Water pressure again but different

[Dave Smith]

I didn't take any notice of the recent topics of water pressure because it didn't affect me at the time but now I wish I had because I find myself in a very similar situation to one of those earlier posters and now I can't find the info posted in their threads.

Our water company are saying that to get compensation the pressure must fall below 1.0bar but I pointed out that that makes no sense unless the person in question has a pressure guage, and I haven't (although I'll get one now for future use). I said that it was taking 1min 5sec to draw off 1 litre from the kitchen tap but she just kept referring to the 1bar thing. It appears I'm talking in apples and she's telling me in oranges and never the twain shall meet.

Can anyone tell me what the pressure was likely to have been at a time when it took 1min 5sec to get a litre of water out of the tap? We have a Triton T80xr shower and the "Low Pressure" light was lit. According to the manual, it needs a min of 1bar and a flow rate of 9litres/min, if that helps.

Thanks

[Invisible Man]

I imagine the pressure is the static pressure provided by the water company to your property through a set diameter pipe. It cannot be measured by flow rate because the flow will depend on the various valves, bends etc in your pipework which will reduce the flow the and the dynamic pressure.

You will need to get a pressure at an opening when there is no water flow. Ie all taps turned off and tanks not filling. If this is not below the limit try turning a tap on somewhere else in the building and seeing if the pressure plummets. If it does then there may be something somewhere partially restricting a pipe. That could take a while to find.

[John]

How can flow rate be a different thing to pressure? If opening the kitchen tap usually gives a high-speed, gushing torrent of water (which it does) and then one day I turn it on and only get a trickle out of it, I would describe that as low water pressure and I imagine 90% or more of the population would describe it that way too. Is that not correct?

What happened was that the water main runs along the street and the houses are tee-d off it. From the main a two-foot long pipe goes to a stop c*ck in the pavement outside our drive, and from the stop c*ck a piece of pipe, probably 10 metres long runs under and along our drive, and pops up in our kitchen, behind the washing machine, where our internal stop c*ck is. The kitchen sink tap is about 4ft along from it.

It appears that something had partially blocked the 2ft section of pipe between the external stop c*ck and the tee into the main, resulting in (as I describe above) low water pressure in the house, with just a trickle coming out of the kitchen tap. Are you saying that that situation is _not_ low water pressure?

The problem has now been fixed but surely it's unrealistic of the water company to expect the general public to have access to pressure guages - I can't see my 84-year old granny getting a tool box out and fitting a pressure guage anywhere. The general public perception surely must be that if you normally get a good flow and now you haven't, then there's low pressure. Or, another way would be to say that under normal circumstances I can't press my thumb up against the tap to stop the flow of water, whereas during the time with the problem, I could easily stop the flow. They are the only diagnostics available to most of the public I would imagine.

[David WE Roberts]

This comes up nearly every time water pressure is discussed.

Think of a 50mm diameter water pipe with 10 bar pressure and a 50mm tap on the end. Think of a T connection to a reducer which leads this 50mm pipe into a 1mm diameter pipe with a 1mm tap on the end. Assume that there is a massive water main behind this which can maintain the pressure when you open the tap.

Use each tap in turn to fill a bucket. Which pipe and tap do you think would fill the bucket faster, the 50mm or the 1mm?

Remember that they are both operating at the same water pressure.

HTH

Dave R

P.S. the drop in flow rate suggests a drop in pressure, but you can't use flow rate to measure pressure directly.

[Geo]

Hang a bucket of water from the ceiling. Drill 1mm hole and measure flow rate. Repeat with 10 mm hole. Pressure is identical in both cases but flow rate is not.

[John]

OK, thanks for that Dave, a good illustration and I now understand - I think :-)

So, when the water company say they'll give a few quid compensation for low water pressure, are they using semantics to avoid paying out? Like I said earlier, you can't expect an 84-year old granny to get a toolbox out and fit a pressure guage. The general public understanding is of the amount of water they get out of the tap - high speed gush = good pressure, low speed trickle = low pressure.

Did our pressure fall below 1bar - who knows? My granny only knows that she couldn't use the shower for more than two weeks and had to go to a neighbour's for a shower, to name just one of the inconveniences experienced. It would seem that the water company are dangling an unreachable carrot by saying they'll pay compensation for low water pressure when the vast majority of people don't even know what it is, let alone how measure it.

[SS]

Flow and pressure are completely different things. You can have a very high pressure and a very low flow rate quite easily. And vice versa.

Would I be correct in saying a river has a high flow rate but no pressure?

[Invisible Man]

Depends on the river. It could be very wide and slow flowing or it could be quite narrow but running down a mountain with the odd high waterfall

[John Rumm]

While this may be the interpretation they put on it, it is a flawed appraisal.

Look at it from the suppliers point of view. They supply water to the property at say 3 bar. With adequate capacity in the mains to maintain that pressure even under dynamic conditions (i.e. while water is being drawn). However the owner of the house has 50m of badly scaled 1/2" copper pipe between the main and his tap. The result is the static reading will still be 3 bar at the tap, but the flow rate will likely be inadequate. The property next door however with a 10m run of 25mm MDPE pipe will have no such problems. It seems rather unfair to make this the suppliers problem.

This may be the case, but realistically what else could they do?

[John Rumm]

Not quite - you will need some pressure difference to achieve a flow in the first place. However its fair to say they probably have low pressure and very low flow resistance - hence you get a massive flow rate for a relatively small pressure.

(analogies with any other flow of energy will give similar results - the current flow in a circuit will depends on the "pressure" i.e. voltage pushing it round, and the resistance it meets along the way. The rate of flow of heat will depend on the temperature difference, and the resistance (i.e. insulation) to that flow etc)

[polygonum]

A just what would that pressure be in the middle of a waterfall? Especially one where the water is breaking up?

[Invisible Man]

Whatever the force per unit area is on the rock it strikes. Somewhat variable I suspect

[The Natural Philosopher]

1 bar of course.

[Matty F]

Well this is a d-i-y group. It can't be hard to make one. I can calibrate it at work where the pressure is 110 psi and we use that to test boilers :)

[The Natural Philosopher]

Pressure gauges can be hired at very small expense.

[polygonum]

Why the 'of course'? If there is enough pulling apart of the water, might it not actually be less than 1 bar?

[RobertL]

I assume the pressure is OK now but yo uwant to work out what it was when it was low.

One way: I think that the triton warning light would be a good indicator that the pressure was less than 1 bar at the Triton.

Another way: You know that the 'open tap' flow rate was 0.92 litre/ min. Measure it again now (x litre/min say). If the supply pressure now is assumed to be the usual 4 Bar you might calculate that the supplied pressure during the blocked period was 4*0.92/x bar.

BTW if the blockage was on your land then I think you are responsible for it, but I am not certain.

Robert

[Martin Bonner]

No.

Imagine that you have two taps mounted on the wall. One is connected through a very thin tube to a tank at the top of the mountain. The other tap is connected to a water-butt the other side of the wall.

If you open the two taps a tiny crack, water will spray out of the tap connected to the tank at the top of a mountain in fine spray. The tap connected to the water butt will just drip slowly. This is because the water from the mountain-top tap is at high pressure, but the water from the butt tap is at low pressure.

Now open the taps fully: It is difficult to get water to flow quickly through a narrow pipe, so not much will flow out of the mountain-top tap; on the other hand, there's no particular resistance to water flow from the butt, so it will flow quickly.

[PeterC]

Is this what I've seen referred to as 'over-pressure' for air flow? The wind is at 1 bar but exerts a force required to accelerate it around obstructions. The jet from a 'pressure washer' must be at atmospheric pressure and it's the velocity that gives it the energy to remove crud - pressure without velocity wouldn't do much!

[Onetap]

It is right, but you can't equate low flow to a certain pressure, since it depends on the pipes and fittings in your installation. The flow rate is whatever causes a frictional pressure loss that equals the available pressure difference.

They know that, but if you complain about low pressure, it's not unreasonable for them to ask what the pressure was.

The other one is the head of water. 1 bar static is equivalent to 10.2 metres head of water, so if the water isn't getting to the storage tank in most 2 storey houses, there's probably les than 1 bar and complaints will follow.