Pipe Slope 3

[djfitch]

I would like to know some of the theory behind pipe size and it's minimum slope, particularily for 4" and 6" PVC SDR 35. It seems that every municipality has their own theory/regulations.
The reason I am posting this is I have a commercial office/maintenance building with 20 people, with two rest rooms, The sewer line from these is 4" with 1% slope with several sags and flat areas (which I feel is a result of the minimal slope), after 60 feet it changes to 5" terra cotta, again with a 1% slope. It picks up a few wash sinks and continues for 140 feet to the septic tank. The septic tank currently has 4.5 feet of cover and most likely will be replaced.
My thought is to run 6" PVC at 1% slope (grade set with laser), to reduce depth of installation and septic tank, and hopefully achieve a better flow.
Again I would like to hear the book theory as well as individual's theory and experience.
Thanks

 

[dicksewerrat]

One percent slope is just fine. Sags, flats is a result of bad installation. 4" inch pipe is adequate for two wash rooms and the rest of the fixtures. Relaying the pipe will help the drainage. Why would you replace the tank?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[djfitch]

The tank needs to be replaced because it does not meet current code and the capacity is too small for the facility. I agree that 4" pipe is adequate but would 6" perform better?
I concur on the bad installation comment.
Thanks

 

[dicksewerrat]

A 6" inch pipe will not really do anything for you. And the smaller pipe will flow with more depth and clean itself better.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[bimr]

The minimum slope for larger gravity-flow sanitary sewers is based on Manning's equation with a minimum velocity of 2 ft/sec.

The slope of drainage systems is set by the local plumbing code which is may or may not have any science behind it. Here is a typical one:

Large Piping. Horizontal drainage piping larger than 3 inches but less than 8 inches in diameter shall be installed with a grade of at least ? inch per foot. For piping 8 inches or larger in diameter, the grade is determined by the number of drainage fixture units connected to the drain pipe.

 

[bimr]

The minimum slope for larger gravity-flow sanitary sewers is based on Manning's equation with a minimum velocity of 2 ft/sec.

The slope for building drainage systems is set by the local plumbing code which is may or may not have any science behind it. Here is a typical one:

Large Piping. Horizontal drainage piping larger than 3 inches but less than 8 inches in diameter shall be installed with a grade of at least 1/8 inch per foot. For piping 8 inches or larger in diameter, the grade is determined by the number of drainage fixture units connected to the drain pipe.

 

[dicksewerrat]

House runs, under floor are usually set at 1/4 inch per foot or 2%. Once it leaves the bldg., it is set by the slope needed to meet the main in the street or easement. 1/8 inch per foot comes out to 1%. Do you have a quantity that the system delivers per day? Peak flow?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[bimr]

This is a directly quoted from the IL State Plumbing Code:

"f) Small Piping. Horizontal drainage piping of 3 inches diameter or less shall be installed with a grade of at least 1/4 inch per foot.

g) Large Piping. Horizontal drainage piping larger than 3 inches but less than 8 inches in diameter shall be installed with a grade of at least 1/8 inch per foot. For piping 8 inches or larger in diameter, the grade is determined by the number of drainage fixture units connected to the drain pipe. "

It was further suggested that the information is available in the local plumbing code whereever the poster is located.