I want to put a drain pipe under a sidewalk for rain drainage. I am trying to determine the volume of a 6 inch pipe and a 4 inch. I know that one 4 inch wont be enough, but am wondering if TWO 4 inchers carry as much or more water than a 6 inch pipe. I have never been any good at mathematics. I think I have to multiply 4 or 6 by 3.14, but I am not sure. I will either use a 6 inch or two 4 inchers, which ever holds more water. Or I may even go to an 8 incher. Alot of water runs down the hill and keeps putting mud over the sidewalk. I need lots of drainage volume.
Bill you used diameter instead of radius. It is more like 27 sqin for the 6 inch pipe and 12 sqin for the 4 inch pipe not counting the small change. Also about 48 sqin for the 8 inch pipe. Way more than using 2 of the 4 inch pipes.
And what we learned is that Bill gave the correct formula and then made the simple mistake of substituting diameter for radius and then not thinking about what the range for the answer would be. No criticism of Bill is intended as we all make mistakes, but what most people don't do is then consider if the answer makes sense.
For example, a simple 6 x6 grid of inch squares has 36 square inches. Slap a 6" diameter pipe down on the grid and the pipe won't cover it, so you know the answer has to be less than 36 square inches (no where near 100 square inches). Using the same logic a 10" pipe wouldn't have 100 square inches of area. By the same token a 4" pipe can't have an area of more than 16 square inches.
Actually it is the 4" pipe is 2x2x3.14= 12.6 and 6" pipe is 3x3x3.14= 28.3
The answer is that a single 6" pipe has a larger area than two 4" pipes.
Sorry for being pedantic, but a simple visualization is often all that is needed to realize an error in a mathematical problem.
Assume similar pipe types such as tile or concrete and a slope that is the same for both the 4-inch and the 6-inch pipes.
The maximum (pipe full) flow for the 6-inch pipe will be about 15 percent greater than for the pair of 4-inch pipes.
I computed on the basis of Manning's Formula. I'll spare you the details but you can Google it if you're curious. Been a long time since I did a lot of that as a young engineer so I checked a nomogram in one of my old handbooks and got verification. Interesting exercise but I had to scrape through a lot of mental rust.
Yes the single 6" pipe has slightly more area than two 4" pipes
but................
how much water shed area (roof and / or ground) are you draining with this pipe?
I drain ~800 ft^2 of roof & another 800ft^2 of yard with a single 4" line. The line drops about 20" over ~130 ft.
This setup handles almost all the various levels of rainfall I get in SoCal. This year we had nearly 36" of total rainfall with some REALLY heavy , in the 2" per hour range. With the soil at or close to saturation, all this needed to be drained.
I did experience some flooding (temporary pooling) due to poor local soil slope but the water would generally drain off in a few minutes (
Quick calculation ---pi/4 washes out: Area of a 6" pipe-6^2=36. Area of two
4" pipes is 2*4^2=32.
36/32=1.125--so 6' pipe has about 12.5% more flow area than two 4' pipes. Just agreeing with your numbers--mental exercise for me too MLD
And, now it falls to me to give you pi in the face.
===================== Has anyone had time to calculate pi beyond the first 4 BILLION places? They did it with the aid of a computer in the eighties, but after that, I don't know.
For those morbidly curious, assuming this number is correct, that's pi to 6000 decimal places.
Note that pi to 50 places allows you to calculate the circumference of the universe (assuming you have accurate measurements) in electron diameters, with several decimal place accuracy.
The inside diameter of a pipe should not exceed the outside diameter, otherwise the hole will be on the outside of the pipe. Seriously though, another way of figuring the area of a circle is that it will be about
78.5% of that of a square the same size as the circle's diameter,regardless of whether it is an inch or a mile. Larry
Well, since you went to this extreme, and I really do feel sorry for your typing fingers, I have to provide this mathematical fact. I once saw this nonsense on a website, but do not recall where it was. Anyhow, someone calculated the average human penis is six inches long. Divide the world population of humans in half and that's roughly the amount of males on earth. OK, now if every male cut off his penis and laid them end to end to form a chain, they would encircle the earth
12.8xxxxxxxx times. The number was longer, but I only remember the
12.8 rounded figure. Either way, I think any woman would find this fascinating, and if it really happened, some blonde woman would walk (or swim) around the globe 12.8 times just to see each and every one of them.
pipes is 2*4^22. 36/32=1.125--so 6' pipe has about 12.5% more flow area than two
4' pipes. Just agreeing with your numbers--mental exercise for me too MLD
Most of the answers address the problem presented. But the question asked was about the "volume"of the four and six inch diameter pipes. Roughly, with a cross section of 12 square inches, a foot of four inch diameter pipe has 144 cubic inches, or 1/12 cubic foot.The six pipe has a rough cross section of about 27 square inches, or about 1/5 cubic foot per foot of pipe. Or simply, the six inch pipe for the same length has a volume of a bit more than twice the volume of the four inch pipe. Last time I looked, four inch pipe is a lot less than half the price of six inch pipe. So if cost or distributing the flow are factors, the smaller pipe(s) may be the better answer.
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