Pressure Reducing Valves

[bernibiker]

We have just installed a new mains water pipe to our house, as we have been on well water up to now and it's run dry(ish)!.

The run is 300m, and we have used 40mm PE pipe.

Upon connection at the house we measured 5 bar, far more than I was expecting, so we installed a PRV up at the road, just after the meter (the pipe is in 25mm for a short way here).

Unfortunately, the pressure guage failed after this, probably due to dirt getting into it, so are unsure of pressure at house now. The PRV is preset to 3 bar, and we didn't touch that.

So the question is, will we be getting 3 bar at the house, or less?

If I understand correctly, although there are losses in the 300m pipe run, we should still have 3 bar at the house, as the PRV auto compensates for pressure drop to maintain the 3 bar.

If we had 3 bar supply from the street, no PRV, I suppose we would get about 2, 2.5bar at the house.

But with about 6 bar at the street, and the PRV, in theory, do we get 3 bar at the house?!

Thanks for any help!

 

[gaswizzard]

3 bar standing pressure , this is not the same as dynamic pressure (outlet tap open) , PRV should have been installed in the property , taking the 300metre run into consideration you could have a lot less than 3 bar at the outlets.

 

[bernibiker]

Thanks for the reply.

We could not install at the house end, as the piping was arriving in 40mm, then 2 metres of 32mm, then no space in the inside cupboard.

A 32mm PRV was quoted to me at £400!! The 25mm was £80.

I realise that dynamic pressure may differ, but I was under the impression that the PRV will always try to maintain 3 bar on the outlet side subject to the available pressure on the inlet side.

I will get a new pressure guage, and if it turns out we only have 2.5bar at the house , then I can up the PRV to say 3.5, in order to get 3 at the house.

I was just curious about the way PRV's work I suppose.

 

[JohnD]

IIRC, on my PRV, the pressure gauge can be unscrewed and a new one put on. I've never had to do it, but I believe (haven't read the instructions for years) you don't have to turn the water off (sounds risky but maybe there is a sealed plunger). Most of the cost is on the big brass valve.

 

[SimonH2]

I was just curious about the way PRV's work I suppose.

Very simply !
There is a plunger or diaphragm sensing outlet pressure - when the pressure is high enough to generate force to overcome the spring pressure, the plunger/diaphragm moves and this closes the valve. When you draw off water, the pressure drops, and the valve opens again to maintain pressure.

The pressure is sensed at the valve outlet, although there are valves with remote sensing used in industrial control. So you will get pressure drop in your 300m of pipe and the dynamic pressure at the house will be lower. That's why you should have installed the PRV at the house.

Unless you have unusually high demands, a short drop to 25mm pipe shouldn't cause an issue - especially as you have some 25mm pipe (and PRV) up at the supply end. You'll gain more by compensating for the pressure drop in the long supply pipe than you'll lose in reducing the pipe size at the house.

 

[bernibiker]

Thanks for the explanation.

In fact, we installed complete new arrival plumbing inside the house. The mains and the well supplies arrive next to each other, and each has it's own shut off valve and non-return valve.

The well pump is on and set to 3 bar (at the well head, which is 80m away) and run in 53mm pipe.

We didn't touch the mains PRV, so assume it is at 3 bar. And yet, when both supplies' valves are 'on', the only flow comes from the mains. (Seen by condensation on all the brass connectors whereas the well ones are dry).

I think I'm right in saying that whichever supply has the higher pressure 'wins' and supplies 100% because of the non-return valves.

So therefore, either I'm still getting more than 3 bar mains at house, or the pump is not truly producing the full 3 bar.

I may experiment by upping the well to 3.5 bar (simple digital control) to see if the well water then wins the pressure battle!

 

[SimonH2]

I think I'm right in saying that whichever supply has the higher pressure 'wins' and supplies 100% because of the non-return valves.

Mostly, in practice if the supply pressures are equally matched then you may find you get a mix - as you draw from one, pressure drops in the pipe reduce it's pressure and you start to draw on the other as well.

I'm not up on regulations, but I suspect your arrangement wouldn't be allowed in the UK as there's a risk of backfeeding your well water into the mains if a single non-return valve fails. At the very least you'd need to use double check valves.

 

[bernibiker]

I think I'm right in saying that whichever supply has the higher pressure 'wins' and supplies 100% because of the non-return valves.

Mostly, in practice if the supply pressures are equally matched then you may find you get a mix - as you draw from one, pressure drops in the pipe reduce it's pressure and you start to draw on the other as well.

I'm not up on regulations, but I suspect your arrangement wouldn't be allowed in the UK as there's a risk of backfeeding your well water into the mains if a single non-return valve fails. At the very least you'd need to use double check valves.

I understand, but my point is that the two pressures would need to be almost EXACTLY the same for a mixed flow to occur, no? I suspect that the well is arriving at say 2.8 and the mains at 3.1, or something like that.

Untitled

• bernibiker
• 21 Nov 2011
• 1

Above is a picture of where the 2 supplies enter the house, well left, mains right. Each has a non-return valve. Are they double check, or single? I have no idea.

Thanks!

 

[SimonH2]

I think they are double check - the caps on the side being test points so you can test them and work out if one or the other is leaking.

As to mixed flows, suppose you have the supply pressures set as you state - that's a difference of 0.3 bar. Firstly, there is a pressure required to lift the valve in each check valve - and this varies between valves. Say the difference was 0.05 bar - that's cut the difference down to 0.25 bar, or increased it to 0.35 bar, lets assumed reduced.

And the pressure drop does tend to increase a bit with flow rate - you are compressing a spring* a bit more to open the valve more.
* Some have springs, some don't.

Now draw off some flow to get (say) a 0.3 bar drop in pressure along your supply pipe - your difference is now -0.05 bar and so you'll draw some water from the lower pressure supply.
It's not just pressure drop in the pipe, the supply pressure will droop with flow as well - the PRV doesn't hold the pressure exactly steady, as the flow rate goes up, the error will increase until the valve stabilises with a new valve position (and spring compression).

There's various ways of handling things.
One is simply to up/down the pressures so that the well supply is higher than the mains pressure by a reasonable amount. This would be fairly reliable and foolproof - and will leave you with mains pressure during power cuts.
At the other extreme, you could have a motorised valve in the mains linked to the well pump system so that the mains is only turned on when the well is dry (or the pump broken).

 

[bernibiker]

For now, we're just using mains as the well is almost dry, but after some decent rain, we'll put the well up to 3.5 bar and see if it comes through.

Presumably the pressure behind the PRV has some effect? Imagine it were only 3.5 bar, and the PRV were set to 3. When flow starts there is little scope for the PRV to keep pressure to 3 as it doesn't have much 'power' behind it.

However if the town pressure is 6 bar (it is on our case), there is more capacity surely, to keep pressure at 3 bar, whatever the demand from the house?

 

[SimonH2]

How well the PRV holds up the outlet pressure depends on the "gain" of the control element. If the supply pressure is low then the valve will just open further - ie large pressure through small hole gives same flow rate as low pressure through large hole.

In theory, if the valve were perfect, the outlet pressure would be the same whatever the supply pressure as long as the supply was sufficient to meet demand with the valve 100% open.
In practice, such a valve with what would be described in control theory as infinite gain, would be unstable - a tiny drop in pressure would open the vale 100%, at which point the outlet pressure would overshoot (go too high) and trigger the valve to shut off completely.
Digressing a bit, that is effectively what happens in a switching power supply - the power is either on or off, and when averaged out it gives the output required. I'd guess that is how your well pump probably works - when the pressure drops, the pump turns on, when it recovers, the pump turns off, and with an accumulator to even out the supply.


What actually happens is that a low inlet pressure will need the valve to open more, and that will need a larger deviation from required outlet pressure in order to cause the spring length to change by the larger amount that will give the required valve opening.

As another aside, my brother has gone for rainwater collection for non-potable uses (ie flushing the toilets) in his house renovation. There's a lift pump to pull water from the collection tank, but if that runs out, there's a changeover valve to allow the pump to draw from a small tank with a ballcock fed from the mains.