Terraced Garden, Broken Main
A hillside residence is served by a 2-inch service tapped from a 12-inch city main. Static pressure at the tap is 68 psi. From the tap, 55 ft of 2-inch copper runs at street level to the house, then a 1-inch branch rises 42 ft to a hose bibb at the top of a terraced garden. At the bibb the homeowner fills a 5-gallon bucket with diluted herbicide and drops a garden hose with a shutoff nozzle into it. That afternoon a contractor breaks the 12-inch main two blocks away. Pressure at the service tap falls to 6 psi, and the resulting heavy flow toward the break produces about 13 psi of friction loss through the meter and service at the observed flow. The homeowner calls the water department because the bucket is draining and the hose is collapsing.
1. Compute the pressure at the hose bibb at the moment of the break. Classify the event and explain the role of each term in your calculation.
Read the worked answer
Elevation head: 42 ft x 0.433 psi/ft = 18.2 psi, lost to height whether or not water moves. Friction: 13 psi, present only because water is flowing toward the break. Supply: 6 psi remaining at the tap.
Bibb pressure = 6 - 13 - 18.2 = -25.2 psi, roughly 25 psi below atmospheric.
This is backsiphonage. The non-potable source is not pressurized; the potable side is pulled negative, and atmospheric pressure pushes the bucket contents up the hose. Backpressure would require a pump or an elevation head on the non-potable side, and neither exists here.
Two cautions belong in the report. First, -25.2 psi is not a usable suction lift: no siphon raises water more than about 34 ft from an open surface. That cap is irrelevant here because the herbicide sits at the bibb, essentially at the same elevation, so no lift is needed at all. Second, at zero flow the bibb would still read 6 - 18.2 = -12.2 psi, below atmospheric, so the exposure existed before friction was added. The friction term enlarges the vacuum but does not create it.
2. The handyman installs an ASSE-listed hose-bibb vacuum breaker and points out that the sprayer already has a shutoff at the nozzle. Will this cure the exposure?
Read the worked answer
No. A hose-bibb vacuum breaker is a non-continuous-pressure device. It is listed for use only where no shutoff exists downstream, because when the bibb is open and the nozzle is closed the device is held under continuous pressure, which is precisely the condition it is not approved for. The nozzle shutoff makes the installation non-compliant before the herbicide is even considered.
Two further defects: the device must sit at least 6 in. above the highest point of the downstream hose and nozzle for its discharge to mean anything, and the hazard here is a health hazard. A health hazard at a hose bibb is normally resolved by removing the bibb entirely, by a reduced-pressure principle assembly at the service, or by a pressure vacuum breaker on a dedicated line that cannot see backpressure.
Judgment for the file: the correction is to eliminate the submerged connection and the cross-connection, not to add a fitting that cannot legally be used in that arrangement.
3. The utility wants to require an RPZ at the service. The owner objects that the assembly would sit 40 ft below the garden and therefore could not protect it. Respond.
Read the worked answer
The objection confuses elevation with protection. An RPZ at the service protects the public main; its first check holds the zone at least 2 psi below supply pressure and its relief opens to atmosphere if that relationship is lost. Whether the protected fixture is 40 ft above or below the assembly changes the pressures it sees, not its ability to isolate the main, and the assembly is rated for the resulting head (40 ft is only 17.3 psi).
What containment does not do is protect occupants of the building from their own cross-connections. The submerged hose remains a hazard inside the premises, so fixture-level isolation is still required: remove the bibb, or protect the branch with a pressure vacuum breaker installed 12 in. above the highest downstream outlet.
Practical conditions to impose: install the RPZ where discharge is visible and drainable, protect it from freezing, and confirm that the relief discharge cannot be submerged. Document the hazard survey that justified containment in the first place.