Hydraulics and Pressure Principles
Core Concepts
Field testing depends on pressure relationships. Understand absolute pressure versus gauge pressure, and that atmospheric pressure at sea level is about 14.7 psi. A column of water 1 foot high produces roughly 0.433 psi; conversely, 1 psi equals about 2.31 feet of head. These conversions appear constantly in test questions.
Know static pressure (no flow) versus residual pressure (pressure remaining while water flows). Friction loss increases with flow rate and pipe roughness. The Bernoulli principle explains that where velocity increases, pressure decreases, which is the mechanism behind backsiphonage at a venturi or a partially closed valve. Hydraulic shock (water hammer) creates transient pressure spikes that can damage assemblies and cause false readings.
Understand how differential pressure across a check valve holds it closed, and how a relief valve responds to a differential. Know that a reduced-pressure principle assembly must maintain a pressure differential between the first and second zones, and that this differential is measured during testing. Thermal expansion in a closed system can raise pressure and cause relief valve discharge.
Traps in Questions
Watch for unit-conversion errors: questions may give head in feet and ask for psi, or give a pressure differential and ask whether a check valve should hold. Another trap is assuming flow increases pressure downstream of a restriction; in reality, friction loss reduces pressure.
Worked Example
A test gauge reads 12 psi. Convert to head: 12 psi × 2.31 ft/psi ≈ 27.7 feet. If a question instead gives 30 feet of head, the pressure is 30 ÷ 2.31 ≈ 13 psi.