Water Supply and Distribution Systems
Sizing, Pressure, and Protection
Water supply questions test three linked ideas: adequate pressure at the most remote fixture, correct pipe sizing for demand, and protection against contamination. Learn the flow-pressure relationship: residual pressure falls as flow rises, so a static pressure reading alone does not prove adequacy. Know the minimum residual pressures commonly required at fixtures and the maximum static pressure above which a pressure-reducing valve is needed.
Sizing uses fixture units converted to probable demand, then pipe sizing from tables that account for friction loss and velocity limits. Traps include confusing gallons per minute with fixture units, forgetting to add the elevation loss (about 0.433 psi per foot of rise), and ignoring developed length when checking friction loss.
Thermal Expansion and Materials
Closed systems with backflow preventers or check valves need thermal expansion control. Know which materials are permitted for potable water and where dielectric fittings are required between dissimilar metals to prevent galvanic corrosion.
Worked Example
A building has 8 psi of elevation loss to the top floor and needs 15 psi residual at the fixture. Required supply pressure at the base is at least 23 psi before friction, so add friction loss through the longest run. If static pressure at the street exceeds the code maximum, a pressure-reducing valve is required, and the downstream system becomes closed, triggering expansion control.