Sprinkler Pressure Demand at the Base of Riser (NFPA 13)
The pressure the water supply must deliver at the base of the riser to drive the hydraulically most remote sprinkler.
Example
You enter
- Flow at most remote head (gpm) 26
- Sprinkler K-factor (gpm/psi^0.5) 5.6
- Total flow through governing run (gpm) 260
- Pipe internal diameter (in) 3.068
- Hazen-Williams C (steel 120 wet / 100 dry or preaction; 150 CPVC; 100 CI) 120
- Equivalent length: pipe + fittings (ft) 150
- Elevation of remote head above base of riser (ft) 15
You get
- Start pressure at remote head 21.6 psi
- Friction loss over run 12.1 psi
- Elevation head 6.5 psi
- Demand at base of riser 40.1 psi
Checked against published worked examples from: AXA XL Risk Consulting.
Details, formula, and sources
ScopeThe K-factor start pressure at the end head (P1 = (Q/K)^2), plus the NFPA 13 Hazen-Williams friction loss p = 4.52 Q^1.85 / (C^1.85 d^4.87) carried over the governing run, plus the 0.433 psi/ft elevation head to lift the water. A 26 gpm K-5.6 head needs 21.6 psi, and 260 gpm through 150 ft of 3 in steel adds 12.1 psi of friction and 6.5 psi of lift for a 40.1 psi demand - the point that must fall under the supply curve and the fire pump's rated point. Smoother CPVC (C = 150) trims about 4 psi. One representative flowing path; a full design balances every node and grid loop. A design aid, not a stamped hydraulic submittal; a qualified fire-protection engineer and the AHJ govern.
Formula or tableP1 = (q_head_gpm / k_factor)^2; pf = 4.52 x q_total_gpm^1.85 / (c_factor^1.85 x pipe_id_in^4.87); friction = pf x equiv_length_ft; elevation = 0.433 x elevation_ft; demand = P1 + friction + elevation.
Source and editionNFPA 13 (Standard for the Installation of Sprinkler Systems), 2022, by name; the Hazen-Williams friction coefficient 4.52 and exponents 1.85 / 4.87, the pipe-type C values, the 0.433 psi/ft water column, and the K-factor discharge relation Q = K sqrt(P) are the NFPA 13 hydraulic-calculation method.
Free accessNFPA 13 is viewable free of charge through NFPA's online free-access reader; the Hazen-Williams friction, K-factor, and elevation arithmetic is public.
What governsEstimate. AHJ and licensed professional govern.
API field namesq_head_gpm, k_factor, q_total_gpm, pipe_id_in, c_factor, equiv_length_ft, elevation_ft, start_pressure_psi, friction_psi, elevation_psi, demand_psi
- Hazen-Williams C the pipe-roughness default is the NFPA 13 pipe-type value (black or galvanized steel 120 wet, 100 dry or preaction; 150 copper or listed CPVC; 100 unlined cast iron), editable; the diameter is the actual internal diameter, not the nominal sizeNFPA 13 (2022)
- Equivalent length the entered length is the actual pipe run plus the fitting and valve equivalent lengths from the NFPA 13 fitting table; the tile does not tabulate fittings itselfNFPA 13 (2022)
- Single representative path the demand is assembled over one flowing path at a single total flow; a full stamped design balances every node, branch, and grid loop in the remote area, and the 7 psi end-sprinkler minimum is flagged, not enforcedNFPA 13 (2022)