Water Supply Analysis, Piping, and Backflow Protection
Water Supply Analysis
Candidates must read a flow test correctly: static pressure, residual pressure, and flow at the test hydrant. Use these to build a supply curve (N^1.85 relationship). Compare the supply curve to the system demand curve at the base of the riser. Account for private mains, meters, and backflow preventers, which add friction loss.
Piping and Materials
Know acceptable pipe materials (black steel, galvanized, CPVC, copper) and their limitations. Underground mains require thrust blocks at changes in direction. Aboveground piping must be supported per NFPA 13 spacing tables. Grooved couplings, welded joints, and threaded fittings each have specific applications.
Backflow Protection
Arizona law and local jurisdictions require backflow prevention where the sprinkler system connects to a potable supply. Know the difference between a double check valve assembly (DCVA) and a reduced pressure zone (RPZ) device. An RPZ is required where there is a health hazard; a DCVA is acceptable for non-health-hazard connections. Test annually and document.
Traps
- Confusing static vs. residual pressure when calculating available flow.
- Forgetting that backflow preventers reduce available pressure.
- Assuming a fire department connection (FDC) can substitute for adequate supply.
- Missing the requirement for annual backflow testing.
Worked Example
A flow test shows 60 psi static, 45 psi residual at 1,000 gpm. Using N^1.85, the supply at 500 gpm is roughly 52 psi. If the system demand is 500 gpm at 50 psi, the margin is thin-consider upsizing the main or adding a pump.
Rule of Thumb
A backflow preventer can cost you 5–15 psi; always include it in the hydraulic calculation, not as an afterthought.