What Does this mean "Liquid's diffusion process"

[Zd.Mh]

I'm trying to understand this paragraph:
"After the liquid leaves the impeller, but before exiting the pump, the final transformation of energy occurs in a diffusion process. An expansion of the flow area causes the liquid’s velocity to decrease to more than when it entered the pump, but well below its maximum velocity at the impeller tip. This diffusion transforms some of the velocity energy to pressure energy."
The sentences in Italic had confused me a bit.

 

[bimr]

Diffusion - the spreading of something more widely. Pumps usually have a smaller discharge than the pump intake. The velocity leaving the pump is higher than the pump intake because of the smaller discharge on the dicharge.

I will add that a hydraulic pump doesn't creates pressure in a hydraulic system. The fact is that a pump creates flow, not pressure. Pressure is the resistance to flow.

 

[Zd.Mh]

Thx for answering firstly!

That's weird. The reference said:"An expansion of the flow area causes the liquid’s velocity to decrease to more than when it entered the pump" and you said "The velocity leaving the pump is higher than the pump intake because of the smaller discharge on the discharge". Decrease or increase?!

 

[Artisi]

Increase of velocity in the discharge.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[bimr]

The paragraph is not written very clearly. You can see from the picture that the outlet is generally smaller than the inlet side.

 

[Zd.Mh]

I think you're right. Maybe there's a mistake in the book.
Anyway I appreciate the time you spend to put your thoughts here!

 

[TenPenny]

I think what the sentence may be trying to say is that the liquid leaves the tip of the impeller at a certain velocity, then decreases in velocity as it (circulates in the volute) and exits the pump, but the liquid at this point is still at a higher velocity than when it entered the pump.

A typical volute will have an increasing area from the cutwater to the discharge, and this is where the liquid velocity decreases and converts to pressure.

That's my take on what that sentence is trying to convey.

 

[Rputvin]

I think you're missing the sentence all together- it's describing the behavior between the impeller and discharge, while comparing it to the suction. Not very straightforward, but correct.

"An expansion of the flow area causes the liquid’s velocity to [highlight #FCE94F]decrease to[/highlight] more than when it entered the pump, but well below its maximum velocity at the impeller tip. This diffusion transforms some of the velocity energy to pressure energy"

It decreases in velocity from the impeller to the discharge, but is still greater than the velocity at the suction/inlet.

Fluid velcoities
Inlet = V1
Impeller = V2
Discharge = V3

V1 < V3 < V2