connecting to an existing force main 1

[zzmarkzz]

when connecting a gravity (sanitary) sewer to an existing sanitary sewer manhole force main, can you connect at the existing elevation or is there a required elevation (i.e. 6" above existing invert) you need to follow?

 

[LHA]

Maybe I misuderstand the question, but...

The only way to get gravity flow into farce main is to catch the gravity flow in a wetwell, then pump into the force main at a pressure equal to or greater than the existing head in the force main. You can't just "tap in" to a force main, or the pressurized liquid will come out.

 

[semo]

lha is correct; but, the hydraulics of the total system should be examined before adding another pump to it.

The new pump might overpower one or more of the existing pumps and cause additional problems. The system owner should be contacted prior to any work such as this so as not to harm their system.

 

[zzmarkzz]

sheesh...found out it wasn't a force main (which would of helped my search) so the new Q would be: when connecting a smaller diameter gravity sewer to a larger diameter gravity sewer...what should be the change in invert elevation

i think the A is:

When a smaller sewer joins a larger one, the invert of the larger sewer should be sufficiently lower to maintain the energy gradient. An approximate method for securing these results is to place the 0.8 depth point of both sewers at the same elevation.

 

[LHA]

You must be working in PA, USA...you've cited page 18 the PA Domestic WW Facilities Manual verbatim.

I just match the crowns, and have never had this method rejected. There are two practical reasons I do this. First, it is easier. Second, what if the flow (and therefore flow depth) changes? Are you going to dig up the pipe and move it up a fraction of an inch? I'm not, and I'll bet your client won't either.

There has been endless debate on this website about whether matching crowns or .8 flow depth is better, so do yourself a favor and DON'T ask which is better.

 

[zzmarkzz]

actually, it was cited verbatim from Illinois Recommended Standards For Sewage Works

thanks for your input/insight lha

 

[BobPE]

Well, that is a lot better of a question than the first one with the force main....lol...

Match the EGL's at max flow...It is so simple yet so many engineers get it wrong. For smaller diameter mains crown to crown is a good rule of thumb. The problem with rules of thumb though are that they are not solutions...just guesses....So calc the EGL (at theoritical max pipe flow, not the flows you are expecting to put through the pipe) to the influent pipe, determine the elevation just at discharge to the MH, then calc the effluent main EGL just at the point of entrance and match the two...If you do these calcs this way, then every time you do it will work as engineered....


BobPE