Mobile Water Supply Apparatus Design, Components, and Water Systems
Tank and plumbing
NFPA 1901 treats a mobile water supply apparatus as one with a substantial water tank, commonly at least 1,000 gallons; verify the edition enforced in your jurisdiction. The tank must be baffled to limit surge, vented to allow air in and out during fill and dump, and fitted with an overflow that directs water away from the chassis and roadway. The tank-to-pump line, tank-fill line, and dump valves must be large enough to support the intended fill and dump rates. Dump valves are commonly 8 to 10 inches clear opening on large tenders, with air or electric actuation.
Supporting components
A portable tank, jet siphon, suction strainer, hard suction, large-diameter hose, low-level indicator, and drain are part of the water system. The pump and transfer case drive the tank fill and, at a dump site, the drafting pumper. Weight matters: water at 8.34 lb per gallon plus the tank, pump, and chassis must stay within axle and tire ratings.
Worked example
A 2,000-gallon tank carries 2,000 x 8.34 = 16,680 lb of water. If the chassis payload after body, pump, fuel, and crew is 14,000 lb, the apparatus is overloaded before hose and equipment are added. Reasoning: calculate water weight first, then compare with the manufacturer's gross axle weight ratings, not just the total gross vehicle weight rating.
Traps
Do not assume baffle compartments are separate tanks; they share a common tank and vent. Do not size a dump valve by opening size alone; plumbing and venting can still limit flow. Do not confuse pump rating with dump rate, and do not ignore axle loading when adding water, equipment, or a full portable tank.