Water Pressure theory question

[StephenW]

Scenario: 15mm rising main into loft. Just before feeding the CWT a 22mm tees off to feed the bathroom. Obviously the pressure in the 22mm will be lower than the 15mm but flow rate should be the same.
If at the bath tap end of the 22mm I tee off a 15mm pipe to feed an electric shower, will the pressure in this 15mm be the same as the original rising main (assuming bath tap not running at the same time)?

 

[kevplumb]

yes

 

[StephenW]

Wow - speedy reply with all the detail required! Thanks very much.

 

[jackthom]

Scenario: 15mm rising main into loft. Just before feeding the CWT a 22mm tees off to feed the bathroom. Obviously the pressure in the 22mm will be lower than the 15mm but flow rate should be the same.
If at the bath tap end of the 22mm I tee off a 15mm pipe to feed an electric shower, will the pressure in this 15mm be the same as the original rising main (assuming bath tap not running at the same time)?

Forgive me if I'm taking this too literally or something but I don't understand why you would say the pressure would be lower in the 22mm ?

 

[StephenW]

Maybe a simplistic statement - long time since I did Physics!
I was imagining a situation where you have a hosepipe with the open end pointing upwards at an angle. The higher the pressure in the water the further the water will travel in an arc. If you fed the hose into a length of 4 inch drainpipe then it wouldn't travel so far. The question then is if you sealed the end of the drainpipe and bored a hole in the end the same size as the hosepipe, would the water then travel as far as using just the hospipe (if you follow me). The answer given would indicate that it would. Keep awake at the back!

 

[Lordbothwell]

Quite correct Stephen, but there's many factors that can affect the dynamic pressure of a mains water supply.

 

[TicklyT]

Without any flow, the pressure will be the same at all points in the system, no matter what size the pipes are.

In a dynamic situation the pressure in the 22mm pipe will be higher than the pressure in the 15mm pipe on either side of it (Bernoulli's principle), but the effect will probably be unnoticable in domestic plumbing. It's what makes fittings like injector tees work though.

The association of larger bore pipes with lower pressure has probably come about because a larger bore pipe offers less resistance to flow than smaller ones. It is common to use larger bore pipework on gravity systems, which need all the help they can get to deliver a decent flow rate with the limited pressure available, where a mains pressure system can drive water through smaller pipes with pressure to spare.

 

[bengasman]

...Obviously the pressure in the 22mm will be lower than the 15mm

Clearly not.

If at the bath tap end of the 22mm I tee off a 15mm pipe to feed an electric shower, will the pressure in this 15mm be the same as the original rising main (assuming bath tap not running at the same time)?

Static pressure in a system is the same everywhere when height is disregarded.

 

[StephenW]

Well, always happy to learn stuff !
I also got the answer I needed. My between-the-lines question was whether I would lose significant pressure in the system by including a longish length of a larger diameter pipe. And it seems not. Thanks everyone. Carry on the debate as you wish!