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Pipe material depends on what is available.

As far as pressure is concerned I would just put in a number of break pressure tanks (BPT) to keep pressure within the limit of the available pipe.  As you are probably aware 480m pressure in the village will blow the taps apart, so probably best to keep the pressure in any tap down to a maximum of about 30m/3 bar pressure. 

 

Pipe over ground will need restrained joints or pipes that have end restraint.  Building restraints over movable pipe joints (e.g. standard push fit spigot/socket) would not be impossible but may be impractical.  The brief description suggests either steel pipe with threaded joints, or polyethylene (PE) pipe with butt fusion joints, electrofusion joints or, if pipe is small enough (I suggest 90mm diameter and below), then compression joints.   Note for PE pipe you can use a combination of the jointing methods.

 

As long as it is readily available in country I would probably start by looking at PE pipe. Some observations:

·         PE pipe will be able to flex to the ground surface so less work to get it flat.  You will still need to make sure you avoid air traps.  If you get these then just install a small 25mm plastic air valve in the high spot and a washout valve in the low spot (these would need to be regularly checked).

·         Black coloured pipe usually has good resistance to UV light (50+ years, check with supplier for resistance in Kenya). 

·         PE pipe can have pressure rating up to 16 bar (about 160m), there are also lighter weight pipes.  So within a pipe run (between pressure break tanks) you could vary the pressure rating of the pipe to suit.  This will save weight and pipe cost.

·         So using PE pipe with maximum 16 bar pressure you could have something like 150m below spring have BPT, next 150m down have second BPT, third 150m down have main store tank leaving 30m of pressure to feed into community taps.  You would need to do the friction calcs to work out the details.

·         Within each 150m height drop you could have top third from SDR21 or 26 (max 6 or 8 bar) PE pipe, middle third of SDR17 (max 10 bar) PE pipe and bottom third of SDR11 (max 16 bar) pipe.  On smaller diameter pipes you may only have the choice between SDR11 and 17 pipe.

·         PE pipe usually has additional short term pressure resistance to withstand occasional water hammer pressures.  If in doubt put in pressure dampers or pressure release valves to deal with the water hammer pressures.

·         Pipe can be fixed in place with posts or straps bolted to rocks.  However PE pipe will expand and contract so you need to build in areas where there is flexibility (usually around sweeping bends) to allow the PE pipe to move. 

·         You will also need to make sure that movement areas do not cause rubbing or abrasive damage to pipe.

 

Over 6km you might find that in a hot and sunny climate the water will heat up.  So any covering you can place over the pipe would be good (will also reduce any UV impact issues).

 

P.S.  If there is good flow in the system then you could consider putting in a microhydroelectric scheme to give some power to the community. 

 

Nick