orifice equation 1

[bank]

I am trying to size a restrictor pipe for a detention pond using the orifice equation Q=CA*sqrt(2gh). I have run into a wide range of interpretations as to what head is being represented by the "h" in the equation. The restrictor pipe will be five feet long, and sit inside the outfall pipe from the pond. It will be designed as fully submerged.

Can someone please tell me what the "h" in the equation is measuring?

 

[Ryb01]

H is the vertical distance measured from the centroid of the orifice to the headwater surface elevation

Hope this helps

 

[gibfrog]

1) A 5 foot long restrictor pipe is not analyzed as an orifice, it is analyzed as a pipe with minor losses or a culvert. You are using the wrong equation unless you cap the upstream pipe end and drill a small hole, about 1/3 the X-area of the pipe. The hole may now be analyzed as an orifice

2) For a fully submerged pipe or orifice, h = (Head at inlet) - (Head at outlet), in your case the H at the free upstream surface - the H at the free down stream surface.

3) Ryb01 is correct for an orifice with a free outfall, but yours is submerged.

4) The C in the orifice equation is dependant on the "sharpness" of the orifice and for a well rounded orifice that is as thick as its diameter, it approaches 1.



Clifford H Laubstein
FL PE 58662

 

[bank]

Thanks Guys.

I think the way the restrictor is designed might qualify it for the orifice equation. The outfall pipe from the pond is designed for the actual runoff from the site as if there is no detention. The restrictor pipe is installed inside the outfall pipe on the upstream end. The remaining area of the outfall pipe cross section is sealed, so all water flows through the restrictor. That being the case, only the upstream end of the restrictor pipe is submerged.

The City and County include that arrangement in their design criteria manuals. Both use C=0.80 for the discharge coefficient, but they seem to differ in their definition of H. I wish they were as clear on this as your responses were. Thanks.

 

[gibfrog]

1) For smooth pipe, I suppose that 5 lineal feet of 18" or larger diameter pipe may be analyzed as an orifice but 5 lineal feet of 12" or smaller pipe really should not - it is a pipe flowing full with an entrance loss, if it discharges freely.

2) City & County (& State) regulations trump rigourous engineering analysis. You must always follow all local regulations, ubnless you aotain a design exemption or equivalent. Also, do not assume that because the design equations/outfall structures are the same, that both the County & City use the same H or Del H in the orifice equation. You should ask the PE who will sign seal your work or call the local authority having jurisdiction and ask them what is H (Del H). When I practice in a new jurisdiction, I purchase copies of recently approved projects of a similar nature. In Florida, we have a broad public records law.

3) After following local regulations, remember that you should not unacceptably flood your client's structures (pavement & buildings) or your neighbors' property (at least not above historic levels/frequencies). Then remember to use proper engineering analysis for the design aspects that are not regulated by the Authority Having Jurisdiction. In general, your state DOT can provide guidence on acceptable local drainage practices. There are also FHWA documents and software.

4) Read "Voodoo Hydrology"

http://www.stormh2o.com/july-august-2006/urban-hydrology-methods.aspx

Clifford H Laubstein
FL PE 58662

 

[gte959s]

You can use the orifice equation if both sides of the pipe are submurged, flowing full, and not over 60ft long. Over 60 ft you have factor friction loss. Otherwise you need to do it as a culvert.