backflow-preventers

1. Introduction

Backflow preventers are assemblies with two check valves and two shut-off valves designed to prevent reverse flow in a plumbing system. They protect potable water supplies from contamination by isolating downstream pressure anomalies back into the public main supply.

2. Fundamental Principles

Backflow can occur via backpressure or backsiphonage. Backpressure happens when downstream system pressure exceeds supply pressure, while backsiphonage arises from negative pressure in the supply line. Preventers use one-way check valves and relief ports to block reverse flows and safely vent trapped water.

3. Types of Backflow Preventers

  • Atmospheric Vacuum Breakers (AVB): Single check valve and atmospheric vent that closes when system pressure drops, preventing backsiphonage while in service.

  • Pressure Vacuum Breakers (PVB): Similar to AVBs but include a spring-loaded poppet and test cocks for verification. PVB assemblies are prevalent in irrigation and outdoor potable water applications.

  • Double Check Valve Assemblies (DCVA): Two independent spring-loaded check valves in series, with test ports, suitable for low- to medium-hazard scenarios.

  • Reduced Pressure Zone Assemblies (RPZA): Two check valves separated by a pressure-relief chamber that vents to atmosphere if either check fails, providing high-hazard protection.

  • System Separator BA: A DIN EN 1717-compliant device offering optimal protection up to fluid category 4 by monitoring differential pressure and operating a drain valve upon failure detection.

4. Key Components and Materials

Most backflow preventers share these components:

  • Check Valves: Swing, wafer, lift, silent (center-guided), ball, and cone check types tailor crack-open pressures and closing speeds.

  • Relief Valve/Drain Port: Central chamber venting device in RPZ models.

  • Test Ports/Shutoff Valves: Enable field testing of each chamber under code requirements.

  • Housing: Cast iron, ductile iron, bronze, brass, stainless steel, or plastics (PVC, CPVC, PVDF, PTFE), selected based on fluid compatibility and pressure rating.

  • Gaskets/Seals and Fasteners: EPDM, FKM, or PTFE gaskets and stainless or carbon steel bolts ensure leak-tight integrity.

5. Standards and Certifications

Backflow preventers must meet multiple standards:

  • American Petroleum Institute (API), Manufacturers Standardization Society (MSS), ASME, UL, and FED SPEC WW-V-35C for valve dimensions and materials.

  • ASSE 1013 (vacuum breakers), ASSE 1015/1017 (double checks/RPZ), NSF/ANSI 61/372 for potable water, and cross-connection codes.

  • European: EN 12729 (PVB), EN 1717 (cross-connection protection), and DIN 806-5 maintenance protocols.

6. Sizing and Hydraulic Considerations

Proper selection requires matching the device’s nominal size to the system’s maximum flow rate and pressure rating. Head loss curves from manufacturers guide selection to maintain allowable pressure drop. A table summarizing typical sizing guidelines is shown below.

Device Type Nominal Size (DN/in) Flow Range (m³/h) Max Pressure (bar) Typical Head Loss (kPa)
AVB 25 / 1 0.3 – 12 10 5 – 15
PVB 25 / 1 0.3 – 18 10 10 – 20
DCVA 25 / 1 0.5 – 30 16 8 – 25
RPZA 25 / 1 0.5 – 50 16 15 – 50
 

7. Installation Best Practices

Install backflow preventers on a horizontal run with minimum 5 – 10 pipe diameters of straight pipe upstream and 3 – 5 downstream to ensure laminar flow. Position test cocks and shutoff valves for easy access, and protect devices from freezing or immersion. Elevation above grade is required for AVBs per manufacturer instructions.

8. Testing and Maintenance

Code mandates annual testing of DCVA and RPZA devices, and semi-annual testing for detector assemblies. Field tests use test cocks to measure differential pressures across each check valve and confirm relief valve operation. Testing should follow AWWA M14 or EN 806-5 procedures, with all results documented and reported to authorities as required.

9. Applications and Selection Criteria

Select device type based on system hazard level:

  • Low Hazard (e.g. boiler water): DCVA.

  • Medium Hazard (e.g. chemical feeds): RPZA.

  • High Hazard (e.g. irrigation injection points): PVB or RPZA with additional protection.

  • Ultra-High Hazard (contact with category 4 fluids): System Separator BA or equivalent.

Consider maintenance frequency, allowable head loss, ambient conditions, and installation footprint.

10. Advanced Features and Monitoring

Some manufacturers offer smart backflow preventers with integrated pressure sensors, digital test logs, and remote communications via Modbus or Bluetooth. These solutions enhance predictive maintenance and compliance reporting in municipal and industrial environments.

11. Conclusion

Understanding the operating principles, device types, materials, standards, and maintenance requirements of backflow preventers is essential for reliable contamination protection. Proper specification, installation, and testing ensure system safety, regulatory compliance, and long-term performance.

QTE Technologies offers a wide range of backflow prevention devices from renowned brands. We, a global MRO provider, serve customers in over 180 countries. Established in 2010, we offer over 1 million products across all industries and engineering disciplines. Additionally, you can reach us anytime via 24×7 chat support, phone, WhatsApp or email. Discover what our valued customers have to say about our services on our dedicated review page.

Post Author By QTE Technologies Editorial Staff (with a solid background in both technical and creative writing - accumulated 15+ years of experience).