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Author Topic: Real life math problem
golfgimp
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Icon 1 posted June 19, 2007 09:33      Profile for golfgimp   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
I have 18 water drums, 55 gallons each, interconnected. There is a water line running down the side of my house from these tanks to my garden 150' distance, the drop is 30'. What is the water pressure at the bottom using a 3/4" pipe? Could I increase the pressure using bigger width to smaller width pipe down the hillside like the old hydraulic mining technique?
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spungo
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Icon 1 posted June 19, 2007 10:12      Profile for spungo     Send New Private Message       Edit/Delete Post   Reply With Quote 
Hydrostatic pressure in a liquid at any particular point depends only on the density of the liquid, and the height of the liquid directly above that point. Changing the shape of the vessel has no effect, per se. I'm not familiar with this mining technique you're talking about... sounds weird.

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maximile

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Icon 1 posted June 19, 2007 11:33      Profile for maximile   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
Reminds me of the Venturi Effect. But that decreases pressure as it increases the flow rate.
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Ashitaka

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Icon 1 posted June 19, 2007 11:35      Profile for Ashitaka     Send New Private Message       Edit/Delete Post   Reply With Quote 
you need a copy of pery's chemical engineering handbook
The water preesure is going to be dependant on the 30' height, density of the water and type and thickness of the pipe. Look up bernoulis equation.

Well here it is
 -

v = fluid velocity along the streamline
g = acceleration due to gravity
h = height of the fluid
p = pressure along the streamline
ρ = density of the fluid

for our purposes the water is incompressable so this applies.

For the constant you can try getting an estimate of what it should be in Perry's chemical enigneering handbook. (you look up your type of pipe, how long it is ect. and it will give you an estimate of the constant.)((try a local college library))

I'd try to give you an estimate of the answer, but I don't do american units.

It might be easier just to measure the flow rate if that is all you want to know.

Equations like this are better for the planning stage so if you know what flow rate you want at the garden, you build the system to give it to you.Basically, if you want more pressure, increase h. (that is why water towers are so high up.)

If I wanted to increase the pressure, I would seal the water storage tanks and pressurize the air in the tanks.

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spungo
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Icon 1 posted June 19, 2007 11:57      Profile for spungo     Send New Private Message       Edit/Delete Post   Reply With Quote 
Ashitaka: you say that the pressure will depend on the type and thickness of the pipe, and then produce a formula that relies on no such dependence.

Bernouilli's equation isn't all that relevant as the only variations in pressure are hydrostatic. (You would use to explain how 'lift' was generated by an aerofoil, for example.)

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Ashitaka

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Icon 1 posted June 19, 2007 12:06      Profile for Ashitaka     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by spungo:
Ashitaka: you say that the pressure will depend on the type and thickness of the pipe, and then produce a formula that relies on no such dependence.

Bernouilli's equation isn't all that relevant as the only variations in pressure are hydrostatic. (You would use to explain how 'lift' was generated by an aerofoil, for example.)

Type and thickness of the pipe is in the constant.

Bernoulli's equation has many uses, and I believe that it could be used to solve this problem. Other simpler equations invoiving the pressure of liquids could also be used, I just cannot remember what they are at the moment.

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spungo
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Icon 1 posted June 19, 2007 12:14      Profile for spungo     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by Ashitaka:
quote:
Originally posted by spungo:
Ashitaka: you say that the pressure will depend on the type and thickness of the pipe, and then produce a formula that relies on no such dependence.

Bernouilli's equation isn't all that relevant as the only variations in pressure are hydrostatic. (You would use to explain how 'lift' was generated by an aerofoil, for example.)

Type and thickness of the pipe is in the constant.


Bullsh&t. Bernouilli's equation is derived from the conservation of energy -- ain't no mutha-fsckin' pipe details in it.

As for equations involving the "pressure of liquids" -- I don't know what the hell yer talking about -- hydrostatic pressure is given as

p = rho * g * h

p == pressure differential
rho == density
g == accel of free fall
h == height

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Sxeptomaniac

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Icon 1 posted June 19, 2007 12:23      Profile for Sxeptomaniac   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by spungo:
quote:
Originally posted by Ashitaka:
quote:
Originally posted by spungo:
Ashitaka: you say that the pressure will depend on the type and thickness of the pipe, and then produce a formula that relies on no such dependence.

Bernouilli's equation isn't all that relevant as the only variations in pressure are hydrostatic. (You would use to explain how 'lift' was generated by an aerofoil, for example.)

Type and thickness of the pipe is in the constant.


Bullsh&t. Bernouilli's equation is derived from the conservation of energy -- ain't no mutha-fsckin' pipe details in it.

As for equations involving the "pressure of liquids" -- I don't know what the hell yer talking about -- hydrostatic pressure is given as

p = rho * g * h

p == pressure differential
rho == density
g == accel of free fall
h == height

Yes, but we're not talking about hydrostatic pressures here. In order to work an irrigation system, you need to look at hydrokinetics. The system loses pressure as it operates. There are a lot of factors involved in how much pressure is lost, but diameter of pipes is part of that.

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GrumpySteen

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Icon 1 posted June 19, 2007 12:30      Profile for GrumpySteen     Send New Private Message       Edit/Delete Post   Reply With Quote 
Not to interrupt your discussion, but I'm pretty sure the mining technique golfgimp referred to is one that used a convergent nozzle to trade off a loss in pressure for an increase in kinetic energy (velocity) which allowed hydraulic miners to blast away at tightly packed gravel/dirt/crap that they couldn't move otherwise.

You may return to your regularly unscheduled mathematical tangent.

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spungo
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Icon 1 posted June 19, 2007 12:31      Profile for spungo     Send New Private Message       Edit/Delete Post   Reply With Quote 
That was not a very scientific link you submitted, Sxepto. I would like to know how you think the diameter of a pipe can change the pressure of water running down in, bearing in mind that liquids are incompressible. Are you sure there isn't some confusion here between pressure and volume flux?

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Ashitaka

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Icon 1 posted June 19, 2007 12:38      Profile for Ashitaka     Send New Private Message       Edit/Delete Post   Reply With Quote 
Bernoulis equation applies to flow in pipes, We have flow in pipes. The question is how much does the friction of the water against the pipes slow the water.

ps

liquids like water are mostly incompressible, but they are compressible.

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spungo
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Icon 1 posted June 19, 2007 12:41      Profile for spungo     Send New Private Message       Edit/Delete Post   Reply With Quote 
Friction which is small, and is not taken into account in Bernoulli's equation, and probably not worth worrying about.

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WinterSolstice

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Icon 1 posted June 19, 2007 12:54      Profile for WinterSolstice     Send New Private Message       Edit/Delete Post   Reply With Quote 
Well, now that all the geeks have weighed in, why don't you just simply do it, then tell them the pressure?

I swear, people put too much thought into this stuff.

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CommanderShroom
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Icon 1 posted June 19, 2007 12:59      Profile for CommanderShroom     Send New Private Message       Edit/Delete Post   Reply With Quote 
I am absolutely out of my league as far as being able to tell you what the pressure will be at the exit of your line.

But yes, you will see a change in pressure if you step the pipe down in size. Of course if you go too small you will lose flow. You may be better off experimenting some to find out what will work best for your application.

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Sxeptomaniac

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Icon 1 posted June 19, 2007 13:00      Profile for Sxeptomaniac   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by spungo:
That was not a very scientific link you submitted, Sxepto. I would like to know how you think the diameter of a pipe can change the pressure of water running down in, bearing in mind that liquids are incompressible. Are you sure there isn't some confusion here between pressure and volume flux?

It's practical application of solid science, and the author demonstrates a good knowledge of the principles involved. That person knows irrigation systems inside and out, which is clear based on how well they can explain them.

The diameter of pipe doesn't matter when dealing with a static fluid, but it does affect the psi of a running system, pure and simple. See this page for a look at how pipe size affects a sprinkler system. Note that the author explains Bernoulli's principle at the bottom.

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spungo
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Icon 1 posted June 19, 2007 13:43      Profile for spungo     Send New Private Message       Edit/Delete Post   Reply With Quote 
As I said, there will be no change in pressure (due to flow) in a uniform pipe (which is what we are talking about) - any small changes in the pipe diameter will produce totally insignificant changes in pressure when compared with the increase of pressure with "height-of-liquid-above" hydrostatic pressure.

If you add a smaller-diameter pipe near the bottom, you will have an increased flow rate, but a decreased pressure.

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Metasquares
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Icon 1 posted June 19, 2007 14:21      Profile for Metasquares   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
Discussions like these made me wish I had a better grade-school physics teacher. I must be the only mathematician in the world who can't stand physics.

Anyway, figure out which equation you'll use and I'll be glad to try and solve it [Smile]

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CommanderShroom
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Icon 1 posted June 19, 2007 15:04      Profile for CommanderShroom     Send New Private Message       Edit/Delete Post   Reply With Quote 
Metasquares that made me wish I had just finished school.

Though I did get an eyeful of information when I started looking up spungo's reasoning on his last post. Most of it just went right over my head. But it took me on a nice journey through hydrogeology. Too bad I won't remember a word of it in the next 10 minutes.

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Sxeptomaniac

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Icon 1 posted June 19, 2007 15:07      Profile for Sxeptomaniac   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
Back to the original question, I am curious as to exactly what kind of system he plans to use to deliver the water to the plants. A drip line seems likely. The static pressure of his system calculates out to only 13 PSI, which isn't much. Reducing the line diameter at any point would only further drop the operating pressure. If he wants any more pressure from the system, a pump will be necessary.

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littlefish
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Icon 1 posted June 19, 2007 15:36      Profile for littlefish   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
Oh, you guys!

I could be wrong, but I'm moderately certain that spungo has a PhD in physics. More to the point, he is completely correct.

Pressure is unaffected by pipe diameter. You can get different forces by changing the diameter of pipes, which is totally how hydraulics work. But don't disrespect a bloke who knows what he's talking about please!

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Metasquares
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Icon 1 posted June 19, 2007 16:36      Profile for Metasquares   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
You misunderstand. I hate physics because I had a bad grade-school physics teacher (the only person I know that screams at people for getting questions correct!)

I'm not trying to be demeaning or anything; I know this isn't grade-school physics.

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Sxeptomaniac

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Icon 1 posted June 19, 2007 16:56      Profile for Sxeptomaniac   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by littlefish:
Oh, you guys!

I could be wrong, but I'm moderately certain that spungo has a PhD in physics. More to the point, he is completely correct.

Pressure is unaffected by pipe diameter. You can get different forces by changing the diameter of pipes, which is totally how hydraulics work. But don't disrespect a bloke who knows what he's talking about please!

I suspect that spungo and I were talking past each other to a certain extent. That said, my experience is that academics don't always know what they're talking about when it comes to real-world application of principles.

Spungo summed things up nicely here, though:
quote:
Originally posted by spungo:
If you add a smaller-diameter pipe near the bottom, you will have an increased flow rate, but a decreased pressure.



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Just_Jess_B

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Icon 1 posted June 19, 2007 17:27      Profile for Just_Jess_B   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by golfgimp:
I have 18 water drums, 55 gallons each, interconnected. There is a water line running down the side of my house from these tanks to my garden 150' distance, the drop is 30'. What is the water pressure at the bottom using a 3/4" pipe? Could I increase the pressure using bigger width to smaller width pipe down the hillside like the old hydraulic mining technique?

I know this sounds crazy . . . but why on earth would you even have 18 drums in series? Do you collect rainwater with them and then use them to water your garden? Or are you planning to mine your hillside?

spungo -- Hydraulic mining is the process of using a directed stream of water to remove the side of hills and mountains to get to gold in the hillsides.(pictured here). The erosion damage caused by the initial destruction has had lots of environmental effects in the foothills above Sacramento and in the San Joaquin Valley, itself.
Also, I am impressed. After seeing the smuttier side of spungo for years, seeing this side of you is extraordinary, though not really surprising considering the manner of your humour.

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golfgimp
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Icon 1 posted June 19, 2007 23:08      Profile for golfgimp   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
Well, yes I am collecting rain water for the garden. I am going to trench the pipe in and if I can't get much water pressure I'll go drip. I was thinking a sprinkler would be great but I probably need 35psi..? I don't want to go to the extra expense and work of creating a varied dimension pipe system if it doesn't work
Apparently the strip mining hydraulic system can create enormous water pressure but I don't know if it will work on a short run like that.
Maybe I should contact the "Knowledge Network" and see if they'll fund the experiment [Smile]

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Callipygous
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Icon 1 posted June 20, 2007 03:54      Profile for Callipygous     Send New Private Message       Edit/Delete Post   Reply With Quote 
Blinky blonky blimey O'Reilly!

You don't have to have a phud in physics to know Spungo is on the money. I have an "O" Level in physics, and it's more than 40 years old, but I have the clearest of recollection that pressure has no relation to pipe size whatsoever. But plain common sense should tell you this too. If pipe diameter had any bearing on pressure, one could easily set up a perpetual motion machine by taking two pipes one thick, and one thin to the bottom of the garden and connect them then the higher pressure in one hose would drive the water around and around, and yabber dabber doo! free energy that could be tapped using a water mill.

I am one of the people least qualified to call himself a geek on these boards but crikey mates! it is hard to believe that some of the posts in this thread have come from people with even a passing acquaintance with basic science.

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