liam.mccaffrey
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| posted on 25/9/05 at 08:57 AM |
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Pump Theory
I need some help guys.
We are moving a pump in work and i need to work out the performance difference, the only thing changing is the inlet head or pressure
1.
Basically the term "head" when applied to fluid flow. what does it mean? for example what head would a 10m tank of water provide and how
do you calculate it?
2.
What is the effect on the pump performance of changing the head of water at the inlet of a pump..
3.
My boss thinks that you can use the affinity laws for pumping to work all this out but i thought the affinity laws were to work out pump perfomance if
you were changing the speed or rotor diameter. As these will not be changing i don't think affinity is any use?
Please help i'm in a bit of a pickle here
Cheers guys 
[Edited on 25/9/05 by liam.mccaffrey]
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rayward
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| posted on 25/9/05 at 09:35 AM |
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have just had word, with our mechanical bod, and he said provided that the flow of fluid to the pump is not restricted, the pump performance should
not change, but if the outlet of the pump has to pump higher(i.e to a greater head, its flow rate will be reduced).
HTH
Ray
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liam.mccaffrey
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| posted on 25/9/05 at 09:44 AM |
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my thought was that if the speed of the pump could not change then the output flow would not increase. If the new inlet was at higher pressure would
the pump require less power to run or be more efficicant or somthing?
i am having trouble with the term "head" could someone explain it?
really appreciate your help
[Edited on 25/9/05 by liam.mccaffrey]
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Jonr
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| posted on 25/9/05 at 09:48 AM |
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1. Head is simply the height of water either on a suction side that the pump needs to work or on the discharge side of the pump how far the pump can
actually push the water till it deadheads/stalls.
10 M of head in freshwater is equalt to 1 bar or 14.7 psi.
2. You don't say what kind of pump you are moving? But in simplest terms if your pump is centrifugal and is capable of raising the pressure
by 2 Bar or 20 m of head. By raising the inlet pressure by 10 M of head the pump performance should not change just the discharge pressure will rise
by the increase of head so if the pump is capable of raising the pressure 2 bar/20m head and your feeding it with 1 bar / 10 m of head the outlet
pressure will rise accordingly to 3 bar or 30 m of head. Thats all assuming you are not changing anything else especially on the discharge side.
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britishtrident
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| posted on 25/9/05 at 09:53 AM |
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A lot depends what you are pumping and its temperature --- when pumping hot or volatile fluids a higher inlet head helps prevent cavitation.
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liam.mccaffrey
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| posted on 25/9/05 at 09:59 AM |
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sorry, pumping cold water
right so if i can find out the inlet/ outlet pressures/heads before we moved the pump and then work out what the new locations inlet head/pressure i
can find the new outlet head/pressure.
also will the volume of water transferred change?
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Jonr
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| posted on 25/9/05 at 10:05 AM |
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In theory yes, and you should be about right. But in reality if you are changing the configuration of the pipework then you are going to have a new
set of restrictions and different frictions in the pipe which all add up to make a difference, but if your pump is going to be doing basically the
same job then the pipework ain't going to be that different.
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Jonr
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| posted on 25/9/05 at 10:08 AM |
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volume of water pumpeed should not change, unless in your original installation you did not have sufficient head to adequatley feed the pump. Some
pumps need a minimum head to work properly, others suck happily from a negative head.
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Peteff
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| posted on 25/9/05 at 10:09 AM |
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It's all affected by pipe size as well. I used to work out sizes for a friend who was flushing pipes and needed to know the velocity of water
needed to move a given size particle. If you are only changing the head you shouldn't really make any difference as jonr says.
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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liam.mccaffrey
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| posted on 25/9/05 at 12:20 PM |
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also if the feed tank is above the pump inlet does that affect the head?
is head the same as pressure. ie can you convert between the 2
I'm sorry to ask loads of questions but i need to understand this really bad. If I can help anyone with anyhting please ask.
My speciality is scanning(its part of my job)
On a completely unrelated topic  i have all issues of PPC
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Jonr
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| posted on 25/9/05 at 12:40 PM |
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The head is the total height above the pump inlet, so all the pipework and all the water in the tank, . So if you had a 2 m tall tank full to the
brim. and an 8 m tower on which the tank was mounted and the total height difference between the pump inlet and the top of the water is 10 m then your
head is 10m of water,
Yes you can convert them. 10 M of water is 1 Bar.
"Inches Water gauge " or "/wg is commonly used to measure small pressures, usually a differential pressure across a filter or
something.
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liam.mccaffrey
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| posted on 25/9/05 at 01:42 PM |
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its slowly falling into place
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