Water main sizing advice please

[Craig Chamberlain]

Hi,
I have recently moved to a farmhouse in central Scotland and the other day our closest neighbour accidentally burst our water main with a JCB. The main is a 1” MDPE pipe running about 500m from the water company roadside main to our farmhouse. This was installed about 20 years ago. The farmhouse is perhaps 5-6m lower than the point it connects to the water company main but our house is on two floors so the upper floor is around 3m above ground level. We have a ground source heat pump with unvented cylinder so we can’t use a shower pump and our water tank is downstairs.

My neighbour, who is a builder and architect has decided to take a branch from my pipe while doing the repair, so that he can feed a house that he is refurbishing. His branch is coming from an equal T piece so is also 1”. He says it should not affect my pressure/flow because you can feed 3 houses from a 1” pipe.

So, my question is, should I be concerned about this or am I worrying about nothing? His branch point is around 80m from the roadside mains supply and my house is around another 500m downstream and like I said 5-6m lower than his house.

Obviously I don’t want to fall out with my neighbour having only just met, but it’s going to be much more awkward if every time someone turns on a tap at his house my supply stops!

Advice most welcome!
Thanks,
Craig.

 

[John.g]

Using a little calc which I have found to be quite helpful and based on the following numbers based on neighbour using same volume of water as you then you can see quite plainly that you will be running out of head when total flow exceeds 40 LPM, ie 20 LPM each, with 40 LPM through the 80 M section and 20 LPM through the 500 M length to your house.
I have done the calc first based on no neighbour teed off the 580 M length. The other calcs are based on 30,40&44 LPM through the 80M length and15,20&22 LPM through the 500M length.

LPM LM DPbar LM DPbar LM DPbar DPbar DPDiff
15 580 2.1 80 1.05 500 1.83 2.88 0.78
20 580 3.6 80 1.79 500 3.11 4.9 1.3
22 580 4.3 80 2.13 500 3.71 5.84 1.54

Can't get numbers lined up above but if one takes the 20 LPM number, the pressure drop is 3.6 bar with no neighbour connected and 4.9 bar with neighbour also using 20 LPM, a difference of 1.3 bar.

friction_loss

 

[Brambles]

If you want to calculate it properly, for MDPE pipe e is around 0.0015mm. Whilst in theory, pressure drop is linear over equivalent length - fittings at the flow end will distort this negatively.

 

[Alan Wright]

Hi Craig,
If I were in your position I would seek advise from Scottish Water.
It is the responsibility of Scottish Water to supply water at a minimum pressure and volume. When you bought your house your either paid for or inherited rights to receive a potable water supply at a minimum pressure and volume from Scottish Water. Your neighbour is about to breach this right. Also it is for Scottish Water to approve any connection to their infrastructure. Any new connection is subject to approval and payment of infastructure charges if appropriate. I suspect Scottish Water will be very interested in this.
I am aware of Water Bylaws in England from when I worked for Anglian Water and Northumbrian Water, but I cannot for certain say its the same as in Scotland. I suspect it is the same and if it had happened in my patches the last thing I would want is another DG2 on the list.

Here are a couple of links to get you started but speak to Scottish Water.

Water pressure - Ofwat

What is water pressure? Pressure is the force that pushes water through pipes. Water pressure determines the flow of water from the tap. If pressure is not sufficient then the flow can reduce to a […]

www.ofwat.gov.uk

 

[Craig Chamberlain]

Using a little calc which I have found to be quite helpful and based on the following numbers based on neighbour using same volume of water as you then you can see quite plainly that you will be running out of head when total flow exceeds 40 LPM, ie 20 LPM each, with 40 LPM through the 80 M section and 20 LPM through the 500 M length to your house.
I have done the calc first based on no neighbour teed off the 580 M length. The other calcs are based on 30,40&44 LPM through the 80M length and15,20&22 LPM through the 500M length.

LPM LM DPbar LM DPbar LM DPbar DPbar DPDiff
15 580 2.1 80 1.05 500 1.83 2.88 0.78
20 580 3.6 80 1.79 500 3.11 4.9 1.3
22 580 4.3 80 2.13 500 3.71 5.84 1.54

Can't get numbers lined up above but if one takes the 20 LPM number, the pressure drop is 3.6 bar with no neighbour connected and 4.9 bar with neighbour also using 20 LPM, a difference of 1.3 bar.

friction_loss

Thanks for taking time to work this out - I get similar figures albeit using the later figures for distances which were 100m from main to branch and another 570m to our house from the branch (670m total). I worked it out at 15 lpm each my dynamic pressure would drop by 2.75 bar compared to if I was drawing 15 lpm with no branch installed.

If you want to calculate it properly, for MDPE pipe e is around 0.0015mm. Whilst in theory, pressure drop is linear over equivalent length - fittings at the flow end will distort this negatively.

Hi - yes I came across the figure of 0.0015mm which I then divided by 20.4mm to get the relative roughness of the MDPE pipe. The thing is, this pipe was installed 20 years ago so who knows what the inside walls look like now and how many joints are installed etc. I think this is why I found it quite difficult to get theoretical calculations to correlate with actual static/dynamic readings taken empirically. You also need to work out the Reynolds number and take account of viscosity due to temperature of the water. I found you can make reasonable assumptions in each case to simplify the equations but could never quite construct a model which worked for me at each empirical flow rate. For all I know there could be leaks or kinks or crushed sections of pipe. This is why I chose in the end to focus on the empirical results with a bit of extrapolation.

Hi Craig,
If I were in your position I would seek advise from Scottish Water.
It is the responsibility of Scottish Water to supply water at a minimum pressure and volume. When you bought your house your either paid for or inherited rights to receive a potable water supply at a minimum pressure and volume from Scottish Water. Your neighbour is about to breach this right. Also it is for Scottish Water to approve any connection to their infrastructure. Any new connection is subject to approval and payment of infastructure charges if appropriate. I suspect Scottish Water will be very interested in this.
I am aware of Water Bylaws in England from when I worked for Anglian Water and Northumbrian Water, but I cannot for certain say its the same as in Scotland. I suspect it is the same and if it had happened in my patches the last thing I would want is another DG2 on the list.

Here are a couple of links to get you started but speak to Scottish Water.

Thanks for that - yes I did email Scottish Water and they eventually got back to me to say that they couldn't really comment on the suitability of the pipe to supply two houses and to consult a professional plumber! So that suggests there are no clear-cut guidelines or rules about pipe size/length for a dwelling. What they did say however is that I (as original pipe owner) would have to consent to my neighbour taking a supply from my pipe and that the new build property owner would have to "consider" (emphasis mine) applying to Scottish Water for a new water connection solely for their property. So even here, they didn't use the word "must" - only "consider".

Anyway, the good news is that our vendor received legal advice in the meantime and then approached the neighbour from a position of knowing he could formally prevent him from using the pipe. But in the interests of good relations, they have agreed that the neighbour will install an upgraded 50mm pipe section between the mains and the branch and this will then be branched to supply each property equally. This is acceptable to me since the 50mm pipe will have a cross sectional area of more than double the 25mm pipe and so my supply should actually improve slightly even when both properties are drawing 20 lpm. It also means that the first 100m of piping are shared and so will have shared responsibility for cost of repair or replacement in future.

So thanks again to everyone who offered advice - very much appreciated!
Craig.

 

[Alan Wright]

Thanks for taking time to work this out - I get similar figures albeit using the later figures for distances which were 100m from main to branch and another 570m to our house from the branch (670m total). I worked it out at 15 lpm each my dynamic pressure would drop by 2.75 bar compared to if I was drawing 15 lpm with no branch installed.



Hi - yes I came across the figure of 0.0015mm which I then divided by 20.4mm to get the relative roughness of the MDPE pipe. The thing is, this pipe was installed 20 years ago so who knows what the inside walls look like now and how many joints are installed etc. I think this is why I found it quite difficult to get theoretical calculations to correlate with actual static/dynamic readings taken empirically. You also need to work out the Reynolds number and take account of viscosity due to temperature of the water. I found you can make reasonable assumptions in each case to simplify the equations but could never quite construct a model which worked for me at each empirical flow rate. For all I know there could be leaks or kinks or crushed sections of pipe. This is why I chose in the end to focus on the empirical results with a bit of extrapolation.



Thanks for that - yes I did email Scottish Water and they eventually got back to me to say that they couldn't really comment on the suitability of the pipe to supply two houses and to consult a professional plumber! So that suggests there are no clear-cut guidelines or rules about pipe size/length for a dwelling. What they did say however is that I (as original pipe owner) would have to consent to my neighbour taking a supply from my pipe and that the new build property owner would have to "consider" (emphasis mine) applying to Scottish Water for a new water connection solely for their property. So even here, they didn't use the word "must" - only "consider".

Anyway, the good news is that our vendor received legal advice in the meantime and then approached the neighbour from a position of knowing he could formally prevent him from using the pipe. But in the interests of good relations, they have agreed that the neighbour will install an upgraded 50mm pipe section between the mains and the branch and this will then be branched to supply each property equally. This is acceptable to me since the 50mm pipe will have a cross sectional area of more than double the 25mm pipe and so my supply should actually improve slightly even when both properties are drawing 20 lpm. It also means that the first 100m of piping are shared and so will have shared responsibility for cost of repair or replacement in future.

So thanks again to everyone who offered advice - very much appreciated!
Craig.

Nice Result - Well done.

 

[AWheating]

25mm main is for 1 property. 3 or more would want a 50mm pipe with a manifold with 25mm to each property.