Hydraulics and Backflow Theory
Pressure fundamentals
Absolute pressure is gauge pressure plus atmospheric pressure. A perfect vacuum is zero absolute pressure, roughly 14.7 psi below atmospheric at sea level. Water rises about 2.31 feet per psi, so a 10 psi drop can lift water about 23 feet. These conversions drive many exam calculations: a 20-foot lift requires about 8.7 psi of suction.
Why backsiphonage happens
When supply pressure falls below atmospheric at a point in the system, that point becomes a partial vacuum and atmospheric pressure pushes water from a submerged or connected source back into the pipe. The driving force is atmospheric pressure, not the pump. This is why a vacuum breaker must be installed at least the height of the critical level above the highest downstream outlet: the air-inlet must open before water can reach it.
Backpressure hydraulics
Backpressure arises when downstream pressure exceeds supply. Sources include booster pumps, boilers, thermal expansion in a closed system, and elevation. A device protecting against backpressure must have a check mechanism rated for the differential, not just an air inlet.
Friction loss and flow
Flow through any device costs pressure. Testers and designers use the device's published pressure-loss curve. A common mistake is ignoring that a reduced-pressure assembly has higher loss than a double check, which affects sizing.
Traps
- Using gauge pressure where absolute pressure is required.
- Forgetting that atmospheric pressure is the driving force in backsiphonage.
- Mixing feet and psi without the 2.31 factor.
- Assuming a check valve resists backsiphonage; it does not.
Rule of thumb
Convert early: psi × 2.31 = feet of head; feet ÷ 2.31 = psi. Then ask whether the driving force is atmospheric (backsiphonage) or mechanical (backpressure).
Worked example
A vacuum breaker sits 12 inches above a hose bibb outlet. During a main break, supply pressure drops 15 psi. That is about 34.6 feet of lift potential, far exceeding 12 inches, so the air inlet must open to protect the main.