Determining the Size of Storage Capacity

[Clamont]

I would like to get some assistance in sizing a water storage tank based on the required flow rates from foam application, cooling water and the hose stream allowance following NFPA 15 and NFPA 11.

Please see below required flow rates;

Cooling Water Flow Rate - 394m3/hr (For 2 hours as per IP Part 19)
Water Required for Foam - 53m3/hr (For 30 mins for Flash point between 37.8 Deg C and 60 Deg C)
Hose Stream Allowance - 11m3/hr (For 20 mins as largest tank falls between 10.5m and 28.5m)
TOTAL FIRE PUMPING RATE NOT CONSIDERING PRESSURE LOSSES AND HYDRAULIC IMBALANCES- 458m3/hr

If we multiply the capacities by the required duration we get the below volumes
Cooling Water = 394*2 = 788m3
Water for Foam System = 53*(30/60) = 26.5m3
Hose Stream Allowance = 11 * (20/60) = 3.6m3
GRAND TOTAL WATER STORAGE = 818m3

The concern I have if following the above method is that when the 30 minutes has elapsed for the use of foam and 20 minutes of Hose Stream Allowance all of the water will be diverted to the cooling system and will not last for the required 2 hours.

Can someone please advise that despite the duration required for the hose stream allowance and the foam system these should still be multiplied by the full 2 hours to guarantee that you have a 2 hour coverage for the cooling system as the pump will continue to pump at 458m3/hr (less of course the small additional losses through the increased velocity through the cooling water system).

Thanks in advance