Positive Displacement

• This class of pump differs from the centrifugal class by several important factors 
•  
• Generally self priming whereas centrifugal generally require a priming means such as a belt driven priming pump
• Require the fitting of a safety valve to limit maximum pressure- this pump cannot be started against a shut discharge valve.
  Centrifugal pumps may be started against a shut or partially shut discharge valve. This is especially true for larger pumps where the shutting of the discharge reduces starting and running load.
  It should be noted that the partially shutting of the suction valve on both types of pumps leads to damaging cavitation.
• Positive displacement pumps can handle high differential pressures
• More suited to low to medium flow rates
• May operate with higher viscosity fluids then centrifugal types.source(arab-training.com)