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Centrifugal Pumps vs. Positive Displacement Pumps

Selecting the proper pump begins by knowing

  1. Total head or pressure against which it must operate
  2. Desired flow rate
  3. Suction lift
  4. Fluid characteristics (temperature, corrosiveness, etc.)

The piping system and pump interact to determine the operating point of pumps: flow rate and pressure.

 

Basic Comparison - Centrifugal Pumps
Vs. Positive Displacement Pumps
  Centrifugal Positive Displacement
Mechanics Imparts velocity to the liquid resulting in a pressure at the outlet (pressure is created and flow results). Captures confined amounts of liquid and transfers it from the suction to the discharge port (flow is created and pressure results).
Performance Flow varies with changing pressure. Flow is constant with changing pressure.
Viscosity Efficiency decreases with increasing viscosity due to frictional losses inside the pump (typically not used on viscosities abov e850 cSt). Efficiency increases with increasing viscosity.
Efficiency Efficiency peaks at best-efficiency-point. At higher or lower pressures, efficiency decreases. Efficiency increases with increasing pressure.
Inlet Conditions Liquid must be in the pump to create a pressure differential. A dry pump will not prime on its own. Negative pressure is created at the inlet port. A dry pump will prime on its own.