Ships use centrifugal pumps for fire and flushing systems. Internal-combustion engines use centrifugal pumps to circulate cooling water. There are many types of centrifugal pumps, but all operate on the same principle.

The centrifugal pump uses the throwing force of a rapidly revolving IMPELLER. The rotating impeller creates an empty space at the center hole (eye). The pressure at this point is less than atmospheric pressure (because of the mechanically displaced liquid). This causes atmospheric pres-sure to act on the surface of the liquid being pumped, forcing it into the pump casing and through the hole in the center of the impeller. It is then discharged from the outer rim of the impeller.

By the time the liquid reaches the outer rim of the impeller, it has acquired considerable velocity (kinetic energy). The flow of liquid then slows down as it moves through a volute or series of diffusing passages. As the velocity of the liquid decreases, its pressure increases; and thus its kinetic energy is transformed into potential energy.

Types of Centrifugal Pumps

There are many different types of centrifugal pumps, but the two you are most likely to encounter onboard ship are the volute pump and the diffuser pump.

Figure 13-6.-Simple volute pump.

Figure 13-7.-Diffuser pump.

VOLUTE PUMP.-In the volute pump, shown in figure 13-6, the impeller discharges into a volute (a gradually widening spiral chan-nel in the pump casing). As the liquid passes through the volute and into the discharge noz-zle, a great part of its kinetic energy (velocity head) is converted into potential energy (pressure head).

DIFFUSER PUMP.-In the diffuser pump, shown in figure 13-7, the liquid leaving the impeller is first slowed down by the stationary dif-fuser vanes that surround the impeller. The liquid is forced through gradually widening passages in the diffuser ring and into the volute (casing). Since both the diffuser vanes and the volute reduce the velocity of the liquid, there is an almost complete conversion of kinetic energy to potential energy.