Flowrates through pipes

Oct 05, 2006 4 Replies
R

I just caused myself severe brain damage by Googling for 'flow rates through pipes' and produced awful flashbacks to integration and differential equations. Both of which I thought I'd left behind years ago.



Where can I find - in simple terms - how to caclulate how much water a



40 mm waste pipe can pass in units that I can apply to the area of roof to be drained?

This is is the situation:



I have a waterbut which, courtesy the previous owners, collects the water from my carport roof and the whole roof area of the back of the house. This water was distributed around front and rear gardens by a snakes nest of hosepipes which frequently blocked flooding the garage. I did away with the hoses and, temporarily (5 years ago!) fitted a submersible pump to shift the water, vai a delivery pipe over the roof of the garage, into the back garden out of harms way. On Sunday we had the mother of a thunderstorm accompanied by a power cut. Ooops! Garage nearly flooded.



My solution is to run a 40mm waste pipe from a collector in place of the waterbut through the garage and out to the back garden.



Why 40mm? That the size of the panel between the garage door and the wall of the garage (which is reinforced concrete).



I can fit 2 x 40mm pipes, i.e. one above the other but not 50mm.



TIA



Richard


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O

I think the Building Regs approved documents have tables for calculating required guttering for roofs.

Then you convert the cross-sectional area of standard gutter downpipe into the csa of 40mm pipe, and add on some spare to allow for added flow resistance in the pipes.

The simple method would be to see how many downpipes were used on your house, or are used on a neighbour's similar house, and convert.

Owain

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I

A couple of ways. A) do the equations. B)

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- which is perhaps not as friendly to DIYers as it might be - however, not so bad when you realise that pressure drop of 10000Pa/m = 1m of head per m of pipe, 10Pa/m =

1mm/m head, ... C) get a bit of pipe, and try it, with a funnel and a bucket. 36mm (ID) of pipe, using the above calculator, and entering a flow rate of 1m/s for water, with smooth pipe. 313Pa/m.

So, to drive water along at 1m/s - a volume of about 3cm*3cm*100cm =

900ml/s = .9l/s, you need a head of 30mm per meter. This will move a little under a thousand gallons an hour.

Double the flow rate, and you more than double the required slope, hitting 10000Pa/m (a slope of 'straight down') at some 7m/s, 6l/s or so.

Adding another pipe helps lots.

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R

Gulp!

Standard rear downpipe config was two.

As for calculations from Ian - thanks but I think I might just be able to manage the bucket and funnel test only.

Alternatively I could look on the downpour/power cut coincidence as being slightly more than a maximum credible accident and hope that it doesnt occur again - oh and not leave anything that is susceptible to damp in the garage.

Thanks

Richard

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T

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need 2.1 litres per second capacity per 100m^2 flat roof. for sloping roofs area is (h/2+w) X l

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