Water Supply Principles, Hydraulics, and Flow Calculations
Core constants and relationships
One cubic foot holds 7.48 gallons; one gallon of water weighs 8.34 lb; one psi supports 2.31 feet of water, or 0.434 psi per foot of elevation. Flow rate is volume divided by time, Q = V / t. In a series circuit, flow is the same throughout and pressures add; in parallel lines, flows add. Pump discharge pressure is built from nozzle pressure, friction loss, elevation, and appliance loss: PDP = NP + FL + elevation pressure + appliance loss.
Applying the numbers
Friction loss rises with the square of flow and with hose length. Elevation pressure is added when water is pushed uphill and subtracted when it flows downhill. For dump and fill problems, convert tank volume to gallons and divide by the rate in gallons per minute to get minutes.
Worked example
A 2,000-gallon tank is filled at 500 gpm and dumped at 750 gpm. Fill time = 2,000 / 500 = 4.0 minutes. Dump time = 2,000 / 750 = 2.67 minutes, or 2 minutes 40 seconds. If the dump site is 20 feet above the fill site, the static elevation penalty is 20 x 0.434 = 8.7 psi, before friction and appliance losses. Reasoning: keep units consistent, convert seconds to minutes only at the end, and do not confuse pump capacity with actual dump rate.
Traps
Do not use 8.34 lb for a gallon of foam solution without checking proportioning. Do not forget appliance and elevation losses. Do not assume a 1,000-gpm pump dumps at 1,000 gpm; valve, plumbing, and vent size often limit dump flow. Do not mix gallons per minute with total gallons when calculating time.