Flow/Pressure Question

[dishman]

Can anyone explain to me the following....I understand the physics and fundamentals, just cannot quite understand how they relate...

If I replace my old small diameter water service pipe with a modern 28mm or 32mm supply pipe I have read this will ensure I have good pressure in my property.

I have also read that it is not pressure that is increased but flow (the amount of water that can flow down the pipe). This I understand....but surely if you increase flow you decrease pressure. You have a larger pipe and therefore less restriction so more water can "flow" but at a slower rate? Is it not akin to putting your thumb over the end of a hosepipe? You decrease flow but increase pressure?

Or is it due to the fact that the mains pipes in the road are already a larger diameter so the reduction to 32mm or less has no affect on pressure, just the amount of water that can flow down your pipe?

But this still does not make sense for my hosepipe analogy...unless that is an incorrect way of looking at it....

Anyone?

 

[Newboy]

In simple terms:

1) The pressure at the point where your supply pipe connects to the main is reasonably constant - it does vary during the course of the day as demand for water changes (slightly higher overnight when demand is low and lower in peak demand periods)

2) If you have a restricted service pipe to your property then

a) The static pressure will not change
b) The flow rate will be comparatively low
c) The dynamic pressure will be low - due to the restriction, the amount of water that can flow through the pipe is limited.

Increasing the diameter of the service pipe will result in a higher flow rate and a higher dynamic pressure but will not change the static pressure

 

[Agile]

Nice and clearly explained to you there.

You say that you understand the physics but I have to disagree with that.

Unfortunately, although these are very simple concepts, they are widely misunderstood.

Even many people doing plumbing for a living dont seem to have a proper grasp of the dynamic pressure in supply pipes!

When you open your kitchen tap you are getting the maximum flow rate that is available based on:-

The static pressure in the street main.

The flow resistance of your connection to the street ( and including your kitchen tap ).

Since perhaps 20% of city properties have an old lead supply pipe they can be a very small bore and significantly restrict the flow compared with a nice smooth plastic pipe.

Tony

 

[dishman]

Thank you, it is quite hard finding a suitable explanation (or searching with the correct terms) online.

Thats what I was getting confused on, the difference/relationship between the static and dynamic pressure.

I meant that I can understand the physics, I was just having trouble applying/understanding it in this context.

So it is more like filling a funnel from a running tap. The restriction of the funnel does not affect the pressure coming from the tap water, it just restricts how much can flow through it(hence the funnel fills up quickly as the water cannot pass through fast enough). A wider diameter funnel means more water can pass through and therefore the dynamic pressure is increased. So the closer the dynamic pressure is to the static pressure the better?

 

[Onetap]

I have also read that it is not pressure that is increased but flow (the amount of water that can flow down the pipe). This I understand....but surely if you increase flow you decrease pressure.
Anyone?

It is to do with energy losses due to friction with the pipe wall.

The total energy in a mass of moving fluid consists of height, velocity and pressure components. You can express the total energy, the sum of all 3 components, as head/height i.e., H total = h + v^2/2g + P/ρg according to Bernouilli's equation.

g, ρ h and v are all constant when you've got a liquid flowing in a level straight pipe. The energy you lose from the moving fluid, due to the friction between the pipe walls and the fluid, can only come out of the pressure component, P. That's what happens, the pressure reduces as you go along the pipe. The energy losses due to friction are equal to the pressure losses.

If you put in a bigger pipe, the friction, and so pressure losses, are much less for a given flow; alternatively, you get a much bigger flow for a given pressure difference between the water mains and kitchen tap.

Clear?

Is it not akin to putting your thumb over the end of a hosepipe? You decrease flow but increase pressure?

The pressure at the tap is constant, we'll assume. Unrestricted flow through the hose causes a large pressure loss due to friction, and a low pressure at the hose outlet. Restricting the flow with your finger reduces flow, reduces frictional pressure losses and results in more pressure remaining at the outlet.

 

[dishman]

Ahah!

With some better terminology searches thanks to your posts, I found this, which is the perfect explanation.....it even debunks my incorrect hosepipe analogy (which is the bit that I was not sure about). All understood now and makes sense.

http://www.irrigationtutorials.com/using-a-smaller-pipe-to-increase-water-pressure/

This is all because I am considering replacing the original lead service pipe to my house (which seems to be recommended). Although the water flow/pressure is very good, it seems logical if I am doing a major renovation (including new heating system) to do this.

I always like to know (and think it is important to know) the principle behind things.