Applicabilty of orifice flow
Applicabilty of orifice flow
(OP)
Hello,
I'm working on a project with an orifice plate placed in front of a culvert to control the released flow rate from a retention pond. The size of the culvert is a 36'' RCP and the orifice plate centered on this pipe is a 27''.
The max WSEL above the center of the orifice plate will be 5'.
Would you calculate the released flow rate roughly/simply by using an orifice flow equation (i.e. Q = CdA(2gh)^0.5)?
Does this equation holds for an orifice diameter over 12'' ?
Does it hold for a WSEL higher than 1.5D?
What else to do?
Thanks,
Regards
I'm working on a project with an orifice plate placed in front of a culvert to control the released flow rate from a retention pond. The size of the culvert is a 36'' RCP and the orifice plate centered on this pipe is a 27''.
The max WSEL above the center of the orifice plate will be 5'.
Would you calculate the released flow rate roughly/simply by using an orifice flow equation (i.e. Q = CdA(2gh)^0.5)?
Does this equation holds for an orifice diameter over 12'' ?
Does it hold for a WSEL higher than 1.5D?
What else to do?
Thanks,
Regards





RE: Applicabilty of orifice flow
RE: Applicabilty of orifice flow
RE: Applicabilty of orifice flow
RE: Applicabilty of orifice flow
Howewer : (from page 8-22, Hydraulic Engineering Circular No. 22, Second Edition)
Pipes greater than 0.3 m (1 ft) in diameter should be analyzed as a discharge pipe with headwater and tailwater effects taken into account, not just as an orifice.
RE: Applicabilty of orifice flow
RE: Applicabilty of orifice flow
I was wondering though if I had to check it with another equation/condition. In my case (isn't always?), it's pretty important to access the real (i.e. more accurate possible) flow rate discharged from the pond.
True about anything from Q = 0 to Q at H = 0.2D though!
RE: Applicabilty of orifice flow
The other way to do this without the calculus, if you still want to build up from base principles, is use DOT culvert design nomagraphs. For your inlet control check, select a pipe equal to your orifice diameter, and for your outlet control check, select a pipe equal to your pipe diameter. That's probably how I'd do it, given your situation. It's easy, clean, and produces a record of your procedure that old farts will understand.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Applicabilty of orifice flow
I'll do that.
Wonder what it will look like vs simple orifice calculation.
RE: Applicabilty of orifice flow
With 2/3*Cd*(2g)^0.5 = C
I could have a L for each H. Averaging L for each H... And it wouldn't even be right.
Ouf!
Why then do people use large orifice for detention pond outlet structure. They're calculating it as a small orifice without questionning.
RE: Applicabilty of orifice flow
I imagine some of the people who do it know what they're doing and have accounted for it. Others probably don't know the math and are leaning on hydrology software to think for them. I'd hate to pass judgment on any given engineer or his design without reviewing the design, but I will say I've seen a lot of designs that use large orifices on small reservoirs when they should have used weirs.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Applicabilty of orifice flow
Ideally, your hydrology software will automatically use weir flow under low-head conditions.
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Applicabilty of orifice flow
psmart: HydroCAD won't use a weir flow for a large orifice though (e.g. above a radius of 12''), isn't?
For the design I am refering to, the orifice really looks like an outlet-culvert with a smaller size and a higher inlet elevation than the one entering the basin. I had doubts about the applicability of plain simple orifice flow equation for it and beej76 confirmed what I was thinking.
Now I have to keep Drew08's comment in mind about not over thinking it and keeping it simple :)
But I wonder what would be the outcome of calculations. The difference in Qout/WSEL for a circular large orifice vs small one...
RE: Applicabilty of orifice flow
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Applicabilty of orifice flow
If you're hung up on this, there's software that can do it for you. HydroCAD can do it, for instance (nod to Peter) as can SWMM. For HydroCAD, you use a "pond node" without any actual storage, and you do a 2 stage structure, with the orifice routed to the culvert.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com