On Apr 16, 5:40 pm, wrote:
Hello,

I bought some full bore 15mm isolation valves for my shower, as I
thought the "standard" valves with their narrow centres might impede
the flow. Was I right to be worried?

Yes.

Anyhow, I realise the full bore ones need some space for their
mechanism and would not be 15mm wide but all the same, they don't seem
to have an internal diameter that much bigger, so what's the point?

15mm tube has a 15mm outside diameter, so something like 13mm
approximate ID (cue for resident pedant to provide size to 2 decimal
places) supply .

A straight tube has a pressure loss due to friction; any fitting
(restriction, expansion, elbow, bend, etc.) will have more resistance.
Service ball valves can have a very small bore (6 or 8mm?)and cause a
big restriction to flow. It's not very relevant on mains outlets
(where there's usually excess pressure) but can cause a noticeable
loss of pressure with a shower. They can very seriously knob up a
central heating system where there isn't much differential pressure to
start with. If you've worked through a few systems, estimating the
pressure losses in every metre of pipe and caused by every fitting, an
installation by an idiot with a bumper bag of bargain service valves
is a bad sign. Service valves in inaccessible voids are a very bad
sign.

It's critical on the hot suction side to a pump (where you'd use
22mm) where excess restriction/pressure loss will cause cavitation in
the pump impeller, shredding the impeller.