Pipe sizes

[petehelmet]

So ive got this bathroom in an appartment. There a washhand basin, a shower and washing machine that will be fed to a pump that will in turn be pumped around the room to the soil stack.

I'm looking at the Building regs, they say that pipe from say the sink etc must be 50mm if the run is over 3m and under 4m. So anything smaller would not fit the requirements.

I can accept that. I just don't understand why. Why difference does the size of the pipe make

I spoke to an expereinced person and asked to explain what would happen is the pipe is too small and they just confused me more.

He spoke of negative pressure if you have full bore discharge etc..

if anyone can explain (simply, so i could explain to someone equally as confused) why the large sized pipe is needed and what would occur if a smaller pipe is used. i would be eternally grateful!

 

[Bernie2]

Basically, its to stop a plug of water forming in the pipe and causing a vacuum, which would pull the water out of the trap seals, which in turn would allow drain smells back into the flat.

The smaller 35mm discharge pipe from the basin for instance, whilst it might form a plug to start with in the 35mm, soon finds itself in a much larger 50mm pipe and so the plug of water breaks up.

Its a bit like being squashed in a narrow alley way and then finding yourself suddenly with plenty of room on reaching the main road. The water kinda spreads out in the larger pipe.

That's about it without having to go all technical.

 

[migoplumber]

as bernie says.
as an interesting add on, i was taught at spanish college (so the same regs dont aplly here ) if you have a long run that exceeds the allowable horizontal meterage, we were shown to reduce from 1 1/2 to 1 1/4 to increase the speed of flow to empty the pipe, funny thing is i cant remember them saying anything about empty the traps, even tho now its blatantly obvious that it happens

shaun