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a stupid question about submersible pumps


dave21478
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The situation...a submersible 850 watt pump in a lake, feeding an artificial waterfall through plastic pipe with an i.d. of 25 mm. The pump is rated at between 3 and 5 cubic meters per hour and can pump to 12m or so height-wise.

Total rise from the pump to the head of the waterfall is only about 4 meters.

Will moving the pump further down the lake away from the waterfall reduce the flow of water to the top? the pipe will obviously be longer, put perfectly horizontal along the lake bed at the same height as the pump before it rises to the top of the falls.

Thanks.

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Yes there will be some reduction in flow because of the extra pipe work even if it is horizontal. there is always some resistance in the pipe. The actual effect may well be very small - it all depends on how fast the water is flowing in the pipe and the diameter of the pipe.   

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Replacing the pipe in its entirety will be better than adding the extra length with a coupler because the coupler may introduce a restriction. Furthermore the nature of the restriction is non linear and difficult to predict for us novices.

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[quote user="dave21478"]The situation...a submersible 850 watt pump in a lake, feeding an artificial waterfall through plastic pipe with an i.d. of 25 mm. The pump is rated at between 3 and 5 cubic meters per hour and can pump to 12m or so height-wise. Total rise from the pump to the head of the waterfall is only about 4 meters. Will moving the pump further down the lake away from the waterfall reduce the flow of water to the top? the pipe will obviously be longer, put perfectly horizontal along the lake bed at the same height as the pump before it rises to the top of the falls. Thanks.[/quote]

Yes drastically, because of friction on the pipe walls (irrespective of the horizontal nature of the pipe run)  you would increase the head loss from around 1.5m to 4.5 m excluding the actual vertical rise to the waterfall.

That is a lot of electricity (850w) for what is a small waterfall flow, by comparison a good pump should be able to do the same for around 100-120w  better if the pump was mounted higher to reduce the head from 4m to a lower figure.

Increasing the pipe diameter will achieve what you are after, a 32mm bore pipe would give you much the same head loss as your current shorter 25mm bore pipe run but you'll still be consuming 850w which is the area you could review as any changes would not only pay for themselves but go on to save you money ongoing and of course a little less power from a power station, so a little bit better for us all.

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I was surprised to read your figures which I am sure you have calculated and are correct, I didnt think the straight losses would be anywhere near that significant without bends and elbows, then I thought Ah! the pipe is undersized for the flow but you call it a small waterflow fall, I guess that is a relative term, waterfalls are after all usually a lot bigger than a 25mm pipe bore.
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You gave the height from the pump to the discharge, but the height the water is being lifted is from the water level to the waterfall discharge.

Mounting the pump higher will not make any significant difference to the flow, as the pump is only lifting water from the level of the pond up to the discharge level for the waterfall, whatever its depth.

 

 

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[quote user="nomoss"]

You gave the height from the pump to the discharge, but the height the water is being lifted is from the water level to the waterfall discharge.

Mounting the pump higher will not make any significant difference to the flow, as the pump is only lifting water from the level of the pond up to the discharge level for the waterfall, whatever its depth.

[/quote]

Normally I would agree with the negative head situation but the pipe size is small, very small so the additional pipe run does make a difference even though the head is as you state. Dave hasn't actually said how deep the pump is just 4m from the top of the water fall to some point or other at the pump.  The water is being forced up the narrow bore pipe at quite a rate and therefore a very turbulent flow, thus raising the pump and shortening the pipe run will lower the friction loss in the pipe work which will have a difference in flow.  With a correctly sized pipe for the flow I would agree with you.

All off Dave's point.

 

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Water depth is about 1.8 meters.

Trial and error today showed that the longer run reduced flow by almost half....which is too little for the waterfall to look ok.

I can increase the pipe diameter for the length of the lake*, but not at the section where it rises behind the falls, so a pointless effort. It seems the pump will have to stay closer to the bottom of the falls and I will have to find another solution for the problems I was trying to solve...ie stagnation. The lake is a long dog-leg shape and I was hoping that moving the pump to the far end would create a small current in the water. Plan B is to add a second pump purely to circulate water within the lake.

* - at what point does a pond become a lake? The owner insists on calling it a lake, but I reckon its just a big pond!

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[quote user="dave21478"]Water depth is about 1.8 meters. Trial and error today showed that the longer run reduced flow by almost half....which is too little for the waterfall to look ok. I can increase the pipe diameter for the length of the lake*, but not at the section where it rises behind the falls, so a pointless effort. It seems the pump will have to stay closer to the bottom of the falls and I will have to find another solution for the problems I was trying to solve...ie stagnation. The lake is a long dog-leg shape and I was hoping that moving the pump to the far end would create a small current in the water. Plan B is to add a second pump purely to circulate water within the lake. * - at what point does a pond become a lake? The owner insists on calling it a lake, but I reckon its just a big pond![/quote]

Hi Dave, increasing the pipe diameter across the lake run but not the part where it rises behind the waterfall is not a pointless effort. You will drastically reduce the friction headloss in the pipe run so the flow will be far higher. Yes you will be necking it down at the water fall but that is a short run compared to the whole length.  BTW what is the reason for the pump relocation it surely wasnt to stop stagnation with a 25mm pipe for a whole lake? The whole idea needs a re think.

Did you discuss with the owner using a more suitable pump for the job, a rough calculation was based on 15hours/day would cost €309 /year whereas an eco pump would be around €35 so you could run several pumps to move the water around for the same electricity you are wasting one just one and still be quids in at the end of one year!.

http://en.wikipedia.org/wiki/Pond  or when writing your advertisement [;-)]

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I have to confess to not really knowing what I am doing.

Rich people eh?....

"So, Dave, I want a lake. In the garden."

"Lake? well....I....could start to look up how....."

"well, more of a big pond really. but yeah, a lake"

"um..."

"great. make it this shape *Grabs stick and draws squiggles in the dust at his feet*....put it over there....cut down those trees and make it as large as possible."

"er....."

"Oh, and I want a kind of square projection into the water as the wife wants a chinese pagoda thing to be built there overhanging the water on 3 sides...and we want a waterfall or something at this end - not massive just a reasonable trickle splashing down over rocks to make a nice noise..."

"...!!!"

"Oh, and make it narrower here as I want an arched wooden bridge...just a simple thing made out of really chunky wood like railway sleepers or something."

"...!!!!"

"Yeah, it will need to be strong enough to drive the tractor over too."

"er..."

Great. I will advance some money to cover the costs. Im off to China for 3 months with work. Send me photos when its done."

So I did it. It turned out pretty well, but the circulation needs work. Its a long, almost dogleg shape about 30 meters long, with the overflow at the far end. The waterfall is at the opposite end.

There is a rainwater recovery system that is fed from some of the main building gutters and I have run a pipe down to the lake, so that with the tap open, rainwater flows in. this can be used to keep the level topped up and to allow fresh water into the lake each time it rains. There is also a well nearby for topping up if required.

With the pump I have, which is just a basic thing from a diy shop for about €80, the waterfall effect is perfect, but that was with the pump located close-ish to the base of the falls. The water at the far end was turning pretty manky so I planned to move the pump to the far end to create a small current, but the reduction in flow doesnt look great. I suppose I could upgrade the pipework, or I could put the pump back to the base of the falls and add a second pump at the far end to circulate water somewhat, but as far as flow etc goes, I have no idea what I am doing...its all trial and error.

It uses a rubber liner, and when I did the hole, I made terraced layers to the sides to make shelves. After advice from a garden centre that does ponds, various types of plants are planted in fabric baskets, but will take a while to develop, and it really needs a lot more to oxygenate the water.
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Dave,

Do you think you clients would like a fountain in their pond? This would not only move the water but would also aerate it. Avoid a high spray or you may experience water loss in high winds.

The earlier comments about the power of the pump seem justified but this may be related to the cost of the pump. A good quality, high capacity pump usually costs several hundred pounds/euros.

Alistair
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[quote user="napoleon"]The earlier comments about the power of the pump seem justified but this may be related to the cost of the pump. A good quality, high capacity pump usually costs several hundred pounds/euros. Alistair[/quote]

A good quality high capacity etc, etc, will cost more than a cheap to purchase, expensive to run pump for sure but as the payback is so short it's a no brainer.

[quote user="Theiere"] based on 15hours/day would cost €309 /year whereas an eco pump would be around €35 so you could run several pumps to move the water around for the same electricity you are wasting one just one and still be quids in at the end of one year!.[/quote]

so after 10 years €3090 vs €350!

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