Flow rates /pressure loss reduction

[PaddyLondon]

This is one for serious techies I think:

I have a 12m run in 15mm pipe from the water main stopcock in the pavement to the stopcock in my house.

Water runs at a maximum at 18 litres per minute along this pipe, into the house plumbing with lots of outlets open. The pressure is around 3bar at the stopcock at this flow rate.

If I change the 12m of 15mm pipe to 32mm MDPE, I’ve calculated the pressure loss over that run will be reduced by c. 0.85bar, which all things being equal would raise the dynamic pressure at the house stopcock when water is flowing

If I make this change, what will be the impact on maximum water flow rate? Will it be material?

 

[oldbutnotdead]

Depends what the pipework AFTER the stopcock is but yes you should notice the difference. I'd be surprised if static pressure was much more than 3 bar at the stopcock- have you measured that? And dynamic pressure at the outlet furthest from the stopcock?
As a guide, I've been playing with my water setup. Outside tap delivers 24.6 ltrs/min from about a metre of 15mm just after the stopcock which is at the end of 35 metres of 25mm MDPE to the street. Static pressure is 3 bar, dynamic is 2.8 bar. Kitchen tap (add another 5 metres of 15mm) delivers 18 litres/min, not done dynamic pressure at the tap. Yet. Should do it before I tidy everything up.....

 

[David Costelloe]

... yes you should notice the difference.....p.....

That's encouraging for my project. I've just gone unvented and I am pleased with the results. With several outlets open I get around 18lpm flowing through the water softener according to its display, and two showers can operate well.

I want to squeeze as much as I can before (or possibly avoiding) getting my connection pipe to the main upgraded (£2,600 quote as the allege it requires 6m of work under the road and pavement), which would be the final bottleneck with which to deal. So on my list I have

1. Change the hoses on water softener
2. Replace the 15mm valves, connections and pipes in the manifold above the mains valve, connecting to the water softener and unvented cylinder, most of which is sixty years old (see photo)
3. Upgrade the run to the street stopcock

I wish there was a simple model you could use to forecast the impact of the changes.

 

[PaddyLondon]

I had the internal pipework and hoses improved. Flow rate went from 18 to 20lpm.

Currently getting the externals upgraded to 25mm MDPE.

 

[fezster]

I've given a lot of thought (+reading!) to understand the flow rate coming into my house and I'm not sure I fully get it.

I have a 32mm incoming mdpe with 28mm/22mm running to various showers around the house (relatively high flow rain type shower heads). Depending on the time of day, having 2 showers will either be acceptable or pitiful. Makes sense.

I have a pressure guage on my incoming supply after my 3 bar pressure reducing valve. With a single shower running, the gauge will drop by 0.8 bar to 2.2 bar. With a second shower running, it'll drop by another 0.8 bar to 1.4. Again - all makes sense.

Now the puzzling part. Regardless of time of day, the dynamic pressures will always be the same. I.e. 2.2 bar with single shower and 1.4 bar with 2 showers. I'd have thought that during off-peak times, when the incoming flow rate available is higher, the dynamic pressure would also be higher?? That doesn't seem to be the case.

It could just be I've not accurately measured it (it's been a little non-scientific, as measuring the actual flow rate is difficult - I'm basing it purely on how I feel the shower is performing).

 

[oldbutnotdead]

A bit of physics may come in handy here.
Water is pretty much incompressible. So at any given pressure, the flow rate through a fixed diameter pipe will also be fixed (so, for example, 22l/min at 2 bar, 30 l/min at 3 bar).
Your pressure reducing valve is controlling the max water pressure (and thus the max flow rate) into your house. As long as the pressure in the main never drops below 3 bar your internal pressure drops will be constant, as you've discovered.

 

[fezster]

But the flow rate does not seem to be fixed - that's my point. At peak times, the flow rate is markedly lower, even though the (working) pressure remains the same.