Tenant Improvement Outgrows the Existing Water Supply
A 1990s three-story office building has a wet-pipe system fed by a 6-in city main. A flow test at the hydrant beside the riser produced a static pressure of 76 psi and a residual pressure of 55 psi at 640 gpm. The existing system demand at the base of the riser is 620 gpm at 48 psi, including 100 gpm of hose allowance. A tenant improvement converts an 1,800 sq ft first-floor suite from office to retail, which the designer classifies as ordinary hazard Group 1. The contractor proposes 14 pendent K=5.6 sprinklers on the existing 2.5-in branch lines, designing 0.15 gpm/sq ft over 1,500 sq ft plus 250 gpm hose. The new hydraulic calculation shows 520 gpm at 66 psi at the base of the riser. The ceiling is 10 ft high, there is no basement, and no fire pump exists. The AHJ requires a 10-psi safety margin between available and required pressure. The contractor argues that because 520 gpm is less than the existing 620 gpm demand, the supply must be adequate.
1. Does the existing supply meet the proposed demand with the required 10-psi safety margin? Show the hydrant-curve calculation and state the conclusion.
Read the worked answer
No. Use the flow-test curve to find the available pressure at the new demand of 520 gpm:
P = Pstatic - (Pstatic - Presidual) x (Q/Qtest)^2 P = 76 - (76 - 55) x (520/640)^2 P = 76 - 21 x 0.660 P = 62.1 psi
The proposed demand is 66 psi at 520 gpm, so the raw deficit is about 3.9 psi. With the AHJ's 10-psi safety margin, the available pressure for demand is effectively 52.1 psi, a deficit of about 13.9 psi. The supply is not adequate.
The contractor's comparison of 520 gpm to 620 gpm is flawed because it ignores pressure. A lower flow can still require more pressure if the new remote area has higher friction loss, a different elevation, or a different hose allowance. The controlling check is pressure at the new demand point, not a simple flow comparison.
2. The contractor offers to lower the design density to 0.10 gpm/sq ft to reduce demand. Evaluate this proposal and identify one acceptable hydraulic alternative.
Read the worked answer
The density reduction is not acceptable. Ordinary hazard Group 1 retail requires a minimum design density of 0.15 gpm/sq ft over the applicable design area; 0.10 gpm/sq ft is a light-hazard density and would not be a valid basis for the new occupancy. Reducing density to make the numbers fit is a design violation, not a hydraulic solution, and the AHJ should reject it.
An acceptable alternative is to reduce the pressure demand by lowering friction loss. For example, upsize the 2.5-in branch lines and the cross main serving the new remote area to 3 in, then recalculate. Larger pipe reduces friction and may bring the 66-psi demand below the available pressure with margin. Another acceptable option is to add a listed booster pump sized for the deficit, but that is a supply-side change and must be designed and accepted as part of the system. Any change must be supported by a revised hydraulic calculation.