Centrifugal and positive displacement pumps, both used on board. Selection of these pumps basicaly depends on the behavior and ship board requirements. Centrifugal pumps are most common type of pumps on board ships. Almost all of the pumps on board for seawater or fresh water are of centrifugal type. positive displacement pumps are used for high pressure requirements, like FO supply pump, FO and LO transfer pump, sludge pump etc. Behaviour and performance of these pumps changes distinctly for any change in parameter like viscosity, presure head. Lets see how these pumps behave with the change through graphs.
By looking at the performance chart to the right you can see just how different these pumps are. The centrifugal has varying flow depending on pressure or head, whereas the Possitive Displacement pump has more or less constant flow regardless of pressure.
Flow rate versus viscosity
Another major difference between the pump types is the effect viscosity on the capacity of the pump. Centrifugal pumploses flow as the viscosity goes up but the Possitive Displacement pump’s flow actually increases. This is because the higher viscosity liquids fill the clearances of the pump, causing a higher volumetric efficiency. Remember, this chart showseffect of viscosity on the pump flow; when there is a viscosity change there is also greater lineloss in the system.
Viscosity versus Efficiency 
Viscosity also plays an important role in pump mechanical efficiency. Because the centrifugal pump operates at motor speed, efficiency goes down as viscosity increases due to increased frictional losses within the pump. Efficiency often increases in a Possitive Displacement pump with increasing viscosity. Note how rapidly efficiency drops off for the centrifugal pump as viscosity increases.
Behaviour of NPSHR
Another consideration is NPSHR. In a centrifugal the NPSHR varies as function of flow, which is determined by pressure and viscosity as discussed above. In a Possitive Displacement pump, NPSHR varies as a function of flow which is determined by speed. The lower the speed of a Possitive Displacement pump, the lower the NPSHR.
Operation of pumps at different points on curve
Another thing to keep in mind when comparing the two types of pump is that a centrifugal pump does best in the center of the curve. As you move either to the left or right, additional considerations come into play. If you move far enough to the left or right, pump life is reduced due to either shaft deflection or increased cavitation. With Possitive Displacement pump you can operate the pump on any point of the curve. In fact the volumetric efficiency as a percent actually improves at the high speed part of the curve. This is due to the fact that the volumetric efficiency is affected by slip, which is essentially constant. At low speed the percentage of slip is higher than at high speed.
Pump selection
High viscosity : Viscosity affects cosiderably the flow rate and efficiency of a centrifugal pump. While many centrifugals are cataloged to 1,000 cSt and higher, PD pumps are clearly the better choice when considering the high energy costs.
High Pressure : While some centrifugals can be run in series to boost the pressure, none can compete with PD pumps for high pressure applications. Pressure limits will depend on the design of each pump, but pressure of 250 PSI (580 feet) are not unusual for a PD pump, with some modelsgoing over 3,000 PSI (7,000 feet). The capability for a PD pump to produce pressure is so great that some type of system overpressureprotection is required.
Suction Lift : By their nature, PD pumps create a vacuum on the inlet side, making them capable of creating a suction lift. Standard ANSI centrifugals do not create a vacuum and cannot create a suction lift. There are self-priming centrifugal designs that can lift liquid an average of 15 feet when partially filled (13” hg vacuum). Many dry PD pumps can pull that or better and wetted PD pumps (a pump that is not full of liquid but with some liquid in it) can often reach vacuums of 25 to 28” hg. PD pumps are the logical choice when a suction lift is required.
Pressure Variation : The first graph above clearly illustrates the effect of changes in pressure changes the flow rate of a centrifugal pump. Additional restrictions such as debris in a filter, corroded / rough pipingor a valve left too far closed (or too far open) can have a dramatic effect on a centrifugal pump’s flow rate and efficiency. PD pumps maintain their flow rate and efficiency even with significant changes in pressure
Variation in Viscosity : Many liquids vary in viscosity depending on temperature or due to chemical reaction. A rise in viscosity will alter the flow rate and efficiency of centrifugal pumps. PD pumps become the clear choice for variable viscosity applications.
Cost : PD pumps are costly as compare to centrifugal pumps. So for cost cutting purposes centrifugal pumps comes first in choice.
High flow Rate/Capacity : Where high flow rate at low pressure is required, centrifugal pumps are better option. They can give maximum flow rate that is the reason they are used for ballast, main sea water pump, fire pump etc.
The data presented here is for specefic application. centrifugal was picked at its Best Efficiency Point (BEP) and the Possitive Displacement pump (Internal Gear) was selected to match the flow, viscosity, and pressure. Different applicationshave different curves and efficiencies. These curves are presented as an example of the performance behavior differences of the two pump principles.