storm pipe design question
storm pipe design question
(OP)
I have an open channel that needs to drain into a storm pipe system, the client wants the channel to drain straight into the storm pipe (i.e. no type of box inlet, just a flared end section).
An 18" diameter pipe under pressure will carry the required flows BUT an 18" orifice (flared end section/pipe end) will not allow the required flow to enter the system with the minimal head we have. Due to grading we can't increase the head. So, I looked at a 24" diameter end section and this works just fine. In order to make the transition from the 24" pipe to the 18" pipe I came downstream 15' and specified a manhole and matched the crowns of the pipes.
Now the contractor has found an ADS concentric reducer (no matching crowns) that will reduce from 24" to 18" and wants to eliminate the manhole. I have never looked into these reducers and am a bit nervous about it; I assume there would be some kind of backwater effect? Do you guys have any better ideas for this situation? Do you have any thoughts on this reducer?
FYI: This is a private storm drain system so there is no municipality to please, it's my discrection.
THANKS a ton.
An 18" diameter pipe under pressure will carry the required flows BUT an 18" orifice (flared end section/pipe end) will not allow the required flow to enter the system with the minimal head we have. Due to grading we can't increase the head. So, I looked at a 24" diameter end section and this works just fine. In order to make the transition from the 24" pipe to the 18" pipe I came downstream 15' and specified a manhole and matched the crowns of the pipes.
Now the contractor has found an ADS concentric reducer (no matching crowns) that will reduce from 24" to 18" and wants to eliminate the manhole. I have never looked into these reducers and am a bit nervous about it; I assume there would be some kind of backwater effect? Do you guys have any better ideas for this situation? Do you have any thoughts on this reducer?
FYI: This is a private storm drain system so there is no municipality to please, it's my discrection.
THANKS a ton.





RE: storm pipe design question
Second, there will be head loss at the reducer creating about the same headwater as you calculated as the entrance loss for the 18" pipe.
Third, changes in pipe size should always occur at a manhole or similar structure so as to permit easy access for cleaning and other maintenance. ( Whether public or private this pipe must be maintained by someone.)
Fourth, where pipe sizes do change it is a good rule of thumb to match crowns which will approximate matching hydraulic grade lines.
Fifth, you should consider and inlet structure which minimizes head loss at the entrance. Rounding the inlet helps a lot. Or use 24" PIPE THROUGHOUT THE LENGTH OF THIS CULVERT.
Use your discretion and good luck.
RE: storm pipe design question
BTW - I also agree that reducing the size downstream is a bad idea - just make it a 24" pipe all the way. The increased cost of the larger pipe will likely be less than the cost of adding a manhole.
RE: storm pipe design question
RE: storm pipe design question
I have specified a trash rack at the inlet end to address the concern of something getting into the 24" pipe and plugging the 18" pipe.
I do agree its never ideal to reduce pipe sizes but this was a tough situation due to clearances to other utilites (below) and minimal cover (above). I was always taught to match crowns when reducing, but maybe I was taught wrong, obviously it does cause standing water, not a lot in this situation, there is also the silt concern but that is a maintenance issue. I was also taught to always have a manhole at pipe reducing areas for cleanout purposes; hence one reason I'm nervous to delete it. I guess if the owner wants it and it's his maintenance I am absolved of liability??
I guess it does make sense that a reducer probably has less head loss than a manhole.
It a tough situation, I do appreciate your thoughts and any other feeback/ideas is welcome.
RE: storm pipe design question
RE: storm pipe design question
RE: storm pipe design question
good luck
RE: storm pipe design question
RE: storm pipe design question