Water pressure out of kitchen tap - calculation
John
Can anyone show me how to convert static head of water pressure into
delivery of water at the tap - or even better, please do it for me
because I never was any good at maths :-(
First the numbers, then the explanation - it was taking 58 seconds to
fill a 1-litre jug, so how many metres static head is that?
Explanation - United Utilities have fixed a problem on their
"communication pipe" (whatever that is) that was causing us to have very
low water pressure. According to their 'standards of service' (as
pointed out to me by their own inspector), we may be eligible for 50
quid compensation and, given these tough economic times I thought
"great", whereas in the past I may not have bothered. Anyway, I
digress...
There wasn't enough pressure to open the solenoid valve of the shower
and because there's always a slug of cold water in the pipes that has to
come through first, it was taking seven minutes to get enough hot water
from the combi boiler in the loft to fill the bathroom wash basin. We
then had to stand in the bath and scoop the hot water out of the basin
with a jug and pour that over ourselves every morning for 10 days.
So their 'standards of service' wording is:
"If the water pressure in our communication pipe falls below 7 metres
static head twice within a four week period (each time for longer than
one hour) we will automatically pay you £50 once per annum"
From first reporting low water pressure to UU, it took 5 days for an
inspector to come out and have a look, and then another 5 days before
the guys arrived to dig the hole and fix the problem - a total of around
240 hours.
Having just rung UU to see about this compensation, they first tried to
weasel out of it by saying that we'd only had one period of low pressure
within four weeks but to qualify for compensation we need two periods of
more than one hour each - seems that one continuous period of 240 hours
doesn't qualify, or so they are trying to say!!!
Scott M
> First the numbers, then the explanation - it was taking 58 seconds to
> fill a 1-litre jug, so how many metres static head is that?
Flow and pressure are two different things. You can't tell one from the
other (tho I presume some rules of thumb might be used to take average
uses of pipework, stopcocks & taps in a domestic environ and calculate
an average flow restriction. From that you could have a stab at working
a pressure from a flow rate.)
Anyway, the most useful thing you need to know is 1 bar (14 psi)
pressure = 10m head. So their 7m head is 0.7bar which is Not Very Much.
Also, one litre / minute is Very Poor.
IIWY I wouldn't bother with numbers and just fight your corner that the
water supply was unusable for long periods.
--
Scott
Where are we going and why am I in this handbasket?
John
Thanks Scott, that's just what I'll do.
michael adams
"John" <no...@wanted.com> wrote in message
news:MPG.288e34e98...@news.btinternet.com...
What they seem to be saying there, is that if there's a problem, and
their first attempt at fixing it doesn't work, to the extent that
you can report two separate incidents of pressure loss or whatever
then and only then do you qualify for compenation.
As worded there there is no compensation payable for any delay in
their rectifying a problem.
Although that may be covered in a separate clause.
michael adams
...
John
says...
Darn newsreader keeps cutting at strange places, sorry. Anyway, thanks
for that Michael, I hadn't thought of it that way - think I'll post it
in one of the legal groups and see what they think.
Cheers.
Tim Downie
While you're at it, shoot the bozo who fitted a combi in a house with such
poor supply pressure. If ever there was a case for a storage tank system,
you've got it!
Thinking about it, it's hard to believe that any plumber would have fitted a
combi in those circumstances so have circumstances changed? Did your house
ever receive a better supply pressure? Are your neighbours affected?
If they're not, you may have a problem with you plumbing. Have you checked
that the mains water cock is fully open? Do you have a stopcock in the
street that you can check?
Another simple test you could do is to hold your thumb over the cold tap.
If the pressure builds to the point of forcing past your thumb then it's a
flow restriction you've got rather than a lack of supply pressure.
Tim
Tim
John
says...
You've got the wrong end of the stick Tim :-)
This was a recent problem (first water problem in 28 years) at our house
and the neighbours at either side of us were unaffected. Now that the
problem is fixed, it's taking about two seconds to fill that 1-litre jug
instead of 58 seconds. What had happened (apparently) is that it's a
(cast?) iron main running along the street and some sediment had
dislodged and was blocking the point where our feed pipe joins the main.
harry
There is 0.4336 psi in every foot head.
So,If you have 10ft head you have 4.336psi when there is no flow. (ie
the tap is closed)
As flow in your pipe. increases,the pressure at your tap will fall.
By how much depends on the amount of flow and the size/resistance of
your pipe.
It works in exactly the same way as Ohms law (electrcity). Only non-
linear, you have to use a chart.
You have pressure/ft head=volts.
You have resistance (pressure drop per unit lengthf or size of pipe
derived from the chart)=ohms
You have flow (gallons/liters perminute) = amps
There are tables that give pipe resistances either in ft hd, psi or
bars for each size of pipe per unit length for the maximum recommended
flow for the pipe.
The table also gives resistances for pipe bends and elbows.
Best source of tables is pipe/pipe fitting manufacturers.
It's all about finding the minimum pipe size for a given flow,
pressure and length of pipe.
Newshound
> On Jul 18, 1:18 pm, John<n...@wanted.com> wrote:
>> Can anyone show me how to convert static head of water pressure into
>> delivery of water at the tap - or even better, please do it for me
>> because I never was any good at maths :-(
>>
>> First the numbers, then the explanation - it was taking 58 seconds to
>> fill a 1-litre jug, so how many metres static head is that?
>>
>
> There is 0.4336 psi in every foot head.
> So,If you have 10ft head you have 4.336psi when there is no flow. (ie
> the tap is closed)
> As flow in your pipe. increases,the pressure at your tap will fall.
> By how much depends on the amount of flow and the size/resistance of
> your pipe.
> It works in exactly the same way as Ohms law (electrcity). Only non-
> linear, you have to use a chart.
> You have pressure/ft head=volts.
> You have resistance (pressure drop per unit lengthf or size of pipe
> derived from the chart)=ohms
> You have flow (gallons/liters perminute) = amps
>
> There are tables that give pipe resistances either in ft hd, psi or
> bars for each size of pipe per unit length for the maximum recommended
> flow for the pipe.
> The table also gives resistances for pipe bends and elbows.
>
> Best source of tables is pipe/pipe fitting manufacturers.
>
> It's all about finding the minimum pipe size for a given flow,
> pressure and length of pipe.
Up to a point, Lord Copper.
As Tim says above, this all *assumes* you don't have a serious
restriction somewhere. And from your flow rate, I bet that you have.
Think volts, amps, and ohms (if you can do electrics).