Old plumber wisdom, is he right or not?

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An old plumber who installed 20-30 meters of 1/2'' piping said this size would give a HIGHER pressure compared to the 3/4'' option. Why might he think so?

This is general for all houses in his opinion, please forget my electric heater in this thread.
 
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That depends on where the pipes are and where you measure pressure. A smaller diameter will lead to a higher pressure drop in the pipe section for the same flow.
 
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So why might he say the opposite?
 
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He didn't say the opposite. Both are technically consistent. If I may speak on mfb's behalf I think he was merely noting that pressure and volume in a closed system are usually inversely related. You can experience this with a simple garden hose and an adjustable nozzle.
 
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I propose that while flow is a little less with the thin piping, speed is higher so for the same exit tap thinner piping increases speed at the exit and this the plumber PERCEIVES as a higher pressure.
 
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That depends on where the pipes are and where you measure pressure.
Can't think of an example where the pipes are located in such a common path that the thinner one produces a higher pressure. What were you thinking?
 
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You didn't tell us where the pressure would be measured and where the pipe is, so I kept my answer quite general.

Exit speed and pressure are different things, and typically the outlet won't be directly the end of a longer pipe.
 
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This is a bit of a shock to me. You are engineers, you are not supposed to take advise from an old plumber, yet no one will state it that the old plumber is wrong!

You are comparing two pipes that are exposed to the same input pressure, you only care about the exit pressure.
 

russ_watters

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This is a bit of a shock to me. You are engineers, you are not supposed to take advise from an old plumber, yet no one will state it that the old plumber is wrong!

You are comparing two pipes that are exposed to the same input pressure, you only care about the exit pressure.
If what you care about is the exit pressure, there is no scenario I can think of where a smaller pipe will yield a higher pressure when water is flowing. As said, the pressure drop is higher if the velocity is higher.
 
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For another question, not the present one, I have an idea for measuring pressure without a manometer and would appreciate some feedback on it. A long hose going upwards to the height where flow stops (probably 30 or 40 meters), would effectively measure static pressure at the tap. Given this result, can dynamic pressure be calculated from flow measurements? Ie you measure flow without a hose, with the hose going to the same floor, with the hose going to the next floor up, with the hose going 2 floors up and so on. Will this work if the hose cannot go high enough to zero the flow?
 
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Maybe this is plausible. Thin pipe, higher exit pressure. Maybe when the input pressure does not have a flat enough characteristic.

Image4.jpg
 

Baluncore

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An old plumber who installed 20-30 meters of 1/2'' piping said this size would give a HIGHER pressure compared to the 3/4'' option. Why might he think so?
For pipes of the same material and wall thickness, thin pipes will handle much greater pressures than larger diameter pipes.
How do we know you understood the old plumber and have not misquoted him ?
Did he say the pipe would "give" or could "handle" ?
 
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Definitely give. Pressures high enough not be handled would be way off the chart.
 
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Could simulate the partially open tap as a super thin pipe. Where can we find some data sheets for copper pipes? Ie pressure-flow graph?
 

jrmichler

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Cameron Hydraulic Data has a whole chapter full of tables of pressure loss vs flow rate for copper tube down to 3/8" diameter up to steel pipe 192" diameter. Amazon has it: https://www.amazon.com/s?k=cameron+hydraulic+data&ref=nb_sb_noss_1&tag=pfamazon01-20

They have an excellent discussion of how to use the Moody diagram if you want to calculate a case that's not in the tables.

You do not need the latest edition. My 16th Edition has all the information you need, and so do earlier editions.

I propose that while flow is a little less with the thin piping, speed is higher so for the same exit tap thinner piping increases speed at the exit and this the plumber PERCEIVES as a higher pressure.
True, IF there is no restriction (valve, faucet, or nozzle) on the end of the pipe. In a smaller pipe, the flow velocity will be higher even though the flow rate will be lower. This may easily be perceived as higher pressure.
 

DaveC426913

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Is it possible that the plumber was talking in terms "practical" pressure? i.e. from a house-owner's point of view?

If I turned on my tap and water shot into the sink and soaked my shirt, I would say that the "water pressure" is pretty spectacular - even if it isn't technically accurate. Nobody judges household water pressure by how quickly it can fill the sink.

That's essentially what those "water saver" widgets do in shower heads. They reduce the water volume while increasing the exit speed - making it seem like you've got plenty of pressure for a shower - even though you're using less water.
 
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Does anyone have a copy of that book to tell us the parameters for 1/2'' and 3/4'' copper pipe? Should be like this:

output pressure = input pressure - A * flow rate * length

Or does anyone know what to search for in google to get the A value?

1/2'' pipe:

Po(1/2'') = Pi - A * f(1/2'') * L

3/4'' pipe:

Po(3/4'') = Pi - B * f(3/4'') * L

partially closed tap at the end of 1/2'' pipe:

0 = Po(1/2'') - C * f(1/2'') * l

partially closed tap at the end of 3/4'' pipe:

0 = Po(3/4'') - C * f(3/4'') * l

4 unknowns: Po(1/2''), Po(3/4''), f(1/2''), f(3/4'').

For the plumber to be right, Po(1/2'') > Po(3/4'').
 
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=> f(3/4'') / f(1/2'') > A / B

Also

0 = Pi - A * f(1/2'') * L - C * f(1/2'') * l

( A * L + C * l ) * f(1/2'') = Pi

( B * L + C * l ) * f(3/4'') = Pi

f(3/4'') / f(1/2'') = ( A * L + C * l ) / ( B * L + C * l )

hence for the plumber to be right

( A * L + C * l ) / ( B * L + C * l ) > A / B

A * B * L + B * C * l > A * B * L + A * C * l

B * C * l > A * C * l

B > A

So the plumber is always wrong. UNLESS, Pi is not the same for both pipes.
 
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haruspex

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He didn't say the opposite. Both are technically consistent. If I may speak on mfb's behalf I think he was merely noting that pressure and volume in a closed system are usually inversely related. You can experience this with a simple garden hose and an adjustable nozzle.
That's different - the same pipe but with a constrained output. In this case the lower flow means less pressure drop along the pipe.
 
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Jonathan 212, I have run into the same scenario before. I believe I can also show were the old timers get their perception.
Often old time plumbers did not run calcs. They operate from a series of "Rule of Thumb" basics. One of the things that used to be used (and still is in some circles) is a Residual Pressure. This is a simple test where a small hole is drilled at the distal end of the run. The system is pressurized and the height of the column is checked. This give a rudimentary indication of internal pressure. It is usually done with a low pressure for test purposes only.
Most of the time the "specification" for the test hole is 1/8". Some of the old timers have predrilled threaded caps in the box which are of different sizes for this test.
Interesting observation though. It is (if not combined with calculations to correct for other factors) more correctly a measurement of pressure drop across the orifice. This is also affected radically by pipe size. In Example 5 psi into the system should generate a squirt height of 11.55 ft (5 ft x 2.31 ft/psi) if you put a 1/8 orifice on the end of a 1/2 pipe this is actually what you will observe. If however you use a 1/4 inch orifice the squirt height will be observably reduced. We can reject the possibility that it is an erroneous or defective orifice. These old timers were mostly very careful to have clean, perfectly drilled, deburred, square shouldered test fittings. Check them if you want but they are probably good. But the low flow of an 1/8th orifice usually means clean laminar flow inside the pipe and your source can keep up easily. A 1/4 orifice can move enough water to change the flow pattern and in some pipe paths even start to induce turbulence it may even move enough water to overwhelm the source therefore a reduced squirt is common.
This same observation can be made with a short piece of small diameter tubing replacing the orifice. A short section of straight reduced diameter closely resembles an orifice.
So now lets think of the old timers. When tested smaller diameter gives greater squirt (larger measured height) therefore higher pressure( nobody is actually measuring) and here is the kicker. Old fashioned plumbing was usually pipe all the way to the water stop which was as close as reasonable to the fixture. Then a short piece of service tubing goes to the fixture ( clean wall, rigid tubing, artistically bent to proper alignment). Ta Daah, this is where we used to work and we "measured" it so it has to be right.
It only takes a little bit for the real old timers to "educate" a newer journeyman and in just a few years "This is right cause we always do it like this."

I think you will find that your old timer is either one of the originals or they are one of the field trained ones that have no formal schooling in fluid dynamics. It can be a real challenge dealing with either one. Especially when you introduce Old school syndrome. It is more of an academic exercise anyway. If it is only a toilet at the end of a 20' run of 1/2 inch how much water does it need? But if it is a bathroom with a toilet, and a sink, and a shower you will very much see an OMG in the shower when someone flushes the toilet. Just like we always have--- Just like could be corrected with new thought.
 
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Old plumber is wrong. Simple as that.
The mystery, which we can speculate for days about, is "What would make him think he's right?"
 
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you measure flow without a hose, with the hose going to the same floor, with the hose going to the next floor up, with the hose going 2 floors up and so on.
Problem is that the flow is not going to be constant: high at first, then slowing down as the hose fills and the pressure differential drops. Maybe you could do some fancy math on the flow-vs-time curve, but you could probably go the plumbing supply shop and buy a pressure gauge in less time. :smile:
 
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Of course we would wait for any transients to settle down. The hose is already there but it would be silly if it's hopeless, resulting in 3-4 points on an output pressure-flow plot that are relative to each other and not absolute. But with an assumption for what the absolute curve is, might get somewhere.
 
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Is it possible the zero pressure at the end of an open pipe is wrong?
 
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I'm a science grad and also a qualified plumber (not all that rare!). I've also taught plumbers.
I'm afraid old plumbers are typically not well educated academically, even though their on-the-job learning is not something you'd ever find in a book. There's a reluctance to learn things which appear to go against years of experience, even though the experience leads to some woolly and imprecise assertions.
He knows that the same flow in a smaller pipe will mean a higher speed.
He knows that a smaller orifice (jet size) on the end of the pipe will give a higher pressure on your skin if you direct the flow at it. That's obvious, the skin dents further - if only over a very small area!
What's missing is the distinction between the static pressure in the pipe (where he was wrong of course) and what might be called dynamic pressure, due to water hitting his skin so much faster. The energy in that narrow jet of water, of course goes with v². It doesn't pay to try to be too "scientific" with calculations though - tables of measured pressure drops in pipes of given sizes and given flow rates, usually don't agree with each other very well. It's in practice, impossible to account for every possible variable, and not worth trying beyond the major ones.
There are many reliable "rules of thumb" in plumbing - for which do consult a wise, educated, experienced plumber!
 

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