Fire Pump Operations and Hydraulics
Pump operators live by one equation: engine pressure equals nozzle pressure plus total friction loss plus or minus elevation loss or gain, plus appliance loss. Friction loss is computed as FL = C x Q squared x L, where C depends on hose diameter (about 15.5 for 1¾-inch, 2 for 2½-inch, 0.8 for 3-inch, 0.2 for 4-inch), Q is flow in hundreds of gpm, and L is length in hundreds of feet. Elevation converts at roughly 0.434 psi per foot, or 5 psi per floor of about 12 feet - add it for height, subtract it for a below-grade objective. Standard nozzle pressures are about 100 psi for fog handlines, 50 psi for solid-stream handlines, and 80 to 100 psi for master streams.
Know the pump ratings: 100 percent of rated capacity at 100 psi, 70 percent at 150 psi, and 50 percent at 250 psi. Understand the difference between a pressure governor and a relief valve, and remember that a centrifugal pump heated by churning water needs a small bypass or tank-fill line opened to stay cool. Cavitation sounds like gravel in the pump and comes from inadequate water supply, a blocked strainer, or too much lift - reduce discharge or improve supply rather than increasing throttle. Water hammer comes from abrupt valve movement; change settings slowly.
Worked example: a 200-foot 1¾-inch handline flowing 150 gpm. FL = 15.5 x 1.5 squared x 2 = 15.5 x 2.25 x 2 = about 70 psi. With a 100-psi fog nozzle at grade, engine pressure is 100 + 70 = 170 psi. Traps: forgetting elevation and appliance loss, and confusing residual with static pressure.