Irrigation System Sharing a Domestic Service With a Chemical Injection Point
A homeowner has a 1-inch domestic service from a community water system. The service tees to the house and to a lawn irrigation system. A fertilizer injection pump draws from a 15-gallon tank and injects downstream of the irrigation zone valves. The irrigation main runs uphill from the tee and then downhill to the zones, creating a high point about 8 feet above the tee. The homeowner installed a 1-inch atmospheric vacuum breaker (AVB) at the highest point of the irrigation main, 6 inches above the highest sprinkler head. The AVB is installed downstream of the injection point. During a site visit, the inspector finds the AVB installed with a shutoff valve downstream and a 12-inch riser to a sprinkler head on the same zone. The homeowner reports that the AVB sprays water when the zone valve closes.
1. Evaluate the AVB installation and determine whether it provides adequate protection for this system.
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The AVB does not provide adequate protection here for several reasons. First, an AVB must not have any shutoff valve downstream of it, because a closed valve can trap pressure and prevent the AVB from opening to atmosphere during backsiphonage. The downstream shutoff valve violates this requirement. Second, the AVB must be installed at least 6 inches above the highest downstream outlet, and it is, but the 12-inch riser to a sprinkler head on the same zone means the AVB is not protected from continuous pressure; AVBs are not approved for continuous pressure or for use where backpressure can occur. Third, the fertilizer injection point is upstream of the AVB, so the AVB is exposed to chemical-laden water, which can foul the seat and is not an acceptable location for a device protecting against a health hazard. Fourth, the irrigation main rises 8 feet above the tee and then falls, so the AVB at the high point may be subject to backpressure from the downhill zones. The spray on valve closure is a symptom of water hammer and pressure surge, which an AVB is not designed to handle. The correct protection for a fertilizer injection system is a reduced-pressure principle backflow prevention assembly or an air gap, installed upstream of the injection point and protected from backpressure.
2. Recommend a compliant protection scheme and describe where the assembly should be located relative to the tee, the injection point, and the high point.
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The appropriate scheme is an RPBA on the irrigation branch, installed downstream of the tee and upstream of the injection point, so that the assembly protects the domestic service from both the irrigation water and the fertilizer. The assembly should be located at a point where it is not subject to backpressure from the downhill zones, which may mean installing it near the tee before the main rises, or providing a means to prevent the downhill leg from pressurizing the assembly outlet. The assembly must be installed horizontally or vertically per the manufacturer's instructions, with adequate clearance for testing and maintenance, and with the relief port discharging through an air gap or a properly sized drain. The AVB should be removed or replaced with a device rated for the hazard and the pressure conditions. Because the injection point is downstream of the assembly, the assembly is protected from chemical contamination. The homeowner should also install a check valve or other means to prevent fertilizer from draining back toward the assembly, and the injection pump should be interlocked with the irrigation pump so injection cannot occur without flow.