Electric-driven pumps, usually vertically mounted and fitted with separate motor-driven priming systems. Close-coupled designs have the pump rotor mounted on an extended motor shaft. This can cause difficulties when there is a need to open up the pump, as the motor may also have to be dismantled to gain access.
With owners expecting to shorten port turn round times; the need to get the ballast in or out of the tanks can take on a sense of urgency. Container ships are case in point. With containers stacked perhaps six high they cannot leave port until the ballasting is correct. This means that ballast pumps have to move impressive amounts of seawater.
If the operator says the ballast tanks have to be filled or emptied in a certain time, it is possible to look at this simplistically and divide their volume by the time to calculate the rate. However, as the tank empties, the head will reduce, and so will the effective flow rate. This in turn means the safety margin built in by the pump manufacturer is reduced, and friction losses in the pipework can take the flow out of specification.
A priming system with an adequate air-handling capacity is another important need. The pump/priming system not only has to contend with the depth of the tanks in the double bottom but also with the height of the pump above the tanktop. Air ejectors have limited capacity, so for the larger pumps separate motor driven pumps are required.