Invert Matchups

[gtrainor]

In a manhole when matching an 8-inch sewer line inlet with a larger line outlet, say 18-inch, do you match the pipe crowns or match the inverts (maybe with a 1/10' drop) or what?

 

[Maury]

From a hydraulic stand point, matching the 0.8 full elevation of each pipe is the way to go. Matching the crown of the pipes would also be acceptable. Matching the inverts should be avoided, though I've done it many times when working with flat ground where elevtion is a problem.

 

[gtrainor]

I also forgot to ask: If the larger pipe is put at a lower invert (say 5"), then how is the drop to the flow line handled for the inlet? Is there a steep flow line inside the manhole or does it just drop from the height?

 

[JRW261]

You will want your 8" above the flow of the 18" but not so high its splattering everywhere. Agreed, matching crowns is often acceptable but in some cases still can be extreme. With that in mind I suggest that it is part of the invert, steep if necessary, and with fast inlet flow sometimes a slight bend down stream. Not just a "splash" into the invert, "Splashes" often make a mess of the "benches", leaving solids on them.

 

[gtrainor]

Maury, when you match inverts, has there been any feedback of problems. And what kind of problems?

 

[cvg]

depending on the locale, matching inverts may be the standard. It is commonly done in southern california and crowns are generally never matched. Same method is also used for storm drains. I have not heard a convincing argument against it.

 

[Maury]

I've not heard of any problems attributable to matching invert elevations of different size pipe but I'm still uncomfortable when I have to do it. It just doesn't seem right.

 

[cvg]

the original post stated that an 8 inch pipe would transition to an 18 inch pipe in a manhole. This is an unlikely scenario. Standard practice for design would be to transition in increments. Usually 2 or 3 inch increments which is the smallest manufactured for most sewer pipe. Even if you had 2 8-inch pipes coming in, you would probably only jump to a 12-inch going out.

The transition between pipe sizes is done in a manhole, which is typically about 3 or 4 feet in diameter, and often with a mandatory drop in elevation of 1 or 2 tenths of a foot. This drop is formed by sloping the channel in the manhole base and isn't generally an abrupt drop. This provides a very gradual transition and minimizes headloss and turbulence, regardless of whether you are matching crowns, inverts, centerlines or otherwise.

 

[gtrainor]

Thanks to all who responded. I was really looking for a convincing argument to only match crowns, whether it was an 8 to 10 inch and 8 to 12 inch. It sounds like there really isn't. Thanks.