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Author Topic: how to measure flow rate  (Read 390 times)
andymb
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« on: February 20, 2021, 03:43:19 PM »

a bit of time on the forum has got me wondering if i have a viable hydro project.......

we have an old water tank which holds 28000 litres and the outlet is a 6 inch iron pipe, I cant fit a big enough container under the pipe to measure flow in a given time, and obviously cant see through the pipe to measure the water's velocity. I did think about trying to work the flow rate out from the pressure, but obviously pressure would drop as the water level falls, so i don't think this would work either

has anyone got any ideas?

base of tank to outlet is only 2m but it seems such a waste when this much of it is ready made!

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pantsmachine
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« Reply #1 on: February 20, 2021, 03:47:30 PM »

What sort of drop do you have from tank to house? Should be able to work out a flow rate from the tank vol to drop, pressure depending on pipe dia, You'll be wanting to reduce that 6" dia down a fair bit. That'll boost your pressure to start with.
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andymb
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« Reply #2 on: February 20, 2021, 04:11:45 PM »

wont the pressure drop as the tank empties though?

tank is downhill of the house, 2m fall from bottom of tank to outlet.....I will have to plumb the outlet into an underground drain pipe which is maybe another metre down, so that may buy me a little more height but obviously I wouldn't want to have any working parts below ground

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pantsmachine
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« Reply #3 on: February 20, 2021, 04:38:05 PM »

Yes it will but how fast depends on shape of tank. You haven't mentioned how the tank will be refilled? I don't think you have enough enough stored and replenishing energy for this to work. I hope I'm wrong.
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Alan D
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« Reply #4 on: February 20, 2021, 05:25:16 PM »

If its only getting re filled from roof / gutters the volume the energy produced
is going to be very little. Not worth it.
If you can divert from upstream river / source to downstream river / source
its worth a go.
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andymb
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« Reply #5 on: February 20, 2021, 05:37:08 PM »

the tank gets the rain water from our roofs and part of and our neighbours roof, - there will be quite a bit of roof area when I finally get the rest of the roof on!
even though I only have 2 of the downspouts on, it frequently half fills after a rainy 8 hours or so.....the victorians must have been ahead of their time with regards rainwater harvesting!! once the rest of the downspouts are on I think it'll re-fill pretty regularly over the winter when the PV isn't doing much.

I have managed to get the outlet valve un-siezed so that you can let all 27000 litres go at once

the tank is rectangular - 2.6m wide by 8.5m long, gently sloping stone floor, the outlet is on one of the 2.6m sides at floor level.

i know that i haven't got much head to play with but seems a waste for it to do nothing!
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Philip R
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« Reply #6 on: February 20, 2021, 07:05:08 PM »

If you had a continuous 10 metre head of pressure and 100% efficient turbine, the energy contained as potential energy of about 2.7 MJ or 3/4 kWh. You are not going to get much electricity from such a system. it is not exactly the Three Gorges or Itaipu in terms of flow rate ( endurance ) or head. Forget about power generation.

However, consider using it for rainwater harvesting; flushing the loo, garden irrigation,  car washing, and if you are really keen, the washing machine.

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Philip R
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« Reply #7 on: February 20, 2021, 07:12:30 PM »

A "V" notch weir is a useful means of flow measurement. The height of water in the V notch equates to the flow rate. The only problem is it needs to be calibrated for the geometry and the fluid viscosity. There is reading on Google about it.
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andymb
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« Reply #8 on: February 21, 2021, 08:19:00 AM »

Phillip - Thanks for your very unequivocal answer - whilst i knew that i was never going to have enough power to reliably run for instance lights, or boil the kettle, I was more hoping for a bit of a project and maybe a way to supplement battery charging/reduce generator use over the winter - but there is no point in a "dead end" project, so thanks for saving my time!

re: using it for rainwater harvesting/washing machine........very likely. washing the car?? a project too far Cheesy Grin
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oliver90owner
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« Reply #9 on: February 21, 2021, 08:44:00 AM »

mgh Is the simple formula for potential energy - where m is mass, g is the gravitational field strength anf h is the height.  All units in SI provides the energy in Joules.  One Joule/second is one Watt.  So easy to calculate the potential.  Philip points out that there are inefficiencies in every stage of the process.

The last time I used a V notch, to measure flow, was back in the 1976 drought.  We abstracted water and needed to monitor the flow-rate of the river - just in case it got to the stage where the supply was insufficient.  We did return cooling water to the river,  so it would never have run quite dry.... it was more of a worry of the process requirements falling below the minimum, during normal working hours.
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djs63
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« Reply #10 on: February 21, 2021, 09:21:41 AM »

It doesn’t matter now but could you measure the flow by noting how much the top surface goes down multiplied by the surface area to give a volume?
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Iain
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« Reply #11 on: February 21, 2021, 09:25:45 AM »

Hi

Quote
.e: using it for rainwater harvesting/washing machine........very likely.

I have used rainwater for the last 12 years or so.
It allows for watering the garden if needed but feeds the house all the time, flushing toilets, "rinse" taps for the 2 dowstairs sinks. The washing machine ran for 5 years using the water however the washing machine in the house now uses hot fill so becomes tricky with rainwater( I think I get more benefit using the solar thermal hot water than rainwater in this case), however my washing machine in the garage for overalls uses rainwater all the time.
Worth keeping it simple.
I have a header tank that allows all household use to be under gravity. It is kept topped up using a simple pump and float switch
One third of my household water now comes from rainwater.
I have a Peristaltic pump that doses the header tank with chlorine automatically. I store about 9000 litres and so far has never run out, however has become low once, 500 lts.
Nice little project!
Iain
« Last Edit: February 21, 2021, 09:28:29 AM by Iain » Logged

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andymb
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« Reply #12 on: February 21, 2021, 11:26:12 AM »

djs63 - that makes perfect sense - thank you - I can't believe that I didn't think of that!

Iain - do you find that your washing machines run ok and without any water pressure issues, or do you also have some sort of inline booster pump? also,  to what degree do you filter your water? - I've read that some machines can be very fussy about even the tiniest particles?
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Iain
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« Reply #13 on: February 21, 2021, 04:20:52 PM »

Hi
I have just checked my rainwater pressure. 0.45 bar. I have no issues with it filling, just slightly slower than mains. I don't actually filter my water. There is a strainer on each of the 3 diverters on the downpipes.

The shed roof water goes into an IBC in the shed then overflows into 3 other IBC's still in the shed. So any sediment settles in the first IBC, i have cleaned it twice in 12 years.

I have a header tank in my shed at the top of the garden, this gives me gravity flow to the house. If i didn't have a shed I could have done the same with a header tank in the loft. This is just filled with a pump and float switch.
Each toilet i fitted and extra float valve(on the opposite side of the cistern) so mains and rainwater can never cross contaminate.

When I first did the house washing machine I added extra inlet solenoids for the rainwater and a timer. Then when starting the machine we pressed a button for the timer, this allowed a mains hot water for the wash and rain for the rinse. After a few years of working great I decided to have a modify and ended damaging the control board. We ended up with a new machine and I was banned from modifying it. The repaired machine ended up in the garage for my overalls totally on rainwater.
My header tank is only about 0.5 m above the toilets  and they work. the washing machines have about 4.5 M head of water. It would be higher if the header tank was in the loft but the shed works for me.
You could always put a 25 micron filter  in line. I used to have one fitted but I used to get fed up changing the filter!!  about every 4 months. All has still worked well since removing the filter.
Iain
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andymb
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« Reply #14 on: February 23, 2021, 08:56:10 AM »

Iain - thanks for the info on your set up -all very reassuring - i was expecting the pressure to have to be much closer to 1 bar for the washer to work, and i thought that a much more sophisticated filtration system would be necessary!

i hadn't really thought about using a settlement tank rather than a filter but again, once its said it makes perfect sense really.....

thanks again, andy
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