Why are centrifugal pumps so inefficient?

[parsec]

I have been playing around with pump curves a bit and have noticed that in general, the best operating point yields a pretty poor efficiency relative to the pump input power (75% max). From a second law perspective, there shouldn't be any losses. Where is all of this energy being lost? Is it in turbulence? Viscous friction?

 

[ChemAir]

Usually fluid friction in the passages and channels in the pump and losses due to direction changes going through it. Additional power is lost turning parts (bearings, seals, etc.). A pump is not ideal, and can't meet calculated theoretical head, power, or efficiency.

Measure the temperature on the inlet and outlet, you will find most of it.

 

[Ketch22]

I think you will find your answer in the definitions. In general a centrifugal pump will produce the highest flow rate, however, the pressure curve will fall off quickly. Another style pump such as a multiple stage centrifugal pump will produce less flow but do so at a higher pressure.
Efficiency could be distilled to getting the most bang for your buck. Look at the pump characteristic map, the one that most closely fits your needs with the lowest power requirements is the "most Efficient."
If I need a very large flow at moderate pressures (think engine turbo charger) a centrifugal pump is a great choice as I get what I need. If I need a small flow at a great pressure (think deep well water pump) a multi-stage centrifugal a great choice.
If you look at the differing characteristics and what changes between applications you will see where the energy is expended. The "most efficient" is the one that best matches an applications requirements with the lowest input energy.

 

[parsec]

I just thought it was odd that what seems to be the most ubiquitous pump has such relatively poor performance (thermodynamically). In comparison, other bits of rotodynamic machinery typically have much higher efficiencies.

 

[Ketch22]

I just thought it was odd that what seems to be the most ubiquitous pump has such relatively poor performance .

Even within a single category there can be large variations. The Advertising slicks and even the specifications do not often reflect what you are looking at. In a centrifugal pump the type and direction of the vanes on the impellor can significantly affect the pump curve. Be very cautious that you are not reading a boilerplate pump curve but the actual curve for that impellor/volute combination. Centrifugal pumps are not the most efficient but they can be easily produced, readily repairable, economically priced, tolerant of challenging environments, and tolerant of liquids with a suspended solid level of higher than very low. Kind of makes them the quick and easy item of choice.