Mythbusters, alt.home.repair edition: Pipe Slope

Just because I'm sick of seeing the liquids-outrunning-solids-on-steep- slopes myth on the net....
http://www.pmmag.com/CDA/Archives/830d83d4ebfc7010VgnVCM100000f932a8c0____
"A few years later I took a college course that we nicknamed Sewer 101. You got it; the course was on the design of sewers and sewage treatment facilities. The professor was explaining the installation of sewers in steep terrain. He was suggesting that we change piping materials for sewers that were pitched more than 30 degrees.
I raised my hand wondering about the problems that would be encountered. My mind was abuzz with whether my father was right, or the plumbing inspector was right. I asked, "If you pitch the pipe too much, won't the liquids run away from the solids, resulting in an increased amount of stoppages?"
The professor started laughing as I tried to disappear under my desk. He said, "You're a plumber, right?" He already knew I was because we had talked about it many times. "Yeah," is all I could say.
"I don't know why they always teach plumbers that myth. Maybe it is because they want them to install perfectly aligned sewers."
Of course, I found out that the professor was setting me up. This lead to the presentation of a new equation. The equation calculated the minimum flow rate to keep solids in suspension in a sewer system. The inverse of the equation calculated the maximum pitch before the liquids ran away from the solids. Of course, the answer to the maximum pitch was infinity. In other words, you could never reach that point.
Then the professor asked, "What is the maximum pitch we could have for a project?" That one's easy -- a vertical stack! "Yeah, and in a stack, the solids land at the bottom first." Excuse me?!
"You got it, the solids land first and the liquids come along and pick them up and carry them down the drain."
Most of the students just took this all in. Me, my eyes were bulging. This was mind-blowing stuff. All I could do was think back to that sewer with the 350 feet of distance and 150 feet of vertical drop. The professor just told me (indirectly, of course) that we didn't have to put in one of those vertical drops.
If I knew then what I know now, that sewer would have been 2 feet below grade pitch straight down that hill. The sewage would have been humming when it hit the public sewer connection.
I have long since forgotten that equation. But I learned an important lesson. More than 25 years later, some plumbing instructors are still teaching that if you pitch the pipe too much, the liquid will run away from the solids. But all of you know better.
As for that 3-inch pipe, you can pitch it 1/8 inch per foot without a problem. Of course, at 1/4 inch per foot the flow will run faster."
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On 10/29/2010 5:53 PM, mike wrote:

When I worked at a missile range on a small island I think I remember the plumbers talking about the sewage system being a negative pressure type because there was no way to get any slope on the drains. I do recall that the toilets would geld you if you flushed while sitting.
TDD
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I wonder if the drag experienced by solids verses water has something to do with it.
I have a situation where I've not been able to get optimum slope. I have a 3" darin between the garage and house that goes about 18' between the joists of a deck that connects the two structures.
http://www.jamesgangnc.com/housefront.jpg I had no other way to get the 2nd floor of the garage connected to the septic because the grade around the garage is too low. Tank is in front of the house. The run goes out under a stair landing on the garage side and through the house wall on top of the foundation. I even cut the pt board on top of the house foundation where the pipe is to maximize the slope but water poured in the garage end just barely goes through. The start of the run begins with a 7' straight down drop. So I'm hoping that the additional force from the drop will help propel it along. All the drains in the garage are at the top of the 7' drop so I'm thinking that even if some solids get stopped in the middle the next drain event will pick them up and move along.
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The momentum from that 7' drop basically disappears when it hits the horizontal run. If the pipe were always full it would be a different story.
Where are you that you don't have to worry about running a drainage line outside? It doesn't freeze up in winter? How come you didn't poke a hole through the foundation so there'd be more drop?
R
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Even though I used a one of those T's with a long sweep? You think none of the down velocity is going to remain after it comes out of the long sweep?
North Carolina. I ran it inside a piece of 5" r8 flex duct. I figure the septic tank is always venting warm gas. The tank is about 8' from the house. The dw for the house is in a crawl that never gets much below 50. On the garage side it goes in right above an electric hw tank. It's marginal but we seldom have lows that remain below freezing during the day so it's not going to take much to keep it from freezing. It's a block foundation plus I wanted to keep the pipe in between the deck joists to further reduce it's exposure to freezing.
The only other solution would have been to pump up to the septic tank and the garage just as a 3/4 bath that will be used occasionally. The grade at the garage is lower that the top of the tank. The main house has 3 full baths.
I did put a clean out directly in line with it on the garage end.
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Most of the momentum will be lost in turbulence, and the rest will be lost in the low slope situation. I'd try to block it intentionally and see if there will be a problem down the road.

Hind sight and all that, I would have poked a hole in the block wall. Easy enough to do and there'd be no question about the slope. If it ever becomes a problem you could still do it. You'd only need two or three slip couplings and half a day. Everything else could be reused.

At every change of direction - cleanouts are your friend.
R
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It's closed tank. Never smell anything. The house is at a lake and in order to be close to the corp land surrounding the lake the houses are built on the back side of the lots. Since it's lake that's usually the low end of the lot as well. So the field is a pump up. Tank is huge, I guess because it has a divider and a pump inside it.
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Maybe you guys need lessons in how to build them. Most tanks are close to the house. Properly designed systems don't smell or leak.
Colbyt
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