Irrigation Chemical Injection and Backflow Protection
A 2-inch domestic water service enters a 40,000-square-foot commercial building and immediately feeds a 1-1/2-inch irrigation branch. The irrigation system serves six zones, has a 1-1/2-horsepower booster pump downstream of the point of connection, and injects a liquid fertilizer through a chemical injection pump into the irrigation main. The existing backflow assembly on the irrigation branch is a 2-inch double check valve installed in a below-grade vault. The property manager wants to replace only the check valves and continue annual testing. The domestic service also has a pressure-reducing valve set at 55 psi and a 100-gallon gas-fired water heater. The local water purveyor requires approved containment and isolation protection. The manager asks whether the existing double check valve is acceptable and what changes are needed.
1. What hazard category applies, and what backflow assembly is required?
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Fertilizer injection makes the irrigation system a health hazard; a double check valve is only for low hazard and cannot protect against a toxic chemical. The booster pump can create backpressure, and the injection pump can create backsiphonage. The correct isolation protection is a testable reduced pressure principle backflow preventer, an RPZ, on the irrigation branch, with no bypass, installed above grade in a drainable location, not in a below-grade vault subject to flooding. If the fertilizer is a high-hazard chemical, also use an air gap or approved chemical gap at the injection point. Verify the purveyor approved assembly list and local requirements.
2. What testing and maintenance is required, and does replacing the check valves fix the problem?
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An RPZ must be tested by a certified backflow tester at installation, after repair or relocation, and at the interval required by the purveyor, usually annually. The test report goes to the water purveyor and the building. Replacing check valves in a double check does not upgrade it to an RPZ and does not satisfy the high-hazard requirement. Maintain records and tag the assembly.
3. How does the domestic service and water heater affect the backflow solution?
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The purveyor may also require containment protection on the 2-inch domestic service. If an RPZ or check valve is installed on the domestic main, the system becomes closed; a 100-gallon gas water heater needs a properly sized thermal expansion tank and a functioning T&P relief valve. The existing PRV at 55 psi does not control thermal expansion. Check pressure after installation and ensure the expansion tank is sized for the heater volume and temperature.