Tenant Fit-Out Hydraulics With an Aged Flow Test and a New Backflow Preventer
An office tenant fit-out adds 22 sprinklers and about 180 ft of 1-1/2 in branch line to an existing wet-pipe light hazard system designed at 0.10 gpm/sq ft over 1,500 sq ft. The building is served by a 6 in service with no fire pump. The approved hydraulic calculation on file shows 470 gpm at 66 psi. The only water supply data available is a flow test taken 14 months ago at a hydrant on the corner: static 74 psi, residual 56 psi at 920 gpm. The water authority now requires a reduced-pressure backflow preventer on the service; the manufacturer's sheet shows roughly 8 psi loss at 520 gpm. The tenant's designer calculates the new system demand at 520 gpm at 72 psi before the backflow device. The project manager says 'the supply is 74 psi static, the demand is 72 psi, we are fine, and the existing calc is already approved.'
1. Evaluate the project manager's reasoning.
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Three things are wrong. First, available pressure is not static pressure. The static reading of 74 psi applies at zero flow; at 520 gpm the pressure is substantially lower, and that flowing pressure is what the demand must be compared against. Second, the comparison is not demand against supply, it is demand plus the required safety margin against supply. Hydraulically calculated systems must show a minimum 5 psi margin between the supply curve and the total demand at the demand flow. Third, the 8 psi backflow loss belongs in the calculation, on the supply side of the device, so it raises the demand to about 80 psi.
There is also a documentation problem: the flow test is 14 months old, and most authorities require current data for a new calculation, especially where the demand is changing. The existing approved calculation is evidence about the old system only; adding sprinklers and pipe invalidates it for this analysis.
2. Using the flow test data, estimate whether the added demand can be supported.
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Use the flow test to build a supply curve. The drop from static to residual is 74 - 56 = 18 psi at 920 gpm. Friction loss varies with flow raised to roughly the 1.85 power, so at 520 gpm the drop is 18 x (520/920)^1.85.
520/920 = 0.565. ln(0.565) = -0.571; times 1.85 = -1.056; e^-1.056 = 0.348. So drop = 18 x 0.348 = 6.3 psi, and the available pressure at 520 gpm is about 74 - 6.3 = 67.7 psi.
Now compare: new demand of 72 psi plus the backflow loss of 8 psi equals 80 psi. Add the 5 psi safety margin and the required supply pressure is 85 psi against roughly 67.7 psi available. The system is short by about 17 psi. This is a clear failure, not a marginal one, and the interpolation is generous because real supply curves steepen.
The corrected conclusion: the fit-out cannot be permitted on the existing supply as calculated.
3. What are the viable solutions, and where must the fire department connection land relative to the new device?
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Options, roughly in order of cost: have the water authority confirm the current main capacity with a new flow test (mains get replaced and results change; if the main is genuinely stronger, the problem may disappear); reduce demand by using code allowances such as relocating sprinklers, using a larger K-factor at lower pressure, or reconfiguring the most remote area, which often recovers several psi; install a small fire pump sized for the shortfall; increase or loop the service; or add a storage tank where the main cannot be improved.
On the backflow preventer: the fire department connection must connect on the system side of the backflow preventer and its associated check valve. If the FDC lands upstream, the check valve blocks engine pressure from entering the system and the fire department is pumping against a closed door. Confirm the FDC is downstream, is sized and labelled for the system, and that the backflow assembly is listed for fire protection service and supervised so a closed isolation valve alarms. Include the device's friction loss curve in the calculation, not a single point estimate.