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I designed gravity systems in Rwanda for several years so interesting to see someone else wrestling with similar issues. My suggestions: • Priority must be to establish the head difference between the spring and the tank – without this, one is truly stuck! • Next given the spring flow and the head difference less a safety margin (the book referred to gives figures), check the pipe size required to deliver the maximum spring flow • Now check velocity in pipe, o if it is lower than the minimum figure in the book, you may get deposits, so will have to ensure there are washouts etc, see below – but do not reduce the diameter unless you are willing to lose some of the flow – usually a bad idea o if it is higher than the maximum figure in the book, you will get pipe damage, so increase the diameter • Air locking is indeed a big issue on low pressure schemes but is not insurmountable! Some strategies we adopted in Rwanda o Have as steep and as constant a downhill gradient as possible immediately below the spring and keep on going until you are below the level of the tank – this may mean a slightly longer pipeline but worth every penny! o Once you are down to that level, try to lay the pipe at a constant fall and then a constant rise of say 5%, with highpoints always below the level of the tank and low points at or near the maximum pressure rating of the pipe o At each high point have a wash out valve and an air valve – the air valve can be as crude as a small hole in an otherwise dead end pipe sticking up from the main line. The wash out valve will allow you to fill the pipe without sucking in air (if you use the washouts at the low points you will suck in air and create more problems than you solve) o At each low point have a wash out valve – I repeat not for filling the pipe – but essential to allow a rapid emptying and therefore cleaning out of sediment in the pipe as well as repairs. o Air locking can occur not only at physical/planned high points but also in accidental high points created by trying to lay pipes horizontal or not paying enough attention to laying them at a constant gradient – it is not rocket science but needs discipline and although the pipe line may appear longer as it will tend to sweep in curves, the actual difference is usually minimal – I can suggest ways to do this if necessary. They can also occur where there is a relative high point in between the pipe line and the hydraulic gradient – so worth plotting out and avoiding • If the pipe can be laid as suggested above, the only place where you might get major problems with half empty pipes are in the first section between the spring and the point where pipe falls below the level of the tank. Some possible strategies o Do nothing until you find out if you really have a problem o Control the flow at the tank (not at the spring!) so that the pipe does flow full – I would do this by inserting a diaphragm with a suitable diameter hole(s) drilled in it – using valves is always problematic as the types one finds easily are not designed for flow control – only for fully open/closed. Disadvantage is of course that you will probably end up losing some water through the overflow at the spring. o Consider an even bigger pipe diameter in this first section so that even if it is only partly full, at least any air entrapped has an opportunity to bubble back towards the spring box

Regards,

Tim Foster