Water flowing back into the grid during a mains fail

[puntloos]

This is my basic setup - my question is: If Mains water pressure falls away - which components (if any) would prevent water flowing from accumulator back into the mains grid? Will the pump prevent this, or is every standard house stop cock built to prevent this flowback?

 

Putting the question slightly differently - if mains pressure drops to zero, how much water can I get from my accumulator before my house water also stops? (the acc is 200L in my case)

 

 

Maybe the answers are obvious (200L!) but I find often there are sneaky things that change the answer dramatically

[JohnMo]

The check valve you show at the left hand dashed box

[ProDave]

The very left hand item in your diagram is an isolating valve and double check valve (non return valve) that will stop back flow.

 

That is a requirement for any new house now. with or without accumulator or anything else.

[puntloos]

Thanks gents, I assumed as much but wasn't 100% sure. 

And indeed if the pressure is gone, can I expect to have that 200L of my pressure buffer to be available while the outage is happening? Of course the pressure will slowly drop available water to a trickle, but could I get a good amount of that pressure vessel water (50%? 90%?) for practical use?

[Alan Ambrose]

No, don't believe so. I'm guessing it has the usual rubber bladder (?) - so the difference between fully stretched bladder and fully relaxed bladder ( 😀 ) may be a smallish fraction of the total tank size. 1/4 maybe? I'm guessing the manufacturer could tell you.

[Alan Ambrose]

Ah, presumably something like this:

 

https://www.anglianpumping.com/product/pressure-vessels/200-litre-accumulator

 

Remember GWS accumulators operate on a 60% volume to use ratio meaning a 300 litre vessel gives 180 litres of powerful water before returning to mains flow.

[Conor]

Your meter box will also have an non return valve.

[ProDave]

37 minutes ago, Conor said:

Your meter box will also have an non return valve.

Ha Ha.  I had that "discussion" with Scottish Water.  THEIR meter box has a SINGLE check valve.  My own boundary box that followed it has a SINGLE check valve.  NEITHER are any good, not even both of them together.  They would not connect me until they could see an additional in line DOUBLE check valve.

 

Makes you wonder why they supply and fit something that is not fit for their own requirements.

[John Carroll]

On 12/12/2024 at 16:26, Alan Ambrose said:

No, don't believe so. I'm guessing it has the usual rubber bladder (?) - so the difference between fully stretched bladder and fully relaxed bladder ( 😀 ) may be a smallish fraction of the total tank size. 1/4 maybe? I'm guessing the manufacturer could tell you.

 

The available volume for any accumulator is (1-((prechargepressure+1)/(chargingpressure+1)) X accumulator volume)

For example, in a nonpumped accumulator, if 1.0bar is sufficient for your needs then a 200L accumulator will be precharged to 1.0bar, and assuming a fully charged accumulator from mains at 3.0bar, then the available volume on mains failure is 1-(1+1)/(3+1) X 200, equals 0.5*200, 100L

A pumped accumulator like yours, assuming you require a minimum pressure of say 2.0bar would be precharged to 2.0bar and  assuming the pumped charging pressure is 4.0bar, then the available volume is,  1-(2+1)/(4+1) X 200, equals 0.4*200, 80L. but remember then, because the diaphragm will now have bottomed out that you you won't get a single drop of any extra water even if 0.5bar is enough in a emergency.