Query concerning water flow rates in PVC pipes of differing diameters

[BruceB]

Hi all. We have a private estate of six house lots (only 4 houses built at present). There is a council water supply pipe of 20mm with council meter. Previously that then connected to a single 50mm pvc pipe that ran 50metres to four individual holding tanks. Individual owners then pump water from their own holding tank to their house. Each water bill was split six equal ways by the Council.

Water charges have rocketed upwards and we wanted to bring in a fairer system of water billing, measured by six individual meters mounted beside the Council meter. We replaced the single 50mm line with six 25mm pvc lines, each running direct to the respective holding tank. Two of those new lines were capped, ready to be connected to proposed new tanks when the further two houses are built). The pipe diameters used were approved by the Council and agreed to by the professinal plumber we used to do the work.

Just weeks later we have some owners hopping mad saying that their pumps are working at a fraction of their capacity, their holding tanks are running dry (and the sky is falling!) because they say the 25mm lines are too small to cope. We have some real armchair experts here, talking about friction in the pipes and so on. So we have 4x25mm instead of 1x50mm, double the capacity. Even allowing for greater friction in the smaller pipes, should this change really have made that much difference to pumping performance? All comments/feedback much appreciated

 

[Mike Jackson]

The total cross sectional area of 4 25mm pipes is the same as 1 50mm so when you take into account the frictional losses the carrying capacity will be less. There will also be a loss of capacity through each individual meter. You then have to take into account the fact that it is unlikely that all 4 properties were drawing water at the same time so it is entirely possible that at any one time an individual property was receiving the full capacity of the 50mm pipe.

 

[snowhead]

How big are the holding tanks?

 

[BruceB]

How big are the holding tanks?

Good question. Can't tell you off the top of my head. I would estimate 2m tall x 3m+ diameter. I will try to find out and report back. In the meantime I'll see what is involved in uploading a pic here

 

[BruceB]

Thanks for that input Mike. Can I clarify that you are saying that the change in the pipes (& meters) could in fact have a substantial impact on the ability of the pumps to draw water as suggested by a couple of the owners?

 

[stevetheplumber]

you havent mentioned lenghth of the runs which will also increase frictional losses is it the furthest house that are loosing their supplies
there must be a reason you had holding tanks fitted i suspect this was due to to low flow rate from the existing main can you not get additional mains in from the water authorities ?

 

[stevetheplumber]

another thought has anyone checked the flow rates into each tankive had plastic pipe that was semi blocked before another thought has any one checked the ballvalves haven got debris partially blocking them it easy to get a bit of mud or gravel in the pipe work, im assuming the tanks have ballvalves with the meters fitted flow rates will be an easy check to do first

 

[BruceB]

you havent mentioned lenghth of the runs which will also increase frictional losses is it the furthest house that are loosing their supplies
there must be a reason you had holding tanks fitted i suspect this was due to to low flow rate from the existing main can you not get additional mains in from the water authorities ?

Hi Steve. From the holding tanks to the pumps and from the pumps to the houses is flexible 50mm hose. The runs are extremely long - I don't have a measurement. Two of the furthest houses have additional holding tanks at around the half way mark. We could possibly get an additional main from the Council but no problems have been raised prior to the change with the meters. I should also mention that all four houses have substantial rainwater tanks and the mains water just supplements these.

 

[BruceB]

another thought has anyone checked the flow rates into each tankive had plastic pipe that was semi blocked before another thought has any one checked the ballvalves haven got debris partially blocking them it easy to get a bit of mud or gravel in the pipe work, im assuming the tanks have ballvalves with the meters fitted flow rates will be an easy check to do first

I have given the feedback from the owners to the plumber that did the work, and that was his suggestion to us. He is adamant that pipe diameter is not the issue. I don't think any of the owners have had their own plumbers out to look things over, and are focussng just on the pipe diameter issue thus far.

 

[BruceB]

View attachment 9696 Three PVC holding tanks service three of the houses. The fourth house has three concrete water tanks of slightly larger capacity, one of which is visible in the background.

 

[BruceB]

Merry Christmas all
Any final comments or clarification would be much appreciated as I'll need to take action on this matter next week
Cheers
Bruce

 

[Keefy]

Merry Christmas all
Any final comments or clarification would be much appreciated as I'll need to take action on this matter next week
Cheers
Bruce

You mention that the plumber also suggested what stevetheplumber mentioned above regarding checking for debris/blockages and that you don't think any of the homeowners have themselves had a plumber look over it. Has anybody checked to eliminate that possibility yet?

 

[BruceB]

NPK, Hi. One of the owners complaining about the new pipes claims that he has considerable experience with pipes and pumping (but not a plumber). He says that he has checked out the pumps and holding tanks before and after the work was done and that there is no debris or blockages. I understand that he has also employed a plumber to check things out, but I have yet to see a final report. The little I have seen so far though suggests that this guy's pump was in fact operating ok.

Oh and now the plumber that did the work says that he is not really sure whether the pipe diameter is adequate or not, and was just going along with the recommendation of the council inspector (who said to use 20mm lines).

 

[chris watkins]

View attachment flow rate.pdf
Not good news BruceB, hope the plumber you employed has indemnity insurance on the design ?
The attached PDF gives a formula which is used to size the number of supplies, it is only the basses of the design & we don't know whether the properties do actually require the flow rate delivered by a 25mm supply? Also, what are the flow / fill pattens of the cisterns (tanks) will they be refilling all at the same time or can we apply some diversity ?
Maximum of 5 x 25mm supplies from a 50mm, so will be no good when all 6 dwellings are up & running but it should be OK at the moment.