Dribbling RPZ on a Fertilized Nursery Service
A 2 in. RPZ is mounted indoors immediately downstream of the meter on a line serving a 22-zone irrigation system at a wholesale nursery. A fertigation pump injects 12-0-0 liquid fertilizer through a Venturi into the mainline about 40 ft downstream of the RPZ. The site maintenance man reports that the RPZ relief valve has been dribbling continuously for about a week and that he has tied a rag around it to keep the floor dry. He also mentions that the decorative fountain pond is refilled by dropping a garden hose into it from a hose bibb on the irrigation mainline. The RPZ's last recorded test was 14 months ago. Your firm holds the testing contract and did not install the system.
1. Classify the hazard, decide whether the RPZ is the right device, and evaluate its location relative to the injection point.
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Hazard: high (health) hazard. The mainline downstream of the RPZ carries 12-0-0 fertilizer, so any backflow into the potable main is a health event, not an aesthetic one. The nursery's pond water, chemical mixing and multiple hose connections only add to it.
The RPZ is the correct device for containment at the meter: it resists both backpressure and backsiphonage and is the standard choice for a high-hazard, permanently connected irrigation service.
Placement is correct as described - the assembly sits upstream of the injection point. If the injection point were ever moved upstream of the RPZ, the assembly would be protecting nothing because the chemical would already be in the potable line. Any future re-piping must preserve that order.
The same logic applies to the injection equipment itself: the make-up water feeding the fertilizer mixing tank needs its own protection (an air gap is best, an RPZ is acceptable), and the injection pump should be interlocked so it can only run while the irrigation pump or a zone is running. Otherwise a power failure can leave a slug of fertilizer pressurized into a dead main.
2. Diagnose the continuous relief discharge. What will you test, what may you do, and what may you not do?
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A steady trickle from the relief port means the zone between the two check valves is being pressurized above the relief differential (typically about 2 psi). Ranked causes:
- Fouled or damaged first check (debris on the seat, torn seat disc) letting supply pressure bleed into the middle zone.
- Second check leaking or held open, so downstream water - irrigation main, elevation, a closed downstream valve, thermal expansion - pushes back through it.
- Relief valve itself fouled or with a weak spring, weeping at normal differential.
- Thermal expansion in a closed downstream leg.
Diagnostic: close the downstream isolation valve and watch. If the dribble continues, the first check or the relief is at fault; if it stops, the second check is passing. Confirm the relief setpoint with a differential gauge and open the assembly per the manufacturer's service procedure.
You may not: cap, plug or tape over the relief port. The rag is exactly that. The relief port is a safety function, and blocking it turns a nuisance leak into a contamination path. Close the upstream valve, tag the assembly out of service, notify the owner in writing, and repair with the manufacturer's parts kit. Only a certified tester may return it to service, and the report goes to the AHJ or purveyor per local rule.
3. Address the hose in the pond and the 14-month test interval.
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Two separate issues, both real.
The hose in the pond. A hose dropped below the pond waterline is a textbook backsiphonage cross-connection: if main pressure drops, pond water is drawn up the hose into an irrigation main that is already carrying fertilizer. Because the RPZ is upstream, the potable main is still protected - but that protection exists only if the assembly is working, which is precisely why a failed assembly cannot be left in service. Correct fix: fill the pond through a fixed air gap (fill line terminating at least two pipe diameters above the overflow rim), or at minimum a hose-bibb vacuum breaker, and never submerge the hose end.
The test interval. At 14 months the assembly is overdue; annual testing is the norm, and many jurisdictions are stricter for commercial and industrial users such as a nursery. Combined with the continuous relief discharge, the assembly cannot be certified as-is. Document the last test date, notify both the owner and the purveyor in writing, tag it out of service, and put the site on a tracking schedule so the next test does not depend on the maintenance man remembering.