Four-Story Office Pressure and Backflow
A four-story office building is served by a 2-inch water service. The street pressure at the meter is 55 psi, the meter and backflow assembly lose 10 psi at peak demand, the developed length of piping to the top floor adds 18 psi of friction loss, and the highest flushometer valve is 40 feet above the meter. The break room has a carbonated beverage dispenser with a carbonator, and the landscaping contractor wants to tap the domestic line downstream of the meter for irrigation. No booster pump exists. The owner asks whether the top-floor toilets will flush properly and what backflow protection is needed.
1. Calculate the residual pressure at the highest flushometer valve and determine whether a booster pump is required.
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Convert the elevation to pressure: 40 feet x 0.433 psi per foot = 17.3 psi. Subtract the losses from the street pressure:
55 psi street - 10 psi meter/backflow - 18 psi friction - 17.3 psi elevation = 9.7 psi residual at the top floor.
Flushometer valves require a minimum of 15 psi at the fixture supply. At 9.7 psi, the top-floor flushometers will not operate reliably. A booster pump is required. If the fixtures were flush tanks, the minimum would be 8 psi, and 9.7 psi would be marginal but possibly acceptable; with flushometers, it is not.
2. What capacity should the booster pump be selected for, and what operating setpoint is reasonable?
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The booster must add at least the deficit: 15 psi required minus 9.7 psi available = 5.3 psi at peak flow, before any safety margin. The pump should be selected for the peak demand flow of the building, not just the static pressure deficit, and should be set to maintain at least 15 psi at the highest flushometer under peak flow. A reasonable design setpoint is 20 to 25 psi at the top floor to account for variations in street pressure and friction. The pump must also be protected against low suction pressure and must not create a backflow hazard.
3. What backflow protection is required for the carbonator and for the irrigation tap?
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The carbonator is a health-hazard connection because carbon dioxide can create a cross-connection with the potable supply. It requires an air gap or a reduced-pressure principle backflow assembly; a simple dual check is not adequate. If an air gap is feasible, it is preferred.
The irrigation tap is downstream of the meter and creates a separate cross-connection. It needs its own backflow protection. A reduced-pressure principle assembly is appropriate where a health hazard may exist; a pressure vacuum breaker may be acceptable for a non-health irrigation hazard where it is installed and maintained per the code. The device must be located so it protects the domestic line and must be tested and maintained. The existing meter-side backflow assembly does not protect the domestic system from the irrigation branch.