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The 150m fall over 1600m gives a hydraulic gradient of 0.0938 (1 in 10.67) which as you indicate is far steeper than required, so natural flow is likely for at least parts of the pipe. I calculate that for 6 litres/sec a 90mm EXTERNAL diameter polyethylene pipe (assuming 71.6mm internal diameter) would only need a hydraulic gradient of 0.032 (that is a total head-loss of 51.2m rather than 150m).

If you really need to use this pipe then at the lower end of the pipe you could add head-loss to use up the extra 98.8m of head, to ensure that the pipe flows full. Putting a partly closed valve on the pipe, or inserting a section of small diameter pipe that gave a high head-loss, or inserting a small diameter orifice in the pipe are ways of adding head-loss. However if you have stand-posts served by the pipe between the spring and the tank you will need to control the dynamic head above ground (i.e. pressure above ground under flow conditions) so that it is not excessive at the taps (e.g. less than 20m head) so you are probably going to have to use one or more break-pressure tank(s).

You need to bear in mind that without a break-pressure tank, if flow stops (for example because the tank becomes full and a float valve at inlet closes), that the pipe work will be subjected to the full static head (i.e. 150m head at the lower end) - this is another reason for providing one or more break-pressure tanks.

A 50mm internal diameter pipe (63mm outside diameter) over the whole length would give too great a head-loss for the desired flow rate. If you really want to get the right flow for the 150m head-loss then you could use the 71.6mm diameter for part of the length and a 50mm diameter for the remainder. However, this assumes that there are no stand posts on this supply pipe so pressure in it does not also need controlling.

The whole system will need a proper hydraulic design carried out when the topography is known and a suitable site for the break pressure tank(s) can be located.

regards,

Brian Skinner