Axial Flow pump - Flow / BHP
Axial Flow pump - Flow / BHP
(OP)
Hello,
Can someone please explain why BHP decreases as flow increases on a Axial Flow pump curve.
Thanks
Can someone please explain why BHP decreases as flow increases on a Axial Flow pump curve.
Thanks





RE: Axial Flow pump - Flow / BHP
I'm afraid that's as deeply as I can explain it; basically impeller design determines the shape of the performance curve and rising HP with lowering flow is a characteristic of the axial flow design.
One of the pump design experts we have frequenting this forum could give a much more detailed answer, but you might have to get pretty deep into pump hydraulics to fully answer your question.
RE: Axial Flow pump - Flow / BHP
RE: Axial Flow pump - Flow / BHP
hp=gpmxft/3300/eff (imp.gallons) or
kW=l/sxm/102/eff ( metric terms).
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.)
RE: Axial Flow pump - Flow / BHP
http://www.pumpsandsystems.com/topics/pumps/centri...
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.)
RE: Axial Flow pump - Flow / BHP
RE: Axial Flow pump - Flow / BHP
RE: Axial Flow pump - Flow / BHP
"Can someone please explain why BHP decreases as flow increases on a Axial Flow pump curve." First there are some very important unstated assumptions in this statement. The main one is that the head or "back pressure" remains constant. This is often true for axial flow pumps, which are used to raise water up a fixed height.
The impeller of a centrifugal pump takes low velocity water at its axis and spins it up to a higher velocity at its periphery. Accelerating the water to a higher velocity takes energy. More flow requires more power. At zero flow little power is used because the water just spins in the pump casing and there are only some friction losses due to turbulence and liquid shear. The pressure at the outlet of the pump will be at maximum, and it is caused by the centrifugal force of water spinning in the casing. There is very little circulation of high pressure water back to the low pressure of the inlet. With an open outlet, the water simply leaves the pump at high velocity and little pressure. But this also means high flow entering the pump, which then has to be accelerated up to the higher velocity, using more power.
An axial flow pump does not generate centrifugal forces (in reality they do to some extent because the impeller is spinning,and "mixed flow" pumps will use this effect). It works like a boat propeller. The blades are paddles and work on water like paddles on a paddle wheel, except they are arranged differently on rotating shaft. A paddle wheel is relatively simple to understand. When a boat with a paddle wheel is going full speed (water is flowing past the boat) the paddles do not have to push hard on the water, as they are both moving at close to the same speed. Now tie the boat to shore so it cannot move (water flow rate is zero) and the paddles use a lot of power trying to push through the water at the same rpm. Water is simply churned and recirculated from behind to in front of the paddle wheel. In a boat propeller or axial pump impeller the angle of attack of the blades changes with flow rate. At high flow the angle will be close to zero, and at zero flow the angle of attack will be much higher, given a constant rpm.
So at dead-head a centrifugal pump will use minimum power and an axial flow pump will use maximum power. And that is the best I can do for an explanation in a short post. I did look for better ones on the internet, but while most of the explanations technically correct, they are not very illuminating, as some of the statements are open to many interpretations by anyone who doesn't already understand what is being said.
RE: Axial Flow pump - Flow / BHP
Axial flow pumps are dynamic pumps, meaning they utilize fluid momentum and velocity to generate pump pressure.
Artisi posted the equation that demonstrates power consumption in pumps:
HP = gpm *ft/3300/eff
Back to the pump curve. If one increases the flow by a factor of 4 (10,000 to 40,000), the head decreases by a factor of 0.52 (38 ft to 20 ft), and the efficiency increases by a factor of 2.7 (from 30% to 82%). If you multiply these factors, 4 * 0.52 / 2.7 = 0.7 (the HP decreases with an increase in flow).
At 10,000 gpm, the brake HP is 320. At 40,000 gpm, the brake HP is 246.
The increase in pumping efficiency with flow is one of the major reasons for the decrease in HP with flow.
RE: Axial Flow pump - Flow / BHP
Go away from that zone and things get worse. Axial pumps are more like propellors and my assumption has always been that at higher head requirement, more flow spills back off the blades or creates lots of swirl between the blades. When velocity or flow is high it works well, lower velocity and you run into partial stall.
see http://www.codecogs.com/library/engineering/aerody... or
http://thesis.library.caltech.edu/2624/1/Linhardt_...
if you want a bit more theory
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Axial Flow pump - Flow / BHP
RE: Axial Flow pump - Flow / BHP
There will be minor centrifugal forces coming off the blade of an axial flow pump, but this (normally)corrected by the diffusing blades / section of the pump unit which are located directly following the impeller.
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.)
RE: Axial Flow pump - Flow / BHP
There are many intelligent pump experts that use the terminology so it does not cause my nose to get out of joint:
Igor Karrasik's as well as the Hydraulic Institute's definition of a centrifugal pump:
"A centrifugal pump is a simple machine consisting of a set of rotating vanes enclosed within a housing or casing."
[img https://books.google.com/books?id=KEzjBwAAQBAJ&...]
"An axial flow pump is essentially a high capacity low head centrifugal pump."
http://www.peerlessxnet.com/documents/tibs/TIB-20_...
per A.J. Stepanoff's "Centrifugal and Axial Flow Pumps":
"Centrifugal pumps comprise a very wide class of pumps in which pumping of liquids or generation of pressure is effected by a rotary motion of one or several impellers. In the early stage of centrifugal pump development, pumping was ascribed to centrifugal forces. Later this class of pumps was extended to include axial flow pumps, and the conception of the centrifugal action of the impeller was inadequate to explain the operation of axial flow pumps. However, treatment of axial flow pumps as a class by themselves was not justified, because hydraulically they represent one extreme of a continuous series of pump types. This continuity applies to both theoretical treatment and design methods. Some intermediate types are called mixed-flow pumps. In these, the flow through the impeller has both radial and axial components and the impeller resembles a ship propeller."
Per "Pumping Station Design" by Garr Jones:
"In colloquial usage in the United States, a "centrifugal pump" is any pump, in which the fluid is energized by a rotating impeller whether the flow is radial, axial, or both (mixed)"
RE: Axial Flow pump - Flow / BHP
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.)
RE: Axial Flow pump - Flow / BHP
The actual reference should be to the impeller design or geometry.