Pipe diameter and pump size for water distribution

Hi all

I’ve been reading up quite a lot on boreholes, pumping water, pressure, flow rates etc. but I think I reached the limits of my Google skills. I understand Volts and Amps, but this water thing has me stumped.

The scenario.
I plan to build a off grid camp site in the bush, the bathrooms will be distributed over a area of 3 ha more or less. To keep things simple, let’s say I place the water tank/s slap bang in the middle of the campsite, so absolute max distance to the furthest bathroom will be 150m.

1. From what I read a 10m tank stand gives you more or less 1 bar of pressure already. So even though it’s quite expensive, I think going with a tank stand will be a given since I want to keep this off grid and even avoid pressure pumps if I can though I doubt I’ll be able to.
2. What diameter and class HDPE pipe should I use from the tank to each bathroom to get the maximum amount of flow / pressure at the furthest bathroom which will be 150m away? From what I understand a bigger diameter pipe will give a better flow rate, but only up to a point and the bigger the pipe, the lower the pressure. How do you calculate this, how do you find the balance.
3. Each bathroom will have a shower, basin and toilet, decent flow / pressure for the shower will be essential even though the gas geyser can work on 0.25 to 5 bar, I want a decent shower.
4. Is it a pipe dream hoping to do this without a pressure pump and if so, what size pressure pump will I need keeping in mind that I’ll have to eventually drive this with solar and batteries? I should of course also keep in mind that there will be times that 2 or more showers will be used at the same time.
5. The dumbest question of them all, should I run a pipe from the water tank to each bathroom call it “parallel” piping, or can I run the piping from one bathroom to the next in “series”?

TIA

I’m no expert, but this is much more complicated to calculate that it is to just do.

The water pressure you will get from the height of the tank and you can easily do that without the preasure pump. Your pump will just need to refill your tank from storage when it is drained below a point.

The pipe will limit your flow. Now if there is no flow the presure will be the same everywhere. But if someone opens a tap that will reduce the presure at that point a bit as the water from the rest of the system flow to fill the gap where the water left from the tap.
If the tap has a 15mm diameter and the pipe has also 15mm diameter it should be fairly constant, but the resistance in the pipe mutiplied by the distance will reduce that. So a larger diameter pipe (20mm) would be better to use.
If 2 taps open at the same time, then it would be better to have 40mm pipe, but then how often does that happen?
You need to work out a contention ratio for your usage and how critical it is to have high presure on them.

Toilets and taps are not that critical to pressure (they just fill the container a bit slower), while showers especially using gas geysers you need lots of pressure.

About the serial vs parallel connection that too can both work. If the farthest tap opens and no other, then pressure will be good. If 2 taps open at the closest bathroom then the farthest will have little pressure if they are close to the pipe limit.

But the good news is that it is easy to add. I will suggest use 40mm or 50mm black PVC pipe to begin with. They are very cheap and easy to work with. Of you find it is too thin, just add an extra length.
Also you can start with larger pipe closer and reduce to smaller father, but then your fittings and pipes are not always the same and you will end up always needing the fitting that you don’t have. Best to take a diameter and stick with it.

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OK, I can help. I have been doing a bit of research on this myself.
Only two factors are important pressure and flow rate.
You might want a bit more than 1 bar for your shower, so I suggest the highest point dictates your tank stand site. (Unless of course, you live on a flat site).

Now the interesting bit the friction losses in the pipe are not linear. So decide on the flow rate you are likely to need, say 3 showers at once. Google tells me a shower is typically 7.9 Liters/minute. So 3 would be 23.7 LPM. This converts to 0.000395 cubic metres/second.

Now, this is a very useful rule of thumb. Water should flow faster than 2m/s and slower than 3m/s in the pipe.
Slower than 2m/s and you have over-engineered your pipe and faster than 3m/s and the friction losses climb exponentially.

So, the volume of 1 metre of pipe is going to be filled twice in a second at a 2m/s flow rate and thrice at 3m/s.
0.00395m3 / 2 = 0.001975m3 ( This dictates the max. size the pipe should be)
0.00395m3 / 3 = 0.001317m3 ( This dictates the min size the pipe should be)

V = pi r^2 * 1metre = pi r^2
SQRT(0.001975/pi) = r = 0.025m
So Diameter = 0.05m = 50mm ( max pipe size)

V = pi r^2 * 1metre = pi r^2
SQRT(0.001317/pi) = r = 0.020m
So Diameter = 0.04m = 40mm ( min pipe size)

Remember: You’ll have to look up HDPE inside diameter for the class of pipe you choose, it is not the same measurement convention as LDPE. It really looks like 50mm HDPE once you subtract the wall thickness is bang on the money though.

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Wow ok you did some proper research, makes much more sense now. Earlier today on a Aussie website I did read something about flow rate per second needing to be on the money, but my brain got tired at that stage.

Am I correct in saying that the length of pipe, 150m in this case isn’t such a big factor as I initially thought, but rather the rate at which water gets used?

Fortunately shower flow rates can be controlled by the size of shower head you install and I see the so called low flow shower heads they sell these days are all around 8 l/min.

Any comment on series vs parallel piping?

I’ll definitely first try it out without a pressure pump, but I have to keep the possibility of a pump in the back of my head. I fortunately see even the smallest pumps, I looked at the Jojo 0.37 kW for example, can supply 38 l/min at 3.8 bar.

I think I’d do neither.
I’d opt for a ring with strategic isolating valves to isolate and find leaks. (Also now you know your pipe is plenty big enough).
As for a pressure pump, I haven’t had many dealings with them. I do know any leaky toilet and they’ll drive you nutty. It seems that it is worthwhile getting a large pressure bellows tank so that it isn’t stopping and starting as often.
Maybe even a number of smaller pressure pumps could be considered?

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Another pro-tip.
If you are going to be trenching this pipe run fencing wire in the same trench.
It’ll improve the earthing on your property and let you find the pipe with a metal detector in 10 years time.
Also worthwhile to run that plastic red and white trench tape as well that’ll help you avoid the pipe in 10 years time.

This might also help , I got this about 15 years ago from an agriculture irrigation man.Class 6 - 12 pipe friction charts.pdf (437.3 KB)

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Would have never thought of the fence wire trick thanks. As for the earthing, do I actually connect it to something like the tank stand or just leave it like that in the ground?
Oh and the tape, do you use normal danger tape for this or is it something special?

I can just give you the farm perspective. You can run inch-and-a-half (roughly 40mm) over long distances without any real flow issues, and since it is not municipal pressure you can use normal “black pipe”, you don’t need high pressure pipe. Even with a tank 4 meters up, the flow isn’t that bad (thinking of the home I grew up in, which had such low-pressure cistern system), and if you need to compensate with a pressure pump, perhaps put a smaller pump at the bathroom rather than one large one for the whole camp?

I know, I didn’t do any math today, but this is a bit outside my normal area of experience

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I was pretty close with my 40mm/50mm prediction
I also suggest if you can afford it to make a ring. That helps with any pressure high/low knocking. (PS. if you just add an extra 1-2m past your last stop it will also help).

The cheapest (lowest rating) black PVC pipe is rated for 10bar if I remember correctly, so if you do not have a deep valley or high hill you need to cross with your pipe you can go with the cheap stuff using it at 3bar. If the pipe do have large height differences you better get a pipe rated for those pressures.

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The price difference between 3 bar LDPE and 6 bar HDPE isn’t actually so much, so I’ll go with HDPE.

There is quite a big price difference between 40mm and 50mm HDPE 6 bar though, so I’ll have to figure out what the disadvantages will be if I only use 40mm pipe for the hole system rather than 50mm. Maybe @Phil.g00 can do that math thing for me again?

So you guys say I must run the pipe in a big circle and then out of the circle branch out to every bathroom?

From what I read every bend, coupling, change in direction, change in diameter even taps also influences the pressure and flow, so I’ll have to design this very carefully to avoid unnecessary joins and bends.

3 bar LDPE is rubbish.
It kinks and then the edges of the kink gets worn with any dragging as you lay the pipe, either making a hole or creating a weak spot.
A ring of 40mm, means that water has two routes to the destination.
So that would mean that V = (pi r^2) * 2 = 0.002513m3

And 0.002513m3 ( V of 2 x 40mm) > 0.001975m3 (V of 1 x 50mm)
So a ring of 40mm would be superior to a radial of 50mm.

EDIT: Just bear in mind I am talking about an inside diameter. HDPE is classed according to its OD, not its ID. LDPE is classed according to its ID. Different class HDPE pipes will have a different wall thickness, factor this in.

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It is probably better just to leave it in the ground unattached at both ends.
It just generally contributes to a low ground impedance, rather than being an active earth conductor.

Thanks a lot for this, I’ll keep the ID and OD in mind, but I’m sure in the real world that mm or 3 will make a negligible difference in the greater scheme of things.

I’m suddenly having second thoughts on the tank stand again. A 12m tank stand goes for around R35k these days due to steel prices and in the end I want to end up with 2 tanks, so that’s R70k just for the stands. R70k can buy a nice solar and inverter setup to run 220V pressure pumps.

On the other hand a tank stand is for ever, zero maintenance, never needs batteries replaced, will work come rain or shine etc.

The tank option will give you much less problems, so if you can go that route it would be the better option.
You only need 1 tank at the top of the stand. When I stayed on the farm we had 1x5kL tank on a stand and 3x on the ground from where you pump the water to the tank on the stand. 3 households ran from this.

I know my house is a bit light on water usage (~8kL/month) but say you have a more heavy usage of 30kL/month. For that you will need to pump water to the top tank 6x over a month or 1.5 times a week. Scale that up for your camp site and you could fill it every day around noon from solar if the campsite use 5kL or less a day or 150kL/ month. What tipe of campers are you expecting?

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Yeah, but I think the wall thickness is closer to 5mm at Class 9 or 10. And you’ll lose double that on the diameter, so your 50mm OD pipe will only be 40mm ID.
Remember, it is the square of the radius that matters, so those outside few mm will equal a large amount of area.
A 12m tank stand is pretty high, but you do want a few metres to ensure to can still get water of a sort. Remember it is good practice to have a bypass valve on these pressure pumps.

Also keep in mind that a 20kl Jojo is nearly 4m tall, if I recall correctly, so you do have to go too high.

I’m working on 5 kl tanks, R5k for the tank and R35k for the stand.
If you go bigger it gets silly expensive, a 10 kl tank for example costs R15k and a 20 kl tank R40k if I remember and that’s not even mentioning the stands. Stands also only go up to 10 kl, nobody makes larger stands.

To put up 2x stands with 5 kl tanks works out less expensive than a single 10 kl tank on a stand, but I never even thought of @Louisvdw idea to have only one tank high up and others on the ground.

I’m planning a off grid bush campsite, so the footprint isn’t that high.
I’ve been advised to work on a average of 100l per person per day, even though I personally think for the type of campsite it’s a bit high, but let’s go with that since people tend to waste water if they’re not paying for it.

It comes to a average usage of 3.2 kl per weekend in the beginning, later 6.4 kl and even later 9.6, that’s why I’ve been working on eventually having 2 tanks all along.

Thanks for the 1 tank on a stand and the rest on the ground tip, never thought of that.

I can then use a small DC pump working directly off solar to pump water between the tanks, it won’t have to work at night time, so no batteries, it can do all its work during the day.

Why don’t you put all the tanks on the ground and use a small 0.25 or 0.37kw pressure pump to get the required pressure.

Thinner pipes and saving the cost on the tanks stands. Add a panel or two to the existing solar plans.

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