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why is 'height' relevant for pumps?

greenink

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2 May 2011
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I'm confused about why external pumps and filters have maximum 'head heights' - the distance below the surface of the tank that you can put them. (My tank is mounted in a wall, pretty high up, and I need a big height gap between the surface of the tank and the filter/pumps).

Why aren't the only relevant factors the pressure differential between the intake and the output of a pump (e.g. if one is much higher), and the length and curvature of the pipe (for flow resistance) regardless of whether it goes up or down? Any filter or pump is a closed circuit (in my understanding) and so should have equal pressure throughout once it's filled with water - so it shouldn't make any difference at all whether the pump is below or at the same level as the surface of the tank.

Basically, for each ml of water you pump up, another ml is coming down, equalising the pressure on the pump... no? :shifty:

Maybe the issue is to do with initial priming...? Can anyone who understands physics properly explain? Or is it something manufacturers just write on pumps for no reason?
 
Yes, but it still has to be pumped back up again :) Take an external filter's rated power. Then when you put tubes on/media in, it drops. Then lift those tubes to the height of the tank and measure again. It will drop more.

If you tried syphoning water down into the filter and you had the outlet at the same level as the inlet, it would not run out the other end - it would stop. With the pump running, there is still a point where it will struggle to deliver the water back up to the highest point.
 
You have a good point - water is in effect being moved around a closed circuit so in theory, once the siphon is started, there is no head of pressure!
I guesse there is lots of restriction within the pipe work that would limit flow?
How about a centre heating pump in your house, that can circulate water two storeys high, although that is a true closed circuit, I don't know how that is so different?
 
Mike, you're correct in your assumptions, if you take and return water to the same body of water, the distance the pump is below the surface of the water is irrelevant to the pumping head.

However, the more pipework in the system the greater the frictional loss. The greater the distance below water level the greater the static pressure on the pump, perhaps it's to do with the pump seals?

Cheers

Shaun
 
I design pumping systems for a living and the pumps we use have a max rated pressure, not to do with the head they produce but associated with the pressure the seals etc can withstand, so i guess it's the same with external canister filters etc?
 
I built a four foot deep tank a few years back, the tank was on a two foot six tall plinth so there was a 6'.6'' head involved. I could not find any filter on the market that would guarantee the 6'.6'' of water pressure would not cause leaks from the canister.
So it was not the pumps flow rate but the integrity of the canister itself that restricted the filters use.
 
To get an idea get your garden hose pipe full of water trailing on the ground,now blow the water out of it. Easy huh.

Phase 2. Fill hose but have an assistant hold the pipe out of an upstairs window. Now blow hard. It's more than likely you cant do it and end up getting soaked.

The higher the pump has to pump the water the harder it gets. Water is quite heavy and gravity is a hmmmm ;)
 
foxfish said:
I built a four foot deep tank a few years back, the tank was on a two foot six tall plinth so there was a 6'.6'' head involved. I could not find any filter on the market that would guarantee the 6'.6'' of water pressure would not cause leaks from the canister.
So it was not the pumps flow rate but the integrity of the canister itself that restricted the filters use.

Aha! That makes sense
 
And here it is:

-----Ursprüngliche Nachricht-----
Von: Formular Internet
Gesendet: Mittwoch, 31. August 2011 21:51
An: technik
Betreff: Frage zu Products

Hi,

Why do you specify a maximum head height for filters? Is this because of the pump strength, or is it something else, like the integrity of the seals?

Thanks

Mike

--

On 1 Sep 2011, at 14:19, "service, eheim" <eheim.service@eheim.de> wrote:

Dear Mike,

Thank you for your email inquiry. This is because of the pump strength.

<image003.jpg>

I hope that this information is helpful for you.

If you have further questions please don't hesitate to contact us again.

Mit freundlichen Grüßen / Yours faithfully

i.A. Robert Penz

Service EHEIM GmbH & Co. KG
Str.5 73779 Deizisau
GERMANY

Phone: 0049 (0)7153 7002-183
Fax: 0049 (0)7153 7002-111
Internet: www.eheim.de

--

Thanks for such a quick reply, much appreciated. So does that mean the seals can withstand slightly greater (30cm) height, accepting that the pump might not work as effectively? My tank is in a wall and about 2m up from the floor, with no way to run pipes except through the wall.

Mike

---

Straße: PLZ: Land: UK

Hello,

That should be no problem.

Mit freundlichen Grüßen / Yours faithfully
i.A. Robert Penz
Service

EHEIM GmbH & Co. KG
Plochinger Str.54
73779 Deizisau
GERMANY

Phone: 0049 (0)7153 7002-183
Fax: 0049 (0)7153 7002-111
Internet: www.eheim.de
E-Mail: Robert.Penz@eheim.de

Very impressed with Eheim, have to say. But still don't understand the physics!
 
^^ Love home experimenting :D

I wish I had seen this thread earlier. but, as you have found out, head pressure (AKA lifting height) relates the the ability of the pump to lift water in an open loop (where the output of the pump is not below the surface of the water that the input is drawing from). In a closed loop situation, it can be thought of as its ability to push past obstructions. You are absolutely correct, once a siphon is active, it doesn't need to "lift" that amount of water, as its being pushed up to the same level as the tank water by atmospheric pressure.

I'm sure that the seals/pump integrity are pressure rated, but that is not what the head rating describes.
 
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