draft (water)

The term drafting water refers to the use of suction to move a liquid such as water from a vessel or body of water below the intake of a suction pump. A rural fire department or farmer might draft water from a pond as the first step in moving the water elsewhere. A suction pump creates a partial vacuum (a "draft") and the atmospheric pressure on the water's surface forces the water into the pump, usually via a rigid pipe (sometimes called a "dry hydrant") or a semi-rigid "suction hose". The maximum height the water can be lifted is directly limited by the atmospheric pressure. Even with a perfect vacuum, at sea-level, with an average of 14.7 pounds per square inch, a pump could lift water a maximum of 33.9 feet. A typical fire department pumper, in good condition, can lift water between 25 and 30 feet, but this number is reduced at higher altitudes, lower barometric pressures, or higher water temperature, and with higher system friction. To reduce drafting friction and obtain a larger flow or higher lift, a larger cross-section of suction hose is employed. For example, using a 5-inch hose, a pump that could lift 500 gallons-per-minute up 23 feet would only be able to lift the same amount of water 12.5 feet through a 3.5-inch hose. Fire engines are often seen carrying two or three 10-foot lengths of suction hose, but the longer the lift, the lower the flow, for a fixed diameter suction hose and a given pump. As can be seen from the limitations described above, it would be pointless to attempt to draft water to more than about 30 feet above its surface. Multiple relays could be used if the need arises, with a suction pump drafting up to 30 feet, and discharging thousands of feet away (with enough power and large enough hose). It is also possible to use a gravity siphon to draft water for a small lift, and this technique is often used in forest fire suppression, where portable reservoirs (e.g., 1,000 to 3,0000 gallons) are filled with water and small hoses are used downhill of the tanks. The nozzle pressure is proportional to its distance below the reservoir surface. Forty-three percent of the distance, in feet, is approximately the number of pounds per square inch pressure (e.g., 100 feet lower: 43 psi). The tank may itself be gravity-fed through hoses from a nearby water source, or by pumps or helicopters delivering water from further away.

Draft (Water)

See also