Hydraulic Calculation Dispute on a Combined Standpipe/Sprinkler System
A contractor submits hydraulic calculations for a 5-story combined standpipe and sprinkler system in a new hotel. The calculation shows the sprinkler demand at the top of the riser as 750 gpm at 65 psi, and the standpipe demand as 250 gpm at 100 psi at the topmost outlet. The contractor added the two demands as 1,000 gpm at 165 psi at the base. The reviewing engineer notes that the building is 60 feet tall, the fire pump is rated 1,000 gpm at 90 psi, and the city supply is 70 psi static. The contractor's calculation does not include the standpipe friction loss from the top outlet down to the base, and the pump curve shows 1,000 gpm at 90 psi but only 750 gpm at 105 psi.
1. Is the contractor's method of adding the sprinkler and standpipe demands correct? Explain the correct approach.
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Adding the two demands as 1,000 gpm at 165 psi is not correct. NFPA 13 and NFPA 14 require the combined system to be analyzed so that the total flow is the sum of the sprinkler demand and the standpipe demand only where they are supplied simultaneously from the same source, and the pressure required is the greater of the two demands at the point of connection - not the sum of the pressures. The correct approach is to compute the sprinkler system demand at its most remote area, compute the standpipe demand at the topmost outlet, then combine flows at the base and determine the pressure required to satisfy both at the point of connection. The pressure is governed by the higher-pressure demand, not by adding 65 psi and 100 psi. The contractor also omitted the standpipe friction loss from the top outlet down to the base, which must be included. The result is a calculation that overstates pressure in one place and understates friction in another - both errors.
2. Given the pump curve and city supply, does the system appear to work? Show the reasoning.
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The pump delivers 1,000 gpm at 90 psi and 750 gpm at 105 psi. The combined demand is 1,000 gpm (750 sprinkler + 250 standpipe). At 1,000 gpm the pump provides 90 psi. The city static is 70 psi, but the residual at 1,000 gpm is unknown and must be obtained from a flow test - the contractor cannot assume 70 psi is available at that flow. The required pressure at the base must include the sprinkler demand pressure at the top of the riser, the standpipe demand pressure at the top outlet, the elevation (60 ft ≈ 26 psi), and the friction loss in the riser and underground. A rough check: 100 psi standpipe + 26 psi elevation + riser friction already exceeds the 90 psi the pump provides at 1,000 gpm before adding city supply. The system likely does not work as calculated. The contractor must re-run the calculation with correct combination methodology, include all friction losses, and verify against the actual flow test.
3. What should the reviewing engineer require before approving the submittal?
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The engineer should require: (1) a corrected hydraulic calculation using the proper combined-demand methodology, with the pressure at the point of connection determined by the governing demand, not by summing pressures; (2) inclusion of standpipe friction loss from the top outlet to the base and all riser and underground losses; (3) the actual flow test report, not just static pressure; (4) the fire pump curve with the operating point identified; (5) verification that the pump can supply the combined demand at the required pressure, or a revised pump selection; and (6) a stamped calculation and a narrative explaining the methodology. Approval should be withheld until the corrected calculation demonstrates the system meets NFPA 13 and NFPA 14 with the required safety margin.