Water Supply, Hydraulic Calculation, and System Layout
The calculation tells the story
A hydraulic calculation proves the supply can deliver the required density over the required area, plus hose allowance, at the most remote sprinklers. Start with the design density (gpm/ft²) for the occupancy and the assumed area of operation, then flow the remote heads. Each sprinkler discharges Q = K√P, so a 5.6 K head at 15 psi flows about 21.7 gpm. Add hose stream allowance, elevation loss (0.433 psi per foot of rise), and friction loss through pipe and fittings using Hazen-Williams with the correct C factor - roughly 120 for steel, 150 for copper and CPVC. Compare the total demand to the supply curve at the residual pressure, and keep a safety margin (10 psi or 5 percent is the customary rule of thumb).
Supply and layout checks
Know static, residual, and flow test data and how they plot. Layout is governed by coverage limits: standard spray heads in light hazard cover up to 225 ft² with 15 ft spacing, ordinary hazard 130 ft² at 15 ft, extra hazard about 90 ft² at 12 ft. Heads sit at least 4 in. from a wall and no more than half the spacing from it, with deflectors set per listing below ceilings.
Traps
- Forgetting hose allowance.
- Using the wrong elevation direction sign.
- Mixing C factors.
- Designing to the average pressure instead of the most remote head.
Worked example
Ten 5.6 K heads at 15 psi flow 217 gpm. Add a 250 gpm hose allowance = 467 gpm. A 30 ft rise adds 30 × 0.433 = 13 psi. If supply residual at 467 gpm is only slightly above demand, the design fails and layout or pipe size must change.