New High-Hazard Storage Behind an Existing Pump
A 12,000 ft2 single-storey warehouse, 24 ft to deck, has an existing wet-pipe system placarded at 0.20 gpm/ft2 over 1,500 ft2, 550 gpm total demand including 250 gpm hose, and 65 psi required at the pump discharge flange. The fire pump is rated 750 gpm at 90 psi, with 150% overload at 60 psi and 105 psi churn. The city main is static 65 psi and flowing 55 psi at 1,000 gpm. A new tenant wants cartoned unexpanded Group A plastics in double-row racks 20 ft high. The consultant proposes 0.45 gpm/ft2 over 2,500 ft2 with 250 gpm hose, still at 65 psi at the pump flange, noting that the incoming main is 6 in and 'will be fine'. Evaluate the proposal.
1. Compute the new system demand and test the existing pump and city supply against it.
Read the worked answer
New sprinkler demand
- 0.45 gpm/ft2 x 2,500 ft2 = 1,125 gpm
- Plus hose allowance 250 gpm = 1,375 gpm total
City supply at that flow
- Known: 10 psi drop between static and 1,000 gpm.
- Friction and supply losses scale with Q^1.85: (1,375 / 1,000)^1.85 = 1.80.
- Drop at 1,375 gpm = 10 x 1.80 = 18 psi, giving 47 psi residual.
- Pump must therefore add 65 - 47 = 18 psi at 1,375 gpm.
Existing pump
- Rated 750 gpm; 150% of rated = 1,125 gpm.
- The demand of 1,375 gpm is 183% of rated capacity - beyond the maximum operating point and effectively past the end of the curve, where head has collapsed toward zero.
- A centrifugal fire pump cannot be operated there: the motor is overloaded, the pump cavitates, and there is no usable pressure left to feed the system. Passing 18 psi through a pump operating at 183% is not a design, it is a coincidence waiting to fail.
Conclusion: the proposal is not supportable. The consultant's 'the main is 6 in' argument ignores two things: friction losses rise with Q^1.85 for every foot of pipe, not just the main, and the 65 psi flange requirement was carried over from a much lighter density. Higher density means higher branch-line and cross-main friction, so the required flange pressure will also rise - the assumption is wrong in both directions at once.
2. Give the owner a ranked set of alternatives, and identify the non-hydraulic constraints the hydraulic analysis will not catch.
Read the worked answer
Alternatives, ranked
- Cut the storage height. A 12 ft stack of the same commodity, or a downgrade to Class IV, brings the density and area back into a range the 750 gpm pump can serve. Confirm the actual density/area from the applicable storage tables - these figures must come from the tables, not from this analysis - and re-run the hydraulics. This is usually the cheapest capital path and preserves the building.
- New fire pump plus main verification. Size the pump to put the design point near its rated flow, not at 183%. Confirm suction conditions with the city supply (static 65 psi is modest), confirm the 6 in main really is 6 in internally, and recompute the flange pressure at the new density rather than reusing 65 psi.
- ESFR. No in-rack piping, but ESFR generally requires high discharge pressures and high simultaneous flows; check it against 1,375 gpm before assuming it is the answer, and confirm the ceiling height, deflector clearance and aisle geometry suit the listing.
- In-rack sprinklers. Can reduce the ceiling density but adds piping inside a rack that a tenant will rearrange; it also transfers maintenance risk to the tenant.
Non-hydraulic constraints
- 20 ft of storage in a 24 ft building leaves 4 ft; with deflectors at about 22.5 ft the clearance is only around 2.5 ft. Verify the minimum clearance for the chosen sprinkler type - it is not automatically satisfied.
- Water supply duration increases with the hazard class; the existing supply may not hold it.
- Branch-line and cross-main sizing designed for 0.20 gpm/ft2 is unlikely to deliver 0.45 gpm/ft2; the pipes, not just the pump, may need upsizing, which feeds back into the hydraulic calculation.
- The placard, hydraulic nameplate and the AHJ record all have to change, and the tenant's rack layout has to be a controlled document - the density assumes a storage arrangement that a warehouse manager can destroy in an afternoon.