Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

gravity flow rates

gravity flow rates

gravity flow rates

I'm back checking the flow rate from a building with a gravity line connecting to a manhole. The flow rate I have developed from the building is giving me a less than 2 f/s velocity in the pipe. Should gravity sewer pipes be sized for the peak flow rates and checked for the daily to meet a 2 f/s velocity?

RE: gravity flow rates

I am going to disagree. It depends on what the design standards are for your project location.

Every jurisdiction I have dealt with in California sizes sewers to flow half full at peak dry weather flow. For examples, the City of Los Angeles: https://eng2.lacity.org/techdocs/sewer-ma/f200.pdf, Section F250. They also want a minimum velocity of 3 fps (Section F253). Some agencies, such as the City of San Diego, allow trunk sewer to flow deeper, but typically no more than 75%. The City of San Diego Sewer Design Guide, Section, says, "New sewer mains 15 inches and smaller in diameter shall be sized to carry the projected peak wet weather flow at a depth not greater than half of the inside diameter of the pipe (dn/D not to exceed 0.5). New sewer mains 18 inches and larger shall be sized to carry the projected peak wet weather flow at a depth of flow not greater than 3/4 of the inside diameter of the pipe (dn/D not to exceed 0.75)." (https://www.sandiego.gov/sites/default/files/legac...)

I don't know if Los Angeles set these criteria because it has a lot of combined sewers (it's been more than 30 years since I dealt with any sewers in LA), but I see the same standards in other jurisdictions that don't have many (if any) combined sewers.

The moral of the story is: check with the agency having jurisdiction.

"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

RE: gravity flow rates

Agree with cvg regarding his comment which applies for municipal wastewater collection systems. fel3 makes some good points in regards to municipal wastewater collection systems in hot and arid regions where there will be concerns with hydrogen sulfide.

It must be recognized that decreased slopes may cause additional sewer maintenance expense and special linings or materials should be considered for corrosion protection.

However, you appear to be dealing with a service sewer. Service sewers are the drains that run from the building to the municipal sewer in the street. Service sewers generally have a minimum slope of 1%. You should check this service sewer requirement with the local AHJ.

RE: gravity flow rates

The simple, general requirement for any mechanical construction is that it must be constructed to the purpose and load it is meant for. eg. for maximum load.

The answers in previous posts point out some guidelines. Worst case: what happens if the receiving municipal pipeline gets clogged or run full?

RE: gravity flow rates

This is a service line. In a nutshell, if I have an X" pipe at X% slope, flowing half full, I know a velocity and discharge. As long as what is coming out of my building is less than that half full discharge, it is ok. My concern was designing a pipe with a velocity that matched the outflow from the building.

RE: gravity flow rates

The 1% slope for a sewer service should provide a higher velocity than 2 f/s, probably over 3 f/s.

Most municipalities require a minimum size sewer service for a residential development to be 6-Inches in diameter and the minimum size sewer main for commercial and industrial developments to be 8-Inches in diameter.

The reason for over-sizing service sewers is for ease of cleaning.

RE: gravity flow rates

@65nerd - It also depends on the type of occupancy that will take overtime. Since that is not in our hands or the agency, they design conservatively. So if the design criteria is set for min 8" pipe @ 0.035 slope to achieve 2 fps velocity when pipe is 70% full, then that will dictate the design. If you design the pipe to match with the current outflow from the building now, that can and will change in the future. 8" PVC SDR 35 or 26 is fairly inexpensive in grand picture. Hope that helps.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close