It's The Solids That Run Away
There are certain facts you learn when you start out in the plumbing profession: hot on the left, it runs downhill; payday is on Friday; and wear an orange hat around an open septic tank. (That last one is my father's. He always said if you fall in, we'll know where to find you.)
Another important fact: You need to pitch the drain enough so the solids don't fall out of suspension. And don't pitch it too much, or the liquids run away from the solids. One of the big debates always was 3-inch pipe. Can you pitch it 1/8 inch per foot, or does it have to be 1/4 inch per foot? Plumbing codes still disagree on that one. But there is a correct answer. I'll give it to you at the end of the column.
While minimum pitch was always established by the code, the maximum pitch was never stated. I always wondered at what pitch liquids ran away from solids. Everyone had a different answer.
This debate on pitch always comes up when we have to install sewers with changing terrain. If the building drain was at one elevation, and the public sewer connection at another, you had to install drops in the sewer line to accommodate the elevation change.
Words Of WisdomMy father and one plumbing inspector, who is now deceased, always argued about the offsets that should be installed in the sewer line.
My father, being a smart businessman, wanted to install the drops with a 45-degree offset. The inspector always wanted 90-degree offsets. The inspector's reasoning was that you needed a vertical pipe so that the sewage flowed properly. He surmised that at a 45-degree pitch, the liquids ran away from the solids.
My father knew that, at 45 degrees, the liquids didn't run away from the solids, and the installation used less pipe. For every 10 feet of drop, a 45-degree offset used 6 feet less of pipe, plus you didn't need a cleanout. That meant more profit for my father. Smart thinking.
The plumbing inspector never could find a section of the plumbing code to justify his requirement, other than that famous section that read, "I'm in charge, you'll do it my way." Back then, I thought that was in every plumbing code.
One project I will never forget, in the early-1970s, we had a sewer that was about 350 feet long that ran down the side of a hill. The vertical drop was about 150 feet. I'll admit that, over the years, the distances may have increased, but that is how I remember it.
For this project, my father had given up arguing with the plumbing inspector. We planned about 15 vertical offsets for the job. Every offset was a 10-foot drop. That was the easiest thing to do since it was installed in extra heavy cast iron. Just throw in a 10-footer.
You had to see this sewer when we were done. It looked like someone was playing with tinker toys. A horizontal run of 20 feet of pipe followed by a 10-foot vertical drop. Another 20 feet of pipe and a 10-foot drop. Of course, the entire length of the sewer had to be left uncovered so the inspector could see it. Fortunately, he didn't ask for a water test. I don't know how those fittings at the base would have handled about 70 psi of pressure.
Those of us who installed the piping were very proud of our work. But we had to admit there were a lot of cleanouts and offsets.
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."
Myth To RestOf 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.