# plumbing ?

Discussion in 'Off-Topic' started by Mathematics!, Feb 23, 2010.

1. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
I know in electrical that the wire gage determines the amount of voltage drop per square ft (all materials being equal)

I know that house in america run on 120vac or 240vac power company supply voltages

What I want to know is in plumbing is their a standard pressure/force that the water companies supply to a house? (Their must be an equivalent to the power companies but for water)

Also I am assuming the size of the plumbing pipes (copper pipes have a reason to be a certain diameter ...etc?
Is this for how much Pressure drops pure cubic foot of copper pipe...?

Thanks for anybody that can shed some light on this.
Is their a reason why you should use a 3/8in or 1/2in copper pipe or can you usually use them interchangable?

I am assuming their has to be a similar reason to why you use different pipe sizes similar to the smaller gage (AWG) to do longer runs in electrical house circuits...

Is their an equivalent to ohms law in plumbing? if so how much pressure do you need to have go to a sink or shower???

Last edited: Feb 23, 2010
2. ### gerty AAC Fanatic!

Aug 30, 2007
1,153
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It's not so much pressure as it is volume. High volume 1/2 - 3/4 pipe is what's normally found in a house. The filler for a toilet is usually 3/8" because fill time isn't an issue. Water pressure is usually around 60 psi for a residence.
The line feeding the house from the meter is usually 3/4".

3. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Is this what the standard that the water supply company tries to give all their customers +- a few psi...?
I would think at some low pressure point pressure would start to be a concern....

Yes I know increasing the volume would make it so that more water will flow thru the pipe faster water current (all other things taken to be the same)

But say you are going straight up 100ft with a water pipe. I don't care how big you make the copper pipe. Their is still going to be a min value for the pressure to be used to raise the water straight up with a certain current...?

This is why I am asking if their is an ohms law equivalent for water pipes.
we have water current , pressure , volume , distance, and vicousity (water resistance ) to name a few measurement we could use to cook up a math formula...

And for the 1/2" , 3/4" , 3/8" pipes is their a place where you can look up pipe resistance (vicousity) per ft or volume just like the equivalent AWG resistance ohm per ft ...etc

Last edited: Feb 23, 2010
4. ### Wendy Moderator

Mar 24, 2008
20,766
2,536
I can't answer the question directly, but I do know it is so variable that it might as well not be regulated.

My cities water pressure is so high that most hoses will burst if they are capped off with a sprayer. I have to use industrial strength for my lawn. It has both good and bad points.

5. ### gerty AAC Fanatic!

Aug 30, 2007
1,153
304
Pressure is a component of where you live relative to the source and the customers ahead of you that are using water from the same line. A house next to a water tower would have a higher pressure than one at the end of the same line. 60 psi is what I've seen in my area, don't know if there is a
'standard' or not. Your utility company should be able to answer that. that being said 2 homes on the same line, one at the beginnning and the other at the end will have two differnt pressures, possibly stabilizing in the middle of the night with no one else using it.

If you are going straight up 100' then you have to overcome that gravity
stuff. If you exceed the height of the water tower, and they're usually at a high elevation,then you will have very little pressure unless your utility has pumps in place to compensate.
As far as an 'Ohms Law' for water, a given diameter pipe will flow at a given
gallons per minute,at a given pressure, and I have no idea what that might be.

Apr 5, 2008
15,648
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Hello,

If water pipes go up in holland more than 13 meters they need to be presurized with a "hydrofoor".
The perssure must be obtained at the higher floors of a building too.

Greetings,
Bertus

7. ### Wendy Moderator

Mar 24, 2008
20,766
2,536
That reminds of a story about New York, most of the multistory buildings have a local water tower (mainly for fire suppression).

8. ### Paulo540 Member

Nov 23, 2009
188
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10 meters of water is one atmosphere (approx.)

So, converting that to Imperial, you get ~14.7 psi per 34 feet

100 feet / 34 x 14.7 = 43.2 psi. This is the pressure at the bottom of the column of water, regardless of diameter. So, you'd probably want a pump that could do a bit more than that and the gpm would dictate how 'fast' it pumped.

9. ### retched AAC Fanatic!

Dec 5, 2009
5,201
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Finding resistance in a pipe:
hloss_serial = Σ [(λ1 (l1 / dh1) + Σ ξ1) (v12/ 2 g) + .. + λn (ln / dhn) + Σ ξn) (vn2/ 2 g)]

Here you go. Good luck. I had to deal with this crap for two years.
http://www.engineeringtoolbox.com/total-pressure-loss-ducts-pipes-d_625.html

p.s. most pumps are rated in head pressure, this is in feet, or meters.. You can convert this to psi, but most equations I had to do were in feet as head pressure

Jul 7, 2009
1,585
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There is no uniform standard for pressure. Typical city supply pressures are in the range of 50-100 psi, but you can see spikes higher than that. I have my house on a well and my lawn water comes from an irrigation pump; both supply water at about 40 psi. With high city pressures (or fluctuating pressures), it can be wise to put a pressure-reducing regulator in your supply.

Pipe sizes are governed by the application and whatever guidelines or codes that apply. The typical sizes I've seen in the houses I've lived in are 1" pipe for water coming in from the water meter or well (sometimes 1.25"), 3/4" for distribution, and 1/2" to go to fixtures. Steel pipe is common in older houses; sweated copper is used in nicer/more expensive installations, and some codes (like mine) will allow certain types of plastic pipe to be used.

The screwball system of pipe sizes in America came from pipes that were originally close in inside diameter to their nominal sizes (e.g., 1/2" pipe had an ID of 1/2"). As pipe materials got better and stronger, wall thicknesses were reduced to give the same strength -- it would have been more problematic to change the outside diameters, as that would mean significant tooling changes. Thus, today's schedule 40 steel 1/2" pipe has an inside diameter of 0.62" and an outside diameter of 0.84" -- you'll not find a 1/2" dimension anywhere near the pipe.

The required pipe size is determined from the needed flow and available pressure. The principle is pipes with larger inside diameters have smaller frictional losses. Calculating flows is in general not trivial because of the need to calculate the friction losses; for incompressible flow, look up stuff like the energy equation (a generalization of Bernoulli's equation) and the Darcy-Weisbach formula for the friction factor (CE's will use something similar called a Fanning friction factor). If you're just interested in water flow, engineers often use the Hazen-Williams equation instead because it's mathematically simpler to apply. One also needs to account for friction losses in pipe fittings too. A good reference is Perry & Chilton's "Chemical Engineering Handbook" published by McGraw-Hill.

This anal retentive stuff is ignored by practical plumbers and they just follow the guidelines/codes they were trained to follow.

11. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
OK,
But is their a max pressure rating that a certain size copper pipe can take before bursting?

For example
if I have a 3/4" copper pipe how much pressure can I apply before the pipe breaks?

Is their a simple equation for this at least?

12. ### gerty AAC Fanatic!

Aug 30, 2007
1,153
304
That depends on several things like is it hard or soft copper, what is the wall thickness.
I used to work for an air conditioner manufacturer, the copper used in a ac unit has a thinner wall than that used in a house. We had to burst test them at 600 psi (air) before charging the unit with freon. Copper has a higher bursting strength than cpvc pipe.

Sep 30, 2009
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14. ### Paulo540 Member

Nov 23, 2009
188
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I actually made a spreadsheet to calculate this a long time ago. It also has a temperature derating function. As a previous poster said, it will depend somewhat on the temper of the copper so you always want to add in a safety factor of some amount. I'd use 2-3.

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15. ### loosewire AAC Fanatic!

Apr 25, 2008
1,584
435
Can you walk the dog,swing on the porch and yo=yo while
you remodel your house,and solder copper pipe. I try to
guess your next project,low voltage out door lighting or?

16. ### Duane P Wetick Active Member

Apr 23, 2009
408
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Here's an interesting figure to remember; 1 foot of water weighs 0.433 lbs./ sq. in. So if you have a water tank 110 ft up in the air, the water pressure at ground level will be 0.433 x 110 or 48 lbs/ sq. in. Conversely, if you go down in a mine shaft 900 ft, the weight of a water stream coming down to meet you will be 0.433 x 900 or 390 lbs/ sq. in.

Cheers, DPW [ Everything has limitations...and I hate limitations.]

17. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Almost but I have done all the electrical stuff , roofing , painting.

I have plumbing , floor/tileing , installing a door

Thats about it only the plumbing seems a little trick right at the moment.
Because I am still trying to find a few pipes...etc

18. ### retched AAC Fanatic!

Dec 5, 2009
5,201
312
What is the 'total' project? Just fixin' up the ol' homestead? Or preparing for sale?

19. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
recently purchessed a home that needs alot of fixing

20. ### jpanhalt AAC Fanatic!

Jan 18, 2008
5,694
904
How about some pictures? I feel we almost live there too. John