Hydraulics and Backflow Principles
Core concepts
Backflow is the reverse flow of water in a piping system, caused by backsiphonage (negative or reduced supply pressure) or backpressure (downstream pressure exceeding supply pressure). The governing relationship is pressure difference: flow goes from higher to lower pressure, so the exam question often hinges on identifying which side has the greater pressure.
Know the hydraulics that drive test values. A column of water exerts about 0.433 psi per foot of height. A 10-foot column produces roughly 4.33 psi. Use this to estimate pressures in a differential-pressure test or to explain why a device must be installed at a certain elevation. Also know that pressure loss through a device increases with flow, and that a device's rated capacity is at a specified pressure loss.
Traps
- Backsiphonage does not require a main break; a large demand upstream (fire flow) can drop pressure enough to reverse flow.
- Backpressure can occur with a boiler, a pump, or a thermal expansion loop even when supply pressure is normal.
- Confusing static pressure with residual pressure is a common error; residual pressure is measured while flowing.
- Questions may ask why an atmospheric vacuum breaker cannot be used on a line with a downstream shutoff valve: it has no means to maintain a differential under continuous pressure.
Rule of thumb
0.433 psi per foot of elevation; 2.31 feet of elevation equals 1 psi. To convert a pressure to head, divide psi by 0.433. To check whether a device will hold a test differential, use the column height, not the pipe diameter.