The Accidental Smallholder Forum
Smallholding => Techniques and skills => Topic started by: PaulB321 on October 16, 2019, 04:18:11 pm
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Hi
We have a newly erected 72ft polytunnel and are exploring different irrigation methods. At the moment we have an IBC tank collecting from the house roof connected to a standard hose pipe (1/2") running down to the tunnel. The IBC is about 7m higher than the tunnel and roughly 70m away. So we've got roughly 0.7bar but the flow from the hose down at the tunnel is pretty average. This is ok for now as we don't yet have much planted, but i'd like to achieve a better flow if possible.
So, I wanted to ask is it worthwhile running blue polypipe of 25mm or 32mm down there? Once I connect a standard hosepipe to the polypipe in the tunnel will I get appreciably more flow than just running the 1/2" hose all the way? I don't want to buy 100m of polypipe to find the flow increase is minimal.
Over the winter we're going to be experimenting with drip lines too - likely homemade old hose pipe with holes due to budget and reading drip lines tend to get clogged and nibbled by critters (we have a large mouse and vole population) so don't want to invest too much at this point. I'm guessing I wouldn't necessarily need the additional flow for this but I don't know. Also going to look at olla pots too if anyone's ever had an experience with them?
Either way we always want to have the option to hand water with a hose. I read the helpful post over at https://www.accidentalsmallholder.net/forum/index.php?topic=95211.0 (https://www.accidentalsmallholder.net/forum/index.php?topic=95211.0) but any thoughts on the best pipe to run down there for connecting a standard hose would be appreciated.
Paul
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what is the actual m2 size of you tunnel?
I think your problem will be volume as much as flow. In our wee propagation tunnel we use misters for our benches covering about 18m2 - I have about 12 mister heads and run them about 10 mins twice a day. Each one runs at about 47ltr/h so just for the irrigation of 18mm2 of seed and module trays we use around 187 ltrs a day (12headsx47ltr/h)x0.333hrs=187 ltrs a day
Our large tunnel at 240 m2 obviously doesn't need quite so much water as it doesn't dry out like a seed/module tray but still takes a good whack to irrigate.
The overhead system which has only 2 lines (it should really have 3 and doesn't quite reach the edge of the tunnel) has around 30 spray heads going at a rate of around 20ltr/h giving 600ltrs for a 1 hour water - at 2.5lts/m2...
We can easily use 1000 ltrs of water on an irrigation day and obviously in your case with direct harvested rain water you will find that the times you need the most water when its hot and dry you have the least.
To directly answer your question increasing the size of the pipe from 12mm2 to 25mm2 will increase flow but not pressure - if you then go back down to 12.5mm2 to actually direct the water i cant see that you will have better flow.
Hope thats useful
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How about running the water into ibc tanks by the poly tunnel them use a water butt pump to distribute the water
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I find leaky hose invaluable in the polytunnel, moisture under roots, less evaporation.
Just have to remember where it is when/if digging. ?
And my water supply is nowhere near 7m higher than PT.
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Thanks all for the info...
The tunnel is about 180m2 so will definitely be quite thirsty. We have a couple of IBCs which we intend to hook up in parallel and should be able to get hold of a few more from a friendly farmer neighbour who has a few knocking about.
I'd rather not try a water butt pump - we had a battery powered one (no mains nearby) that didn't have much more flow than what we have with gravity, and it broke just a year after we got it.
Leaky hose is definitely under consideration - have read they're easily damaged by rodents so as we a lot of mice and voles I may just test with holey hosepipes for now. I think most of our irrigation will come from leaky or holey pipes under mulch to reduce evaporation and how much water is needed. Still like to be bale to hand water too though.
From more reading online i'm still unsure whether to bother getting polypipe - lots of conflicting info as to whether flow will be increased once I reduce down to hosepipe. Some articles say the additional flow will mean less dynamic pressure loss (static pressure always stay the same) so should increase flow once pipe size is reduced. May see if i can get hold of a length of polypipe to do some experiments.
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We switched from 40 metres of 15mm to 40 metres of 19mm pipe to fill one water butt from another. The difference in flow rate was certainly noticeable, so changing to 25mm is going to be worthwhile.
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We switched from 40 metres of 15mm to 40 metres of 19mm pipe to fill one water butt from another. The difference in flow rate was certainly noticeable, so changing to 25mm is going to be worthwhile.
Thanks for this. I think the unknown for us is whether the flow will be substantially any better once we size the pipe back down to standard 12.5mm hose to water by hand?
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We had micro misting sprays . Three 2 minute session a day kept the polytunnel growing well as most of the time it's the leaves that take the moisture in . Two of the spray times were 21.00 and 05.00 hrs with the third at 11.00 hrs . the late and early times were to allow drips to soak in the soil rather than have them in sunlight here it would tend to evaporate off.
This year in the glasshouse I have had some amazing cucumbers , straw berries and tomatoes in the glasshouse . The tomatoes were on micro spray heads set to trickle about a litre of water into each tub twice a day and the cuc & strawberries micro misting sprays located high up in the roof area aimed towards the plants .
Once a week all plants got half a litre of tomato feed by hand from a bucket & jug
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Short answer yes, pipe diameter will make a huge difference due to the frictional pressure loss increasing enormously in smaller pipes.
Rough calculations show that a 70m pipe with 7m head with 10mm diameter will flow at about 3.5 l/min.
25mm pipe (ID=20mm) will flow at about 23l/min
32mm pipe (ID =26mm) will flow at about 45 l/min.
Yes, in all pipes you will have 0.7bar pressure but only when the water is static (tap shut and no flow), as soon as water starts to flow you have to account for friction which increases exponentially with flow rate
So, for example with 32mm pipe you can have;
45 l/m @ 0 bar
30 l/m @ 0.4 bar
15 l/m @ 0.6 bar
0 l/m @ 0.7 bar
If you ever use sprinklers then pressure matters if you want them to sprinkle instead of just dribble.
Normal garden hose is only much use for short runs because of its small ID. I've got a 30m hose hooked up to my tap, the water coming out of the end of the hose is pathetic even though the water coming from the tap has 3 bar.
*all calculations are approximate.
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^ Yes, that's right. For each doubling of the pipe diameter, the cross sectional area of the pipe quadruples, hence why you can often get much more flow for just a small increase in diameter.
The other thing that works in your favour is that for example, 32mm pipe only costs about 50% more than 25mm pipe, even though as per Oor Wullie's calculations, it will give you nearly twice the flow.
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Thanks oor wullie & Womble... 32mm pipe it is :thumbsup: