storm pipe design question 1

[AlpineEngineer]

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.

 

[RWF7437]

First, it is generally a bad idea to reduce pipe size in a storm drain in the direction of flow. If for no other reason, any debris large enough to pass through the 24" pipe may NOT be small enough to pass through the 18".

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.

 

[saruman26]

Matching pipe crowns is not necessarily a good idea where the downstream pipe is of smaller size. This would result in some water permanently standing within the system.

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.

 

[cvg]

matching crowns is generally of no use when the pipes are flowing under pressure. The concentric reducer will probably not have an appreciable difference in headloss than your manhole under peak flows. In fact, the gradual flare of the reducer versus the expansion, surcharge and then back to orifice flow within your manhole - it may actually reduce your headloss. I hope you have included this manhole loss in your calculations so that it does not affect your inlet headwater.

 

[AlpineEngineer]

Thanks for the input guys. I did consider (using Storm CAD) the manhole and how it affects inlet headwater. To go to 24" pipe for the whole length is undesirable due to the length and some clearances with other utilites.

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.

 

[civilperson]

24" equivalent low head arch or elliptical pipe will have the flow characteristics of the 24 with vertical clearance close to the 18" pipe.

 

[saruman26]

Good point, civilperson. A 19"x30" elliptical RCP (24" round equivalent) can easily be substituted here.

 

[RWF7437]

In what ways is a 19" x 30" elliptical RCP "equivalent" to a 24" circular pipe ? Do they have the same flow area ? Do they have the same conveyance ( capacity )? Would they have the same entrance loss coefficient ? Depending on he flow condition; i.e. inlet or outlet control, full or part full flow, any differences might be significant.

good luck

 

[cvg]

note that 19" x 30" eliptical has a greater waterway area than a 24" round pipe and would provide an additional 5 inches of clearance. An 22 1/2" x 36 1/4" arch pipe also would work but would provide little benefit in the way of vertical clearance.

 

[saruman26]

Most round pipe sections have an 'equivalent' elliptical section that is of similar (probably not exact) capacity and flow area that will do the same job, but can be more useful in some circumstances.