Light Hazard Office Fit-Out With a Marginal Static Supply
A sprinkler contractor is bidding a 42,000 sq ft light hazard office tenant fit-out in a two-story building. The owner's civil engineer reports a city main static pressure of 62 psi, residual 48 psi at 1,500 gpm during a flow test, and a 6-inch ductile main at the street. The building has a 4-inch dedicated fire service and a wet-pipe system with a new 1,000 gpm fire pump rated at 65 psi churn and 55 psi at 1,000 gpm. The contractor's preliminary hydraulic calculation shows a required demand of 950 gpm at 58 psi at the base of the riser, before adding pump churn and safety margin. The contractor is considering deleting the fire pump to cut cost and asking the owner to accept a 4-inch service instead of 6-inch.
1. Should the contractor delete the fire pump? Walk through the supply-versus-demand analysis and identify what must be verified before any pump deletion is proposed.
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No - the numbers do not support it. The available residual at 1,500 gpm is 48 psi. The system demand is 950 gpm at 58 psi at the riser base, plus friction loss in the 4-inch service and the pump churn/margin. At 950 gpm the city supply will be well below the 48 psi residual measured at 1,500 gpm (the supply curve is steeper at lower flow but the static-to-residual drop shows the main is already stressed). Even ignoring service friction, 58 psi required exceeds what the supply can deliver at that flow. Deleting the pump would leave the system undersized and likely fail the hydraulic calculation. Before proposing any change, the contractor must: (1) obtain the actual flow test curve, not just two points; (2) run the calculation with the pump removed and confirm the residual at the demand flow; (3) verify the service size against NFPA 13 and local requirements; (4) document that the proposed change does not reduce the level of protection below the approved design. A cost-driven deletion that fails the calculation is not a value-engineering option - it is a code violation.
2. The owner asks whether the 4-inch service is acceptable. What is the correct response and what documentation should accompany it?
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The 4-inch service may be hydraulically adequate only if the calculation proves it delivers the required flow at the required pressure with the pump in service. Service size is not a code minimum in NFPA 13 per se; it is governed by the hydraulic calculation and the water purveyor's requirements. The contractor should not agree to a 4-inch service based on the preliminary numbers. The correct response: keep the 6-inch service unless a stamped hydraulic calculation, using the actual flow test and the fire pump curve, demonstrates the 4-inch service meets the demand with margin. Documentation should include the flow test report, the pump curve, the hydraulic calculation, and a written statement that the design meets NFPA 13 and the authority having jurisdiction's requirements. Any deviation should be submitted to the AHJ for approval, not decided informally between contractor and owner.
3. During a pre-bid walk, the contractor notices the existing riser has a 2-inch drain and no test header. What should the contractor flag, and why does it matter for this project?
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Flag both. A 2-inch main drain is undersized for a system of this size; NFPA 13 requires main drains sized to the system, and a 2-inch drain on a large light hazard system will not allow a proper drain test at the required flow. The absence of a test header means the contractor cannot perform the forward-flow test of the fire pump at its rated capacity, which is required at acceptance and periodically thereafter. Both deficiencies affect the acceptance test plan and the ITM program. The contractor should include correction of the drain and installation of a test header in the scope, note them in the bid, and confirm with the AHJ whether the existing riser will be reused or replaced. Bidding without addressing them shifts risk to the contractor after award.