pressure vs diameter

Hi folks,

Had a search on the forums but can't seem to find the info I need, so here goes.

I have got my hands on an above ground 2000 liter water tank that i would like to use in the water supply system for the house.

currently we have metered water direct to the house (looking into free alternatives) which we often suffer outages of service with, fallen trees, frozen pipes and so on, so a reserve of 2000 litres should be a huge help to cover the gaps when there are problems.

We live half way up a hill, the water is pumped 40m up to us from the meter and reaches the house at 3.5bar.

I've done a few crude calculations to see how far up the hill behind the house I need to position the tank,  the plan being to re-route the mains water up to the tank and then from the tank back to the house without the need for a pump.

So my question is if I use a narrower diameter tube to get it up the hill, can I expect to put the tank a little higher as only a dribble will be needed to keep the tank topped up then use a larger diameter tube (currently we have 25mm mdpe) back to the house to maintain flow rate and maybe even improve our current pressure?.

Way out of my depth here so any help hugely appreciated.

thanks all!

You really are asking two questions.

The first is actual friction within the pipe to get it to the tank further up the hill. That merely needs to be overcome by a larger pump, a simple task if you have grid-power close by. (An off-grid cabin with limited electrical production would be another matter.) Diamter really would not matter a whole lot if you don't really require the extra volume (flow).

As for the water coming out of the tank, a larger pipe would indeed improve volume (flow) going to your home, but the position of the tank higher on the hill would mean more pressure. The diameter of the tube means nothing in reference to pressure. A 2" pipe has the same pressure as a 1/2 inch pipe at the same elevation.

Here is a simple table that converts head (the height of the water above your house in this example) directly to pressure:

http://www.engineeringtoolbox.com/hydrostatic-pressure-water-d_1632.html

As an example, if the vertical height of the water (measured from the top water surface) above the pipe where it enters the house is 16 meters (52') higher, this will produce
precisely 1.6 bar, or 23 psi.  If the house is two stories,  you will get less pressure on the second story, because the height difference is lower.

While this would work, most people would find that pressure inadequate.

You could always add an inexpensive pump to pressurize the water from the tank to 4 or 4.5 barr in the house, regardless of the head pressure.



By the way, you could just use one pipe to the tank.  Water can travel either way in that pipe depending on what's going on.  If the city pipe is supplying more than you are using, the excess goes into the storage tank.  If the city pipe is supplying zero, then the water flows the other way and the tank supplies water to the house.  Then you just need one big pipe and a check valve on the city pipe so water can't flow from your tank into the zero pressure city pipe.

Your flow rate, on the other hand, depends on both the pressure, AND the size of the pipe.  For example, a 25mm (1") pipe will allow for a far far better flow rate than a 12mm (1/2") pipe at the same pressure.

thanks for the replies guys...really appreciate it!

I think I'm expecting this to be a little easier than it is...my brain just doesn't work with calculations.

I think I may have to go simple and hook a new tube to the end of the supply at the house where the guage shows 3.5bar and run the tube up the hill and see how high I can go before water stops coming out, that's then where I need to put the tank and hope when the water gets back to the house we haven't lost too much of the 3.5bars we have currently.

Adding a pump into the system isn't going to work, firstly no electric point and secondly electric is hugely expensive here.

36 meters.

Your tank has to be 36 meters higher (absolute vertical height difference, not the length/distance up the hill, which will be much longer than 36 meters.) than the point where the water comes out of the faucet to end up with 3.5 barrs pressure.

The size of the pipe has no effect on those numbers at all, the end.

That's about the height of a 10 story building just to get a feel for the scale of that height difference.


Ahhh...but if you want a good flow rate, you had better make the pipe 25 to 32 mm or more, inside diameter to preserve good flow rates.


There, not so complicated now.

Troy Rhodes wrote:36 meters.

Your tank has to be 36 meters higher (absolute vertical height difference, not the length/distance up the hill, which will be much longer than 36 meters.) than the point where the water comes out of the faucet to end up with 3.5 barrs pressure.

The size of the pipe has no effect on those numbers at all, the end.

That's about the height of a 10 story building just to get a feel for the scale of that height difference.


Ahhh...but if you want a good flow rate, you had better make the pipe 25 to 32 mm or more, inside diameter to preserve good flow rates.


There, not so complicated now.

😂 now that is language I can understand Troy...thank you! 😊

Give me a bucket of stucco and a trowel and I'm in my element...plumbing and electrics...not a hope...should have paid more attention at school!

Thanks for taking the time to reply...AGAIN. 👍

If it is impractical to install the tank that much higher than the house, a pressurizing pump would use tiny amounts of electricity to give you 3.5 barr.

You tell me your cost per kilowatthour, and I will give you an approximate cost in electric to run it for a year.  I predict less than $20 U.S.

They even do this in campers (recreational vehicles) from a small 12 volt pump, a battery and a small solar panel if you don't have convenient grid electricity.

If you're looking for cheap water pumps, Shur-Flo makes about a million different pumps, though many of them are similar in specs.  I've used dozens of them and have found them to be good, cheap pumps.

You can get them in either 120 or 12V, from 30-120psi, about 1 gpm, for about $120ish; a bit more for the 120 psi pumps.  They're on-demand, which means they start when the pressure drops below the set-point and they shut off when the pressure is reached.  In combination with a pressure tank, I think they could be a very affordable option.  I used them in my business, and I use the 'retired' pumps for irrigation, fodder systems, compost tea, etc.

You can find them on ebay and amazon, but I've purchased most of mine from Superior Supply, just outside of Atlanta.  They're an RV place, carry several models, and they're priced well.