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Rainfall: Either choose a Location

Pipe Size

This is a very simplified method of calculating a pipe size, and takes no account of losses such as entry and exit losses, and losses due to bends and fittings (it only calculates pipeline losses). These losses are assumed to be compensated for, by allowing water to rise up a bit into the down pipe or catch pit.

Also if the theoretical pipe diameter is greater than 4mm of an available pipe diameter, the next higher available pipe size is selected.

The velocity is calculated on this next higher pipe size.

The Formula used is Colebrook-White (k=0.015) as per the Australian Plumbing Code.
The proper method of designing storm water pipe systems is based on the Hydraulic Grade line.
To learn about this, and storm water design principles in general click here..

What if we use this method to size a pipe from a down pipe, and find that the resulting pipe is smaller than the downpipe itself. (Assuming the Downpipe was sized in the Downpipe calculator)

A Down pipe size is more dependent on the perimeter of the pipe (circular weir) at the entrance, than the cross sectional area. And as such does not flow full.

So, we can put in more DP's to reduce their size, or we can steepen the grade of the underground pipe to decrease it's size, or we can enlarge the Downpipe at the entrance to the diameter required by the downpipe calculator, thereby providing a "Funnel" into the vertical pipe. Or we could just enlarge the underground pipe and say nothing to anybody.

Also a pipe line can never reduce in size downstream. Its against all codes and requirements and laws. It is also a potential blockage problem.

Note: Don't use this to design syphonic systems though, as a syphonic systems need special fittings to get it started, and also to remove entrapped air.

How to Enter a Known Flow
The flow formula is :-    flow = Area * Intensity / 3600 (Where coefficient of runoff is 1)

Make Area equal 3600, when divided by 3600 will cancel out, leaving the intensity equal to the required flow. For example, say your known flow is 20 L/s, enter 3600 as the area, set the location to "I prefer to enter a know Intensity", and enter 20 as the intensity. the flow will be calculated to be 20 L/s.

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Copyright Ken Sutherland 2014