connections.
• Shutoff Valves (2): Gate or ball valves upstream and downstream of assembly. Allow isolation during testing and maintenance. Bronze or iron construction matching pipe size, typically 2"-12" for municipal service applications.
• Housing/Body: Cast bronze or iron construction with flanged or threaded connections. Designed for 175-300 psi working pressure. Includes drain valve for winterization and maintenance access ports for internal inspection.

• Flow Rate Requirements: Continuous flow: 5-2,500 GPM typical municipal range, Peak demand multiplier: 1.5-2.0x average daily flow, Minimum flow velocity: 3 ft/sec to prevent settling, Maximum velocity: 15 ft/sec to limit head loss
• Pressure Parameters: Working pressure: 175-300 PSI (standard municipal range), Minimum inlet pressure: 15 PSI above downstream pressure, Head loss at rated flow: 3-12 PSI depending on size, Pressure differential across assembly: 1-5 PSI minimum for proper operation
• Physical Sizing: Pipe sizes: 3/4" to 16" for municipal applications, Installation clearances: 12" minimum around assembly for testing, Vault dimensions: 6'x8'x6' typical for 6-8" assemblies
• Performance Standards: Backpressure protection: Must close against 1 PSI differential, Annual testing required per AWWA C510, Service life: 15-20 years with proper maintenance, Temperature range: 33°F to 140°F for potable water applications

• What is the required flow rate and corresponding pressure loss budget? Calculate peak hour demand plus fire flow requirements. A 6" DCVA typically handles 750 GPM with 8 PSI head loss, while 8" units handle 1,200 GPM with 6 PSI loss. Wrong sizing leads to inadequate fire protection or excessive pumping costs.
• Should the assembly be installed above or below ground? Above-ground installations cost $8,000-12,000 less but require freeze protection in cold climates. Below-ground vaults add $15,000-25,000 but provide year-round protection. Consider local frost depth, available space, and maintenance access requirements.
• What hazard classification determines the protection level needed? High hazard (industrial connections, irrigation) requires Reduced Pressure Principle assemblies, not DCVAs. Medium hazard (commercial buildings, hospitals) allows DCVA installation. Misclassification exposes the system to contamination and regulatory violations.
• How will testing and maintenance access be provided? Assemblies require annual testing with 15 PSI differential pressure. Inadequate access increases testing costs from $200 to $800+ per test and delays compliance.

• Division 40 - Process Integration
• Section 40 05 23 - Backflow Prevention Assemblies
• Primary specification section covering installation, testing, and performance requirements for potable water backflow prevention systems in municipal applications

• Material/Equipment Verification: Verify ASSE 1015 certification and local code compliance, Confirm bronze/stainless construction for municipal applications, Check test cock specifications and accessibility
• Installation Requirements: Minimum 12-inch clearances for testing access, Proper support and anchoring for larger assemblies, Drainage provisions for relief valve discharge
• Field Challenges: Limited space in existing pump stations and vaults, Coordination with electrical/controls for monitoring systems, Freeze protection in northern climates
• Coordination Issues: Early coordination with testing contractors for commissioning, Integration with SCADA systems for monitoring, Lead times typically 4-8 weeks for standard sizes, 12+ weeks for custom configurations

• Watts Water Technologies: Series 909 and 957 DCVAs, dominant in municipal markets with extensive local distributor network
• Wilkins (Zurn): Model 350DA and 375 series, strong reputation for reliability in large municipal installations
• Febco (Watts): 860 and 880 series, popular for retrofit applications in existing municipal facilities
• Apollo Valves: 40-200 series, competitive pricing for budget-conscious municipal projects

• Reduced Pressure Principle (RPP) Assemblies: Higher protection level, required for high-hazard applications. Cost premium of 40-60% over DCVA but mandatory for certain cross-connections.
• Air Gap Separation: Ultimate protection but requires significant elevation difference and space. Used when backflow preventers cannot provide adequate protection.
• Pressure Vacuum Breakers (PVB): Lower cost option for irrigation and non-critical applications. Not suitable for continuous pressure or backpressure conditions typical in municipal systems.

Establish relationships with certified backflow testing contractors early in project planning - they often catch installation issues before commissioning. Specify monitoring systems (pressure switches, flow meters) during design phase rather than retrofitting later. Consider oversizing by one pipe size for future capacity and reduced pressure loss. Watts and Wilkins offer excellent technical support for complex installations, while Apollo provides competitive pricing for straightforward applications.