What the calculation proves
The calculation walks the path from the water supply through the mains and branch lines to the remote area, accounting for friction loss, elevation and the equivalent length of every fitting along the way. The output is a demand point: a flow and a pressure the system needs.
That demand point is then plotted against the supply curve from the flow test. If it sits inside the curve with margin, the system holds. If it sits outside, the design does not work no matter how good the drawing looks.
What a reviewer checks first
- Whether the remote area was selected correctly for the hazard classification.
- Whether the demand point falls inside the supply curve, and with how much cushion.
- Whether the pipe schedule in the calculation matches the pipe on the drawing.
- Whether the flow test data is current and from the right hydrant.
Where rejections come from
The third item on that list is where most submittals die, and it is entirely avoidable. When the drawings and the calculations are produced by different parties, a late routing change updates one and not the other. The calculation describes a system that is no longer the system on the sheet.
Why one team should do both
When the same team models the system, coordinates it against the other trades and runs the numbers, a routing change propagates to the calculation in the same pass. There is no version to reconcile because there is only one version.
Sources
- NFPA 13, Standard for the Installation of Sprinkler Systems.