understanding a pump curve

[dreamer81]

hi, im kinda new here, but ill ask anyways.

We all know what a pump curve looks like
example here

http://www.mcnallyinstitute.com/02-html/2-03.html

i dont understand it though. When the capacity is low, the pressure (head) is high. That doesn't make sense to me.

When the pump only lets 1 galleon/min through, how can the pressure then be high???

hope someone can enlighten me

 

[electricpete]

Imagine you have a "real" voltage source consisting of an internal ideal voltage source in series with an internal resistance.


When the current is high, the terminal voltage is low.
When the current is low, the terminal voltage is high.

Substitute dp for terminal voltage and flow for current and you have a pump (well, not exactly, the plot of the real voltage source would be a straight line on voltage vs current graph, while the plot of pump is a curve on dp vs flow).


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[Barry1961]

One way to think of it is by power. Power in this case is pressure times flow(psi*gpm). For there not to be a magic increase in power when one increases the other decreases.

There are of course other factors in the equation, such as how efficient some pumps are at low speed.

I think one rough horse power formula for fluid flow is (psi*gpm)/2638.

Barry1961

 

[rbulsara]

dreamer:

Look up the formula for the pump HP. I am not sure u understsand all the definitions of the terms, like capacity.

Pump HP is proportional to product of flow rate and the Head. So a pump with given power (HP) rating will deliver a higher flow rate (capacity) against a lower head or lesser flow for higher head. The product of the two will be a constant.

A pump can deliver say 2 gpm to 5 floor or 5 gpm to 2nd floor (this is a crude expample..I must admit, to make the point clearer..)

 

[dreamer81]

ok, looking on pressure as the height the pump can lift the water, makes it more clear to me.

 

[quark]

dreamer,

You have to take it the other way round. When the pressure is higher, the flowrate is low. Any fluid moving device first overcomes resistance and then matches the flowrate accordingly as per its characteristic(i.e its curve).

Suppose, you selected a pump with a higher head than what, actually, is required then the pump gives more flowrate.

 

[dreamer81]

Ohhh...

for instance a water hose.
When u put ur thumb in front of it, and only let a little slip out, the pressure is high, but almost no flow....

and when u take ur thumb off of it, the pressure is low, but the flow is high...

thanx i get it now

 

[dreamer81]

An additional comment:

I am going to make an application that can load pump curve and power curve coefficients into a frequenzy converter.

Has anyone any experience with this? Or know of a similar program???

 

[quark]

Danfoss already did it for parallel pumping operation and developed good logic for load sharing.