Gravity flow water supply
I've got another question for you if that's ok. I'm not asking people to check my detailed designs, but I could really use some feedback on the principle of the method that I think I need to use... here it is:
It is a gravity flow system from a spring to a reservoir and then to a distribution line in a village. The system was initially designed by a Kyrgyz engineer. The diameters of pipe were chosen without information from a topographical survey. They were effectively 'guestimates' made with the help of a Tajik engineer who had some experience in gravity flow systems (the Kyrgyz engineer did not have previous experience).
The polythene pipes have all been ordered and delivered, but not yet installed. I asked the Kyrgyz engineer to perform a topographical survey which he has now done. He also measured the total flow available from the spring as 2.5 l/s (population of village ~800 people so should be ok)
The topographical information is only for the distribution network. I am trying to find out if any information is available about the section between the spring and the reservoir so I can analyse this section too (this part will be metal pipe with no tap-stands so assuming there is enough drop to get it to the reservoir, I hope there won't be problems).
The height difference between the reservoir and the last tap stand is 160m, over a distance of 4.5km. The polythene pipes are rated for a pressure of 60m (6 atm). Therefore I think the design needs two break pressure tanks so that static head in any part of the system never exceeds 60m. These were not included in the original design.
My process involved firstly analysing the original design based on 12 tap stands set to share 2.5 l/s flow, i.e. about 0.21 l/s for each tap stand. I determined head losses from the flow nomograph in 'A Handbook of Gravity-Flow Water Systems' by Thomas Jordan, and then checked these using the Hazen-Williams formula. It seems clear from the head losses calculated, that if break pressure tanks were included, the lower part of the system would experience negative pressures and therefore would not function properly. (The total head losses are ~160m, about the same as the actual total height drop, so when break pressure tanks are included these head losses cause negative pressures).
Therefore I have tried to re-design the system, using the pipes available. I have tried to keep residual head in the pipeline and at tap stands > 7m where possible (minimum recommended by Jordan). For the lower part of the system, using single 32mm diameter pipe (what is available for this part) causes too much head loss. Therefore I have suggested using two parallel 32mm diameter pipes to halve the amount of flow in each pipe and therefore reduce head losses. Is this idea practical? (Given the situation of pipes already ... (more)
We are getting onto dangerous ground here, and the short answer is no. In theory one could design a system in which the frictional losses “burnt off” the excess head – but this I suspect would need a far smaller pipe diameter, may well give rise to excess velocities leading to scour, and would in practice almost certainly self-destruct for a whole host of reasons (air-locks, human error, water hammer etc.). This is an excellent example I am afraid of where advice at distance is possibly dangerous. Our colleague in Kyrgyzstan needs an experienced engineer on site. That has a cost – but so does 6km of 90 mm pipe. I will try to find time to play with the limited figures we have received, but would really need the full pipe run profile to give more definite suggestions. I also have difficulty understanding why there are problems with break pressure tanks.
Also given it a bit more thought. The preferred option would always be a properly designed system with break pressure tanks etc... Such a system would be designed to have pipes running full with velocities in the safe range (no deposits, no scour). If they really can not have break pressure tanks and need to use 90 mm pipe, they will have to design the system to run almost like an open channel – with the pipe never running full and frequent tees in the pipe with say 2 metre lengths of open ended pipe sticking up at high points to ensure that there is some guarantee that the pipe will be destroyed by excess positive or negative pressure. Another alternative would be to use a smaller diameter pipe – but having looked at the figures while 90mm is too big, 63mm is too small. I do not know if there is an intermediate size available there. But relying on pipe friction to keep the pressure in the pipe within limits is dangerous in my view – one blockage, severe water hammer etc and the pipe could explode.# Hope they find a good solution which addresses their needs safely.
The most obvious solution is to use a higher rated pipe for the section from the final break-pressure tank to the storage reservoir (if not the whole system). Polyethylene comes in 10 BAR and 16 BAR ratings, as well as the 6 BAR that is being suggested at the moment. Perhaps 6 BAR PVC pipes are being used, in which case a different material could be considered for the final stretch. From my work in Tajikistan I know that MDPE pipes are manufactured in Russia, Iran and (I think) Uzbekistan. However better quality MDPE pipe is available in Pakistan (by Sun International, in Lahore) if it could be transported through Afghanistan and Tajikistan - which, admittedly, could be a nightmare!
Failing this, the continuously flowing tank inlet would work. Factors to consider: 1) There may be a need to close off water in the upstream pipeline for other reasons (e.g. distribution system repairs); therefore you could install a gate valve above the last BP tank upstream of the tank. Beware that if a valve is closed on the downstream side of this BP tank then the weight of water in the pipe between the BP tank and the tank will collapse, and likely damage the pipe itself. 2) People may need to do repairs on the tank at some stage - if you install a Tee above the tank with a "run to waste" option, then people may be able to set up temporary storage and/or tap stands during future repairs. 3) Make sure the tank overflow discharges well away from the tank to avoid undermining it.
As far as I can see there is no reason why you need to have a valve on the end of the pipeline discharging into the receiving reservoir tank. What would be its purpose in any case? It would be a liability. Even if the delivery pipe (and joints & fittings) could withstand the static head when closed there could be other problems of water hammer if it was possible to close the valve quickly (unlikely if a screw down gate valve but something to avoid). Ensure the valve is on the entry to the delivery pipe so that the flow can be diverted to the spring or a break pressure overflow. This gives the opportunity for maintenance on the delivery pipe and fittings when necessary.
Re: Point 2, I’m not sure why the pipe cannot freely discharge into the tank, above the tank, so that there is no possibility of a head of water building in the tank to provide any resistance to flow. The overflow would have to be sized sufficiently to ensure the tank does not fill to overflowing so that flooding and erosion around the base of the tank is prevented. So, in both scenarios (piped directly into the tank or discharging above the tank) the overflow needs to be large enough to discharge at a rate matching or exceeding any incoming flow.
As regards break pressure “tanks”, they do not have to be purpose built tanks, as such. If space is a premium or there are construction difficulties then it is possible to place the delivery pipe inside a larger diameter receiving pipe (even if it is eventually reduced down to the same diameter) so that the pressure is “broken” to atmosphere. However, you may find the system overflowing at this point depending on the flow in each pipe section. But this design is an alternative to constructed “tanks”.
In theory the idea will work (as long as the dynamic hydraulic grade line does not rise to more than 60m above the pipe at any point).
However, in future someone may need to repair the lower tank and so may wish to stop the flow into it. Since there will be no valve at the inlet then you will need to ensure that there is a valve that can be closed at the outlet to the collection point at the spring (this valve will also be necessary to allow repair to the pipe). There should also be an air-admittance pipe just downstream of this valve to stop suction developing in the pipe when the valve is closed and the pipe is draining (see Figure 11.6 in Jordan's book).
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