max flow rate from 1/2" pipe

[Buster]

Hi

I wanted to upgrade my house to a pressurised system and unvented cylinder, however the mains supply pipe is only 1/2" Type D alkathene pipe with a nominal bore of 15mm. The flow rate is 12-14 litres/min at a secondary outlet. I have been told that I need a min of 20 litres/min at a dynamic pressure of 2bar at a secondary outlet for the unvented cylinder to operate satisfactorily.
I was going to get the connection to the mains service pipe replaced but the water company want to charge £1400 . I was considering replacing the mains pipe from the house to as close to the stopcock as I can get with 32mm mdpe (about 100m) to reduce flow resistance.
However this will still leave approx 5m of 1/2" pipe from my property below the road to the stopcock and meter.
Will this 5m section of 1/2" pipe prevent me from achieving sufficient flow rates?
What is the max flow rate that can be achieved through a 15mm bore pipe at what dynamic pressurer?

The mws through the house appears to be 15mm copper. I understand the unvented cylinder will require a minimum 22mm supply but will 15mm outlets suffice or will this require upgrading to 22mm?

Ta

 

[PuraTherm]

The 5 meters of half won't matter if you upgrade your service to 32mm with 100m of run. All piping to the unvented system must be 22mm. The flow rate from the 15mm copper tube is dependant on pressure, number of bends and length of run but don't worry about that. The only 15mm you should have on a properly desingned mains feed system will be for the branches coming off of 22mm.

 

[oldplumber]

you could employ a groundworks team with the right permits to make the connection to the stopcock and they wouldnt cost as much as the water co, but they still need permits for work and insurances. Any increase in size will help you as far as flowrates etc go. 22mm in and 22mm out Ting to 15mm as required will work for cyl. try supply to house first without going into road, pressures etc may be ok and save some money.

 

[Avatar]

I think this is a waste of time mate - no garentee you will increase the pressure either.

Why dont you look at a booster pump set up to do the same thing (for much cheaper, and likely to work!)

Are you sure you really need this, or has a sales rep been quoting figures at you, which may not be relevent to your situation???

 

[Buster]

Water company will not allow you to do the grounds works yourself any more. They employ a utilities company to do grounds works and connection which is why it is so expensive. Also getting the new main in through the floor of the house is going to be a lot of hassle.

I need to verify the 1/2" pipe will provide the required flow; before I dig at the boundary of my property to locate the existing supply pipe and join the 32mdpe to it.

 

[PuraTherm]

I think this is a waste of time mate - no garentee you will increase the pressure either.

Why dont you look at a booster pump set up to do the same thing (for much cheaper, and likely to work!)

Are you sure you really need this, or has a sales rep been quoting figures at you, which may not be relevent to your situation???

You would ned to get permission from building control first to install booster pump. You would be looking at £500 to £700 for the pump alone. Thats with out installation costs. Has to be RCD protected and sited properly wich could mean bringing in sparky. 32mm x 100m of mdpe about £100. A little manual labour and that is pretty big cost savings.

 

[Blackcatgas]

tanks in the loft supplying a salamander esp 120 pump connected to the unvented cylinder will give good results, or an accumulator tank, if youve got the space.

 

[Buster]

Thanks CHRISMUR

That was my initial thought that the short section of 15mm bore pipe would determine the max flow. I was wanting someone to prove it wrong by calculation.
I'm beginning to think that it not worth the hassle and cost to upgrade the mains supply if I need to put a break tank and pump on to get the required flow.
I would be better keeping the CWS tank in the loft, stick with the vented cylinder and fit a whole house pump on the hot and cold outlet to boost it around the house.

Is there a bypass loop on a twin pump, to allow it to pump hot or cold only when only one tap is on?

 

[TGor]

Doing a "simple"calculation the 100m of 32mm plastic with a flow rate of 20 l/min will lose you abt 2.5 metres of head .. funnily enough the 5 metres of 1/2" copper will lose you the same thus you will have a total head loss due to friction of 5 metres (.49 bar) if you left it at 1/2" right through the head loss would be over 5 bar .. this does not take account of any bends/elbows, valves NRV's etc that may be in the line .. check your available pressure at the main and subtract the losses that will give you the residual pressure at the end of the line. .... hope this helps.

PS provided there is enough available pressure a 15mm pipe will provide the required flow rate however the velocity in the pipe will be increased i.e. in the 32 mm it will be at abt 1metre per second in the 1/2 about 2 metres per second .. this is a little outside the design recommendation of 1.5 metres per second but I would not consider it excessive.

 

[TGor]

Regarding connecting a 100 Metre run to a 5 metre length of existing Water Mains Pipe sized approx. 15mm Internal Bore [?] :

The Flow Rate / VOLUME of Water would be `Governed` by the VOLUME passing through the EXISTING 15mm Bore - You will NOT increase the Volume or Pressure of the Incoming Water Main very much by installing a 100 Metre run of 32mm Poly Water Main Pipe - you MIGHT decrease the `Frictional Loss` slightly with a Larger Bore pipe - BUT definitely NOT enough to `Warrant` the amount of Work involved !!

sorry but this is not quite correct the frictional loss will decrease significantly (see my post above)
a modern property will require a minimum peak flow rate of 20 l/min and a 1/2" pipe run of 105 metres at this flow rate will lose 5 bar due to friction increasing the pipe diameter to 32mm as far as possible will reduce these losses to an acceptable level and provided there is enough head at the main there will be enough supply at the house (what happens inside the house may be another story but that wasn't the original question.

The Water Flow is determined by the `Tapping` Size into the Water Main in the road - AND the internal Bore of the EXISTING Water Main Pipe - 15mm - Increasing any length of the Incoming Mains Pipe would do no good - Unless you Increase the WHOLE RUN - WITH an Upgraded `Tapping`/ Valve - to the LARGER 32mm Pipe Size.

Disagree .. the water flow is determined by the orifice size and the pressure

The VOLUME of Water is equally as important as the Pressure - The Pipe Bore is related to this - BUT it has to be the Correct Size Bore `All the Way` from the Mains `Tapping`/ Valve - to the Entry Valve within your Home - with sufficient Pressure to `Push` the Volume of Water through the Pipe.

See above

I hope that this has stopped you from all the work of Excavating 100 metres of Trench - Installing the Pipe and Back Filling / Re-Instating the Ground - as well as the Internal Works !!

Sorry but I do not think this is good advice as it is not based on sound hydraulic theory if I was in your position I would upgrade the incoming as far as possible.

 

[Avatar]

yes the pumps can handle hot and cold separately.

no dont do the work on the mains, its not worth it, and unlikely to yeild results!

If you want 2 bar, the pump will give you exactly that, no questions asked. Do check that your CWS is addiquate though!

 

[justlead1]

Hi Using Box's formula :- At say 5 bar and 15 mm pipe 5m long (not account for elbows, valves, etc) the discharge would be 1.74 L/Second
At Say 5 bar and 32 mm pipe 100m long " " the discharge would be 2.59 L/Second
It may help to guesstimate the way forward.

 

[Buster]

Hi Using Box's formula :-.

Hi justlead1

1.74L/sec appears a high flow rate, what is box's formula and how do you calculate it?

Thanks

 

[justlead1]

Hi Not to good at it but here goes. Q = Litres/second discharge.
D= Dia. of pipe (mm)
H=Head in meters (divide x10 to bar's)
L= Length of pipe in meters

Q= D to power of 5 x H divided by (25xLx10 to power of 5) Then square root the answer.
Sorry i am not similar with communicating using symbols etc.

 

[Bernie2]

Without getting technical think of it like this.

If you had a store tank 10 metres up in the air supplying a tap the pressure would be metres head regardless of whether it was a 100mm or 10mm tap. So changing a main supply pipe size is not going to do anything regarding pressure if the water supply company only supplies water at 1 bar, its going to still be 1 bar regardless of supply pipe size.

It will however alter the flow rate, a bit like the same store tank scenario a guy baling out water with a teaspoon and another guy doing the same with a bucket.

Increasing pressure does make the water come out faster of course, a bit like the guy with the teaspoon baling out the water at a teaspoon an hour increasing the speed to two teaspoons an hour.

 

[TGor]

Without getting technical think of it like this.

If you had a store tank 10 metres up in the air supplying a tap the pressure would be metres head regardless of whether it was a 100mm or 10mm tap. So changing a main supply pipe size is not going to do anything regarding pressure if the water supply company only supplies water at 1 bar, its going to still be 1 bar regardless of supply pipe size.

Bernie, it will be 1 bar at the main yes .. but if you have a 100m run of 1/2" pipe at a given flow rate you are going to lose a lot of pressure due to friction. So the residual pressure at the property will be lower than at the main .. by increasing the size of the pipe you reduce that friction and lose less pressure so the the available pressure and hence the flow rate is higher at the property

 

[Bernie2]

Yes its true a larger size pipe will reduce the friction head, but it was not exactly the point I was trying to make. I suppose I was pointing out the relationship between flow and pressure not how to increase or decrease the friction head loss. In other words a 15mm pipe would reduce a 1 bar mains pressure down to say .75bar, but a 100mm pipe would not raise it any higher than the original 1 bar.