Dynamic pressure drop at outside tap

[belfastbob]

Hi guys

My newly built house is connected to the water main by about 30m of 32mm o/d MDPE pipe (roughtly 26mm i/d).

At the outside tap near the stop cock, the measured static pressure is a shade over 3bar. If I leave the gauage on the outside tap and turn on an indoor tap my dynamic pressure is as follows:

Flow = 6.4 lts/min, Dynamic pressure = 2.2 bar
Flow = 4.2 ltr/min, Dynamic pressure = 2.6 bar

By my hand calculations (confirmed using www.pressure-drop.com) at 6.4 ltr/min I should expect to see a drop of just 9.3mbar, or 4.6mbar at 4.2 ltr/min. I'm seeing more than 80 times this drop so obviously I have a problem.

As I'm measuring at the outside tap, I suspect that the problem (blocked pipe causing flow restriction?) lies outside somewhere in the 30m MDPE between the mains toby connection and my stopcock.

Can anyone on here shed light on what they believe could be the cause of such a problem?

Thank you in advance

Bob

 

[Agile]

Your measurements are typical of most domestic supplies.

I always like to measure the dynamic flow rate which leaves 1.0 bar in the house pipework. Yours seems to be about 14 li/min which is enough for a combi boiler but rather low for an unvented cylinder to give its best performance.

What you seem to have not realised is that stopcocks and internal pipework all contribute to pressure loss. The loss of the pipe is probably one of the less significant components. The water supplier's meter/stopcock will be significant too.

Tony

 

[Gasguru]

It might be typical of and old scaled up lead service pipe but it's well below the performance you should expect. With such low flow rates there should be a negligable drop.

Check the stopcocks at the meter in the street and the internal one are fully open. There must be some significant restriction...perhaps check valves or blockage.

 

[belfastbob]

Your measurements are typical of most domestic supplies.

I always like to measure the dynamic flow rate which leaves 1.0 bar in the house pipework. Yours seems to be about 14 li/min which is enough for a combi boiler but rather low for an unvented cylinder to give its best performance.

What you seem to have not realised is that stopcocks and internal pipework all contribute to pressure loss. The loss of the pipe is probably one of the less significant components. The water supplier's meter/stopcock will be significant too.

Tony

Hi Tony
I've measured the flow through internal stop cock every 0.3 bar or so, adjusting the flow from the outside tap to give the desired dynamic pressure. I get:

Pressure [bar] Flow [ltr/min]
3.0, 0.0
2.7, 5.1
2.4, 6.0
2.0, 7.8
1.7, 8.9
1.3, 10.5
1.0, 11.9
0.4, 13.6

In all cases, the pressure drop at that particular flow rate is equivalent to over 2000m of 32mm MDPE! I've only got about 20m to the main supply so I can't understand where the restriction is. A stop cock wouldn't drop 1 bar at only 7.8 litres per minute would it?

Your advice is much appreciated.

Bob

 

[Gasguru]

Yes it's hopeless In fact the legal minimum that the water companies have to provide is around 8 or 9 litres/min at 1 bar and you're not far off that. There's a major problem somewhere. If the stopcocks are fully open I'd dig up the main near the boundary and test there to determine if the issue is the meter/stopcock or service pipe to the street main. But having been down this route before the water companies often don't give a stuff and as long as you get their minimum pressure/flowrate defined in their T&Cs you're stuck.

 

[mfarrow]

You need to check yours and the street stoptaps are open (-1/2 turn to stop them seizing).

 

[Agile]

Yes it's hopeless In fact the legal minimum that the water companies have to provide is around 8 or 9 litres/min at 1 bar and you're not far off that. There's a major problem somewhere.

Its not even that!

Its only about 0.6 bar static pressure and about 6 li/min open pipe flow rate.

That's only at ground level too!


Tony