Water pressure

[Andy Burns]

I'm installing an irrigation system for some trees planted a few months ago, a three hour trip to water them every few days is getting a bit old.

Water supply is 25mm blue mdpe terminating in JG speedfit stop tap and

3/4" tap connector, I suppose the pressure is "highish" because when I reduce it down to 1/2" tap connector it has a tendency to blow the hozelock connector off a hose.

For the irrigation pipe, I've fitted a pressure reducer that can take input of 1-10 bar and give output of 1 bar, inside the black screw cap is just a spring.

But when connecting the PRV, I can't get any get flow out of it, with tap wide open (impossible to stop the flow with my thumb) very likely the input pressure is exceeding the 10 bar maximum, so the PRV is shutting off? But I when I close the tap down to a trickle (easily stopped with a finger) I still got no flow, below the 1bar minimum? No amount of adjusting the tap would get flow through the PRV.

AFAIK 1 bar is 10m of head, any reasonable way to estimate the supply pressure? With the tap fully open, at the 3/4" connector the full flow spurts about 18" vertically, restricting the flow as much as is possible by thumb the jet will reach about 4m vertical ... what ball-park pressure have I got? I'm looking to get another PRV to fit ahead of the Claber one but don't want it to be too much of a guessing game what spec I need ...

[Dave Liquorice]

Lets start with the obvious, got the direction of flow right?

Yep.

I don't think so short of having a pipe rising up to the point where the flow stops. Anything with a flow is going to drop the pressure dependant on how much flow and the resistance to that flow there is.

Is this supply the normal water mains? I'd be surprised if that is above 10 bar. There are hi/lo limits to supply pressures. Water pressure guages are cheap in the sheds...

[Andy Burns]

Yes, it has an arrow on the outlet :-)

Yes.

Having had a look at pressure gauges, and other PRVs they do seem to top-out at 10 or 11 bar, I'll buy a gauge.

[John Rumm]

With a pressure testing gauge! (you can get them that screw onto the tap connector)

[Andy Burns]

Well yes, but I was wondering it if was possible to get a sketchy answer from flow rate or how high a jet of water could be produced, trying to avoid too many round trips ...

[The Natural Philosopher]

In principle a fountain cannot put water higher than the head from which it is fed.

Unfortunately, there are any ways in which it can be way less than that.

What about an old tyre coupled up to the mains, and then use a tyre pressure gauge on that...

[John Rumm]

If he really has 10 bar pressure (~150 psi), that would be well over the design pressure of the tyre! (it would probably take it, but might get messy if it went bang)

[John Rumm]

The height would vary with the size of the "nozzle" e.g. an open hose will not project water anywhere near as high as one fitted with a spray of some kind.

ISTM more likely the PRV is defective - I have seen them fail "blocked" before.

[Andy Burns]

As I originally mentioned, this varies from 18" for the open 3/4" connector to 4m for the smallest opening I can make with my thumb over it.

I wondered about that, but other than unscrewing the black cap and finding the spring within, nothing else looked accessible, I should have brought it home to look at ...

[Chris Hogg]

It's not got some sort of flow controller hidden in the fitting, has it, along with the pressure reducer? I have one somewhere, taken off an inherited hosepipe. Damn nuisance. It's supposed to stop the flow if it thinks that the flow is too high, i.e. that the hose has split or the spray head has come off.

[John Rumm]

If that corresponded to only 4m of actual pressure, then you ought to be able to stop the flow from the tap quite easily with your thumb...

Or try it on your supply and see if it behaves differently.

[Andy Burns]

couldn't see one inside the cavity wit the spring, but didn't have any suitable tools with me.

I've ordered a pressure gauge, was going to collect it and check this evening, but didn't realise toolsatan are closing at 5pm these days.

[Dave W]

Here are critical reviews on Amazon saying the valve gets stuck shut.

[Thomas Prufer]

This doesn't *sound* like anywhere near 10 bar... I have ~7 bar, part in 3/4" pipework.

That said, I think you just can't tell that way: Imagine a very high pressure, connected to your nozzle by just a pinhole. No jet, just a dribble -- but the pressure would build up over time, and burst pipes eventually. Connect without the pinhole, and you have a firehose/pressure washer.

Thomas Prufer

[Andy Burns]

Hmmm, thanks. Have checked pressure and it's 2.5 bar so well within input range, I brought the PRV home, no accessible parts, can't blow or suck through it, will try an exchange at B&Q.

[Andy Burns]

Put it on my garden tap, heard it gave a "click" and water started flowing, here it's on 15mm pipe and 1/2" tap, there it was on 25mm pipe and 3/4" tap, pressure on both is ~2.5 bar.

Having removed it again you can suck and blow through it now, seems that the valve locking-up as per the amazon reviews is real.

Maybe it doesn't like having to feed 50m of empty pipe? Though actually I did try it with nothing on the output after it had originally failed, so why did it "reset" itself?