I can't get my head round the physics and should it be blue

[Boogi11]

Hi all.
First the physics. If I have a 15mm pipe and am going install an inline tap/valve (the screw driver type) when fully open, this must reduce the flow as the valve hole is only 6or7 mm. Will this have a negative effect on the supply to my my , bath, sink and cistern, or shall I take them out after the renovation. Or am I missing something and does water flow not work like this.

Secondly, I have just looked at a previously hidden 30mm copper pipe, and is has about 4 inches of blue residue and some kind of build up on it, this pipe has been in site for at least 7 years and I have no leaks, and it will be hidden again soon, what should I do

 

[ChrisR]

The small hole will reduce the flow to eg a bath tap noticeably, but not a loo cistern.
So it depends what system you have connected.

The "Blue" if greenish is copper oxides, chlorides, etc. Clean it off a bit and see if the pipe looks corroded. If not much, just leave it. If it's eaten away it could eventually produce a pinhole, but that's unlikely.

 

[JohnD]

you can get a full-bore valve which has a bigger hole in it so will not restrict the flow

http://www.screwfix.com/search.do;j...vqQWsThLycdWH0p!528728506?fh_search=full+bore