Water Supply: Drafting, Relay, and Mobile Supply
Drafting depends on atmospheric pressure. At sea level that is about 14.7 psi, and since 1 psi supports 2.31 feet of water, the theoretical lift limit is about 33.9 feet. Perfect vacuum is impossible, so practical lift is far less - keep the lift at 10 feet or less and never plan on more than about 20 to 25 feet, remembering that available atmospheric pressure drops as altitude rises. Use hard suction with tight couplings, set the strainer at least 24 inches below the surface but off the bottom, and watch for whirlpools that pull air. If a portable tank runs low, a jet siphon or a second intake keeps flow going.
Relay pumping moves water long distances through large-diameter hose. The source pumper supplies the next pumper in line, which boosts pressure to the next, and so on; the receiving pumper at the fire should hold roughly 20 psi residual at the intake so the relay does not collapse when a nozzle opens. In a water shuttle, dump valves, porta-tanks, and fill-site turnaround time set the achievable flow rate more often than tank size does. Dual pumping and tandem pumping are options when one pumper is not enough.
Hydrant testing uses the residual-drop rule: a drop of under 10 percent indicates three or more additional hydrants could be fed, 10 to 15 percent about two, 16 to 25 percent about one, and more than 25 percent is unsatisfactory. Worked example: static 70 psi, residual 55 psi - drop is 15/70, or about 21 percent, so only about one more hydrant is available. Traps: drafting from excessive lift, and confusing static, residual, and flow pressures.