ESFR Warehouse: Supply-versus-Demand Comparison with a Marginal Water Supply
A 220,000 ft² single-story warehouse has a 40 ft flat ceiling and 25 ft of cartoned, unexpanded Group A plastic on double-row racks. The layout is a K=14.0 ESFR system, 12 design sprinklers at 35 psi. The current water flow test at the hydrant on the service main shows static 62 psi and residual 48 psi at 1,200 gpm, taken at the same elevation as the sprinkler riser. The 8 in ductile iron underground runs 300 ft to the riser, and the 6 in reduced-pressure backflow preventer is published at 8 psi loss at 1,200 gpm. The contractor submits a calculation showing a base-of-riser demand of 42 psi at 1,000 gpm plus a 250 gpm hose allowance, and writes that the supply "works" because 42 psi is less than the 48 psi residual. The submittal is on your desk for review before the permit set goes out.
1. Is the contractor's comparison of 42 psi demand against 48 psi residual valid? Identify each error in the submittal's logic.
Read the worked answer
No, and the errors compound - any one of them alone would invalidate the comparison.
1. Pressures are being compared at different flows. A water supply is a curve, not a number. The 48 psi exists only at 1,200 gpm; at 1,000 gpm the available pressure is higher, and at the true demand flow it is lower. Supply and demand must be evaluated at one common flow.
2. The hose allowance was added to one side only. If 250 gpm of hose is part of the demand, it must also be part of the flow at which supply pressure is read. You cannot add 250 gpm to the demand and then read the supply at a flow that excludes it.
3. Elevation was omitted. The deflectors sit roughly 39 ft above the riser base. At 0.433 psi/ft that is about 16.9 psi of static head. With 35 psi required at the head, the base-of-riser demand is at least 52 psi before a single foot of friction is counted. A 42 psi base-of-riser figure is not physically possible for this arrangement.
4. Losses between the test hydrant and the riser base were not deducted. The 8 psi backflow loss at about 1,200 gpm and several psi of 8 in underground friction sit between the supply point and the riser, so available pressure at the riser base is lower than at the hydrant.
The correct method is to fix the comparison flow first, then read both the supply curve and the calculated demand at that flow.
2. Build the correct demand-versus-supply comparison and state whether a fire pump is needed. Show the arithmetic.
Read the worked answer
Demand flow. Twelve ESFR heads: Q = 12 × 14.0 × √35 = 12 × 82.8 ≈ 994 gpm. Add the 250 gpm hose allowance → about 1,244 gpm, call it 1,250 gpm.
Available supply at 1,250 gpm. Using the 1.85 exponent: 62 − 14 × (1,250/1,200)^1.85 = 62 − 14 × 1.078 = 62 − 15.1 ≈ 46.9 psi at the hydrant.
Bring it to the riser base. Deduct the 8 psi backflow loss and roughly 2 psi of 8 in underground friction: about 37 psi at the riser base at 1,250 gpm.
Demand at the riser base. 35 psi at the head + 16.9 psi elevation + roughly 9 psi of branch line, cross main, and riser friction = about 61 psi. Apply the safety margin (10 percent of demand or 5 psi, whichever is greater) → about 66 psi required.
Result. Roughly 37 psi available against 66 psi required - a shortfall near 29 psi at 1,250 gpm. The system as designed will not work, and this is not a rounding-error gap.
Recommendation. A fire pump is required unless the design is changed. Select a pump whose rated flow is at or above the system demand so the demand point falls on the flat part of the curve, and read the actual curve at 1,250 gpm rather than sizing from the rating alone. Also confirm that churn pressure plus maximum suction pressure does not exceed the 175 psi rating of the downstream components, and that the suction conditions keep NPSH available above NPSH required. Alternatives worth pricing - a seasonal re-test, upsizing the underground, or lowering storage to reduce the design - should be presented to the owner, but do not approve the submittal as written.