precast submersible pump station

Water Resources: How A Submersible Pump Works

Brent Petring Pump Stations

Basic Impeller Theory

The Impeller is the heart of the pump in a high flow pump station. It is the only part of the pump that adds energy to the liquid. Grossly simplifying the process, energy is added by accelerating the liquid from the smaller radius at the impeller inlet to larger radius at the impeller exit. The implications of this are that we can increase the amount of energy input into the fluid by increasing the outside diameter of the impeller or increasing the speed that it operates at.

The Casing

The Energy added by the spinning impeller exits as a high speed fluid is which is generally not very useful for process applications. What we usually require as a pump output, is higher pressure and not higher speed. To convert from higher speed to higher pressure we defuse (reduce the speed) the flow converting high velocity energy into pressure and energy. See Bernoulli’s equation below.

Where:
  • P = Pressure in PSI
  • sg = Specific gravity
  • V = velocity of the fluid in ft/sec
  • g = acceleration due to gravity (32.16 ft/sec2)
  • Z = Elevation of the centerline of the liquid path
Subscripts:
  • 1 = upstream condition
  • 2 = downstream condition

In the pump the elevation change from point 1 (exiting the impeller in this case) to point 2 (centerline of the volute channel) is generally small and is considered negligible in most cases. This leaves us with only the change in pressure and velocity to consider for the two sides of the equation to balance. If there is a decrease in velocity from point 1 to point 2 there must be a corresponding increase in pressure from point 1 to point 2.

  • P = Pressure in PSI
  • sg = Specific gravity
  • V = velocity of the fluid in ft/sec
  • g = acceleration due to gravity (32.16 ft/sec2)
  • Z = Elevation of the centerline of the liquid path
Subscripts:
  • 1 = upstream condition
  • 2 = downstream condition