Centrifugal pumps work by using an impeller attached to a rotating shaft to move liquid from the pump inlet to the discharge outlet. As the impeller spins, it creates lower pressure at the center to draw liquid in and higher pressure at the outer edge to push liquid out. When selecting a pump, the key parameters are capacity (flowrate) and total head. Total head considers the static head from elevation changes as well as friction, pressure, and velocity heads from piping, valves, and changes in pressure or speed of the liquid. The specific gravity of the liquid affects the pressure but not the total head required from the pump.