Minimum Sewer Velocity 3

[CivilizedPE]

I have a question on how to interpret the minimum velocity requirement of 2.0 feet per second for gravity sanitary sewer pipelines. Many regulations indicate that this velocity is measured when the pipe is flowing full. See examples below.

Ten States Standards states:
33.4 Slope
33.41 Recommended Minimum Slopes
All sewers shall be designed and constructed to give mean velocities, when flowing full, of not less than 2.0 feet per second (0.6 m/s), based on Manning's formula using an "n" value of 0.013.

Texas Commission on Enviromental Quality states:
§217.53. Pipe Design.
(l) Minimum and Maximum Slopes.
(1) All wastewater collection systems must contain slopes sufficient to allow a velocity when flowing full of not less than 2.0 feet per second.

The following regulation however, states that velocity is measured during average day flow and doesn't mention if the pipe needs to flow full or partially full.

Seacoast Utility Authority (Florida) states:
SECTION IV - SANITARY SEWER SYSTEM
B. HYDRAULIC DESIGN
2. Slopes: All sewers shall be constructed with hydraulic slopes sufficient to give mean velocities, using average day flow through the sewer, of not less than 2.0 feet per second, based on Manning's Formula.

By the way, this project is neither in FL nor in TX, I'm just using these as examples. But we do fall under Ten States standards and the local jurisdication uses Ten States for sewer design. We are designing our sewer system based on minimum slopes and diameters and so that it doesn't flow full or surcharged under typical conditions. This results in some pipes flowing partially full at less than 2.0 fps. The Ten States reg seems convoluted since some pipes will never flow full.

Does this mean I need to calculate the theoretical velocity if the pipe were flowing full to see if it reaches 2.0 fps per Ten States? Or do we need to slope our pipes such that full flow velocity is at least 2.0 fps? Even this may not guarantee 2.0 fps at actual flow. Also, additional slope will require several more feet of excavation and we already proposing a 20-ft deep wet well/lift station at the end of our development.

Thanks for your help!

 

[cvg]

why would you not follow the standard? I don't see much upside for you and a big downside to the sewer crew that has to clean the flat sewer you designed.

 

[jgailla]

Ten States has the minimum slopes to achieve 2 ft/sec in a table, so you don't need to calculate the full flow except to double check the table.
If your code references Ten States, you can't have a shallower slope, although you could have steeper.
I don't know how it could be more straightforward than Ten States. For X pipe size, here is your minimum slope.
If you are using the average daily flow, you may need to make your pipes steeper if the pipe is less than 50% full. This probably means your pipes are oversized, though.

 

[CivilizedPE]

Hi cvg, my problem is that I don't understand the standard (Ten States). I'm getting hung up on full flow versus partial flow. Can you spell it out for me?

 

[CivilizedPE]

So based jgailla's comment I take the standard to mean that the slope needs to produce a velocity of 2 fps under average conditions (regardless of whether the pipe flows full or partially full). But, the slope should not be less than what is listed in the table in 33.41, except as allowed by 33.42. Right?

 

[rconner]

While I guess the difference in semantics may be a question for the Ten States committee(s), I will note ASCE MOP No. 60, Gravity Sanitary Sewer Design and Construction has some perhaps helpful sewer flow depictions on page 51 of this manual. These are diagrams of what appears to be normally (partially full) flowing sewer profiles/sections. The one headed "Velocity 2 fps" reads in part, "Efficient solids transport..." The next diagram headed, "Velocity 1.4 to 2.0 fps. Inorganic grit accumulation in the bottom. More sulfide buildup as the velocity diminishes."

While it may not be this cut and dried or simple and larger grit and sewer sizes may even benefit from higher velocities, I suspect you don't really want either of the latter, and of course there are normal construction tolerances/variations as well to perhaps consider.

 

[cvg]

I think you need to re-read jgailla post as well as the standard. stick to the minimum slope per the standards and do not oversize the pipe. flushing with high flows can be used to clean the pipe when it gets fouled, but you need to have the minimum slopes to do it properly