Pressure Tanks - Hydraulic Calculation Method

I have a question regarding performing or checking hydraulic calcs for a pressure tank supplied system in a retrofit application.

As an example, suppose our system is located 5 floors below the existing pressure tank for a high rise building in New York City (NYC). The tank was designed and installed under prior codes and utilized the pipe schedule methods. We may have 2 tanks connected that are 9,000 total gallons capacity each with 1/3 air and 2/3 water pressurized at 75 psi (the initial pressure) at the tank.

Our connection to the sprinkler riser being 5 floors down now has 75 psi plus the pressure gained by gravity (5 psi per floor typical NYC arrangement) equaling 100 psi static available.

The jurisdiction now requires all systems to be hydraulically calculated. The calculation are done to the point of connection to the riser and not all the way back to the water source (NYC quirk) and no hose demand.

The contractor provides a calculation stating that the system demand is below the 100 psi (95 psi residual) and less than 400 gpm (light hazard occupancy 30 minute supply), and claims it works. We’ve argued that as the tank drains, the air-to-water ratio changes, therefore the pressure available at the end of the 30 minutes is not the same as initial tank pressure.

Using the formula given in the Annex of NFPA 13, we can solve for what a system demand should be given the existing initial conditions of the tank plus the pressure gained by gravity and say that as long as the calculated system is less than that pressure, the system is acceptable. But is that a correct assumption?

I want to give them all the correct procedure, methodology and theories behind this subject.

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