Hetty, cdawggy is absolutely right, you need to go back to basics and calculate the headlosses. Two pipes giving the same equivalent cross-sectional area as a single pipe may appear to solve the problem but as we all know frictional losses a generated in the non-laminar region around the pipe walls and two pipes certainly have move wall surface area than a single pipe.

However, in this case I quickly set up a spreadsheet and ran both scenarios through the basic Hazen-Williams formula across a range of flow rates and you should be fine for all flows. Here's an example assuming a flow of 3l/s and a Hazen Williams Coefficient of 130.

For the 63mm pipe:

Headloss (m) = [10.9 x Length x (Flow ^ 1.85)] / [(Hazen Williams Coefficient ^ 1.85) x (Diameter ^ 4.87)]

Headloss (m) = [10.9 x 580 x (0.003 ^ 1.85)] / [(130 ^ 1.85) x (0.051 ^ 4.87)]

Headloss (m) = 32.8m

Along each of the 50mm pipes we use half the flowrate:

Headloss (m) = [10.9 x Length x (Flow ^ 1.85)] / [(Hazen Williams Coefficient ^ 1.85) x (Diameter ^ 4.87)]

Headloss (m) = [10.9 x 580 x (0.0015 ^ 1.85)] / [(130 ^ 1.85) x (0.041 ^ 4.87)]

Headloss (m) = 26.4m

i.e. with a flow of 3l/s, along 580m of 63mm pipe you would lose 32.8m of pressure head. If the flow was divided into two smaller pipes of 50mm, you would lose 26.4m along each.

Of course you may want to factor in secondary losses at the junction but it looks like you should be alright.

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