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!
  • Students Click Here

*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


Connecting Forced Main to Gravity Main

Connecting Forced Main to Gravity Main

Connecting Forced Main to Gravity Main

I am designing a sanitary system in which I am being asked to provide a 2" forced main from a duplex grinder pump and connect it into an existing 4" sanitary line that is gravity-fed. I know that the 2" line will require a check valve per the manufacturer specs, but since this is planned to be all underground; are there any further requirements for the forced-main-to-gravity-connection?

In the past we have pumped a force main up to an overhead sanitary gravity pipe where we dropped into the gravity pipe after the check valve.

Am I right in the thought that a similar system would have to be designed for this; but underground?

Thanks (in advance) everyone!

RE: Connecting Forced Main to Gravity Main

Is the 4 inch main owned by the epople that own the 2 inch main? If not the other owner may require a manhole.

Richard A. Cornelius, P.E.

RE: Connecting Forced Main to Gravity Main

In this situation, I'd be tempted to temporarily block the gravity line and install a tee-wye tapped directly into the gravity line. A 2" line doesn't carry much flow.
dicksewerrat has a good point that you may be forced to do whatever the gravity line's owner requires. Sometimes there can be negotiation, because a manhole is proportionally expensive and may not be required in this case. The gravity line owner may want a manhole simply for ownership demarcation purposes.
You also need to find out the material, condition, and age of the gravity line. Tapping into new PVC is a different issue than old asbestos pipe, for example.

RE: Connecting Forced Main to Gravity Main

Thanks for the pointers so far everyone.

At this point I am providing a backwater valve within an access panel per the building owners request. I just wish they said so right off the bat haha!

RE: Connecting Forced Main to Gravity Main

For future maintenance considerations, the proper procedure is to make this connection inside of a manhole. Should the 2-Inch line foul (or the check valve), it will be difficult to locate. The manhole should not be too expensive since it should not be very deep. You also do not need a check valve.

You should also check with the owner of the 4-Inch line to be ascertain if he has any requirements for the connection.

RE: Connecting Forced Main to Gravity Main

assuming that the pump is lower than the connection at the gravity main, a check valve would prevent sewage from flowing back down the forcemain. If you make the connection in a manhole and above the gravity line, than the check valve could be eliminated. Judging by the size of the 4 inch line, I assume you are connecting into your own private line, so you can set your own requirements for the connection. However, it is standard practice to connect in a manhole with an air gap and not to connect a pressure line to a gravity line which cannot take the pressure.

RE: Connecting Forced Main to Gravity Main

Not sure on the configuration of your installation but here is another solution for future reference. Certainly a structure would be needed at the interconnection with the gravity lateral or main, just not sure that it is necessarily a manhole, maybe it can be smaller box like shown in the detail. See attached file from local agency.

RE: Connecting Forced Main to Gravity Main

When houses connect to laterals via gravity, manholes are not required. They just tap into the line with a tee.

I wouldn't think you would need a manhole at the connection. Most grinder lift stations can accommodate a check valve at the pump.

I would put the check valve as close to the pump as you can.

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!


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