Erica Wisner wrote:
The proportion of flow in the two sizes (120 mm vs 150 mm) will be:
r = radius of 120 mm diameter pipe = 60 mm
R = Radius of 150 mm diameter pipe = 75 mm
Ratio of cross-sectional area for flow to go through = pi*r*r / pi*R*R
= (60*60/75*75) (the pi's cancel out)
= 3600 / 5625
Conclusion: even though 30 mm sounds small, you will lose more than one third of the flow area.