Understanding Pump Curves
How Fluids Move Through Impellers
or
How Pumps Transfer Energy into the Fluid
The flow path through the pump impeller determines how energy is transferred from the spinning impeller into the fluid. Radial Flow impellers transfer the energy primarily by centrifugal force, while Axial Flow impellers transfer the energy primarily by diffusion.
The system which describes the shape of the impeller, and therefore the relationship of centrifugal force versus diffusion force is called Pump Specific Speed, abbreviated Ns in the United States system. A knowledgeable person can use pump Specific Speed values to approximate what an impeller looks like, the relationship of power to flow rate, the operating range of the pump, how the pump will react to variable flow rates (above or below BEP), and more.
Radial Flow impellers "fling" the fluid outwards away from the axle at a sharp right angle so that centrifugal force is the primary method of energy transfer into the fluid. On the other extreme end of the scale, Axial Flow impellers push water forward in line with the axle or shaft (thus the name Axial Flow), with little outward movement of the fluid, so that diffusion is the primary method of energy transfer.
Axial Flow and Centrifugal pumps differ greatly in how they consume power in relation to the flow rate.
Radial Flow pumps consume more power as flow increases,
Because,
Radial Flow pumps meet more resistance as flow increases.
Axial flow pumps consume less power as flow increases,
Because,
Axial Flow pumps meet less resistance as flow increases.
To understand how this happens read further and follow the flow of water through the impeller.
Follow the flow path In the illustration above (blue arrows) and notice that the fluid is turned sharply at a 90 degree angle just after the fluid enters the impeller. After the fluid is turned 90 degrees, the fluid is then thrust outwards by the impeller vanes by centrifugal force. Therefore, centrifugal force is the principle means by which energy is transferred from the impeller into the fluid.
The Radial Flow pump meets less resistance as flow is reduced because less slow moving fluid entering the impeller eye is being brought up to speed by the impeller. At low flow rates the fluid becomes stagnant or captured inside the impeller, and therefore little slow moving fluid can enter the impeller to cause resistance.
Under No Flow conditions the pump driver need only maintain impeller speed against the following forces: Disc Friction (impeller body friction with the fluid surrounding the impeller), the small amount of fluid leaking by the wear ring, and there may be turbulence on the discharge and suction sides of the impeller.
Although radial impeller pumps can operate smoothly at low flow rates, they may also suffer damage at low flow rates due to suction side and discharge side recirculation cavitation damage. (See Recirculation)
Follow the flow path in the illustration above and notice that the path is in line with the "axle" or shaft turning the impeller (more accurately described as a propeller). As the propeller strikes and pushes the fluid forward fluid moves forward by diffusion while there is only a slight movement of the fluid away from the axle. Therefore, energy is transferred from the impeller into the fluid primarily by diffusion.
The Axial Flow pump draws more power as flow is restricted because as resistance to flow increases the propeller must push against that added resistance. Fluid downstream of the propeller becomes stagnant (stalls), causing a pressure increase back to the propeller, resisting the thrust of the propeller.
For this reason, axial flow pumps typically do not operate smoothly below BEP, and axial flow pumps may damage themselves at low flow rates due to excessive power draw and vibration.
We have described the two extremes in impeller design, Axial Flow and Radial Flow. There are impellers everywhere in between those two extremes. The range from lowest Specific Speed to Highest Specific Speed are named as follows:
Radial Flow (Ns = 400 - 2000), energy transfer is dominated almost totally by centrifugal force, power curve is proportional to flow rate.
Francis Vane (Ns = 2,000 - 4500), energy transfer is primarily centrifugal with significant contribution by diffusion, power curve is loosely proportional to flow rate.
Mixed Flow (Ns = 4500 - 10,000), energy transfer is about equal between centrifugal and diffusion forces, power curve is relatively flat in response to flow rate.
Axial Flow or Propeller (Ns = 10,000+), energy transfer is dominated by diffusion forces with very little centrifugal contribution, power curve is inversely proportional to flow rate (that is, higher flow = lower power).
Impeller and case design strongly affect the power to flow relationship. Pump testing provides the only way to accurately determine the power to flow characteristic.
Related Subjects on this Website:
Glossary - Full Load Amps (FLA)
Glossary - Service Factor (SF)
Non-Overloading Pumps and Motor Life Expectancy
Written By:
Richard Neff
President of Irrigation Craft
