A centrifugal pump with a double inlet rotor is of a type used for many duties where a large capacity output is required. As the forces acting on either side of the rotor are the same, no thrust action is set upon the rotor shaft when this pump is in operation, except that due to the weight of the rotating parts in vertical pumps. Generally, centrifugal pumps are driven by an electric motor; exceptions being single or two stage boiler feed water pumps, where steam turbines are more commonly used. The exhaust steam from the turbines driving the feed pumps is used in the feed water deaerator to remove air and gases by heating the feed water. For economic reasons, large capacity centrifugal pumps used for cargo discharge in oil tankers are usually driven by steam turbines. The steam turbine has virtually infinite speed control and is therefore considered to be safer for use with cargo pumps. The' steam turbine is fitted in the engine room and the pump driving shaft passes through a self aligning gas and water sealing gland fitted on the pump room bulkhead. Where the design capacity of the boiler plant is limited, as on some diesel propelled tankers, electrically driven cargo pumps may be used. Centrifugal pumps are not self-priming, i.e. they are not capable of creating a vacuum enabling the suction line to fill with liquid and so allow the pumping action to be started. This disadvantage is overcome by using liquid ring vacuum pumps. When only a small number of centrifugal pumps are fitted in the engine room a vacuum pump may be fitted on each centrifugal pump. As the number of centrifugal pumps increases it becomes more economical to fit a central priming system.