Reference for Orifice Coeficients

[bperry]

Can someone tell me the typical values used for orifice coefficients used in the design of outlet structures and cite me a reference? We are talking about a typical 4x4 concrete coutlet structure with a 3"-4" 1st stage orifice. I typically use 0.7 as the orifice coefficient (seen it used by other firms and within reports by our office), however, I was recently asked to cite a reference for this coefficient used by a review agency?

I have typically seen coefficients ranging from 0.6-0.7 for this use, and I realize 0.6 is more conservative and that the coefficient is dependent on the shape/entrance.

Basically I would like to know if 0.7 is reasonable for this type of square-edge 1st stage orifice for an outlet structure, and if so does anyone have the references to cite from?

 

[gibfrog]

In Southwest Florida we use 0.6. Check out Brater & King for orifice coefficient data or some other Hydraulic data book. A lot depends on the geometry, finish and the head condition.

However, why argue with the regulator from the review agency on a judgement issue. Just use 0.6 and make you orifice about 15% larger. If the actual coefficient is 0.7, your pond will drain quicker. If some debris clogs the orifice your pond will drain slower.

How accurate is your drainge model anyways? Is this issue truely worth argueing about? Just because your firm has historically used a certain factor does not mean that you should. Times change.

When there is no significant difference (atleast 25%) and if a reviewer desires that I use a certain coeficient or factor, I use it unless it may cause a problem to my cleint or create a public danger. Recalculating an orifice is not a problem, it usually takes only a few minutes and a few more to change the plans.

I ran into something similar when some reviewer told me to use a Manning n of 0.13 for RCP. He told me that 0.11 was not "proper" due to silting and typical pipe installation defects. Well I had AICP data that showed I could have used 0.10, but I redesigned my system, resloped some pipes and upsized some others. Shorty there after, the local code changed to require a Manning n of 0.13 for RCP. However, for Smooth wall HDPE or PVC, they still allow 0.10, go figure!! (I prefered to use ADS N-12, a smooth wall HDPE, but the contractor wanted RCP, so I recalculated the system) Sorry for the extra verbage.

Best of Luck....


Clifford H Laubstein
FL PE 58662

 

[bperry]

Thanks for the "extra verbage." I use n=0.013 for RCP myself. Again, it is conservative; better safe than sorry. I also use ADS N-12 pipe wherever the agencies will allow.

As to my original question, I have no problem using 0.6 and making the review agency happy. However, at the same time I would like to know for myself where a good source is. I use PondPack for basin modeling and I was getting slightly (very slightly) greater flow rates using 0.6 instead of 0.7.

Thank you for the info and coefficient reference.

 

[cvg]

0.6 is a reasonable number to use for a sharp edged orifice - such as a steel orifice plate

for head gates and diversion gates (square openings in structural walls) the orifice coefficient may be in the range of 0.70 - 0.90

the coefficient will be affected by the velocity of approach and the amount of contraction. For instance, by beveling the edge of an orifice you may increase the discharge by 25%.

 

[gibfrog]

Dear Bperry, here is exactly what you asked for, from books that may should be available in your office or in your co-workers personal librairies.

1) Merrit, Handbook of Civil Engineering, 2nd Ed (an older edition) For fully developed Turbulent flow uses 0.59-0.63 and references Brater & King.

2) Lindeburg, Civil Engineering Reference Manual 8th Ed, turbulent flow, 0.51 (borda), 0.62 (sharp edged), to 0.98 (round edge or smooth well tapered nozzle). In between values not cited.

3) Brater & King, Handbook of Hydraulics 7th edition,has a ton of coefficient data. 0.59-0.63 for turbulent and fully supressed contractions. for an orifice 20 cm wide x 20 cm high, 0.6 complete contraction to 0.68 supressed at bottom and partially at two sides to 0.98 complete supression of contractions. It just goes on and on. It depends on the geometry of the orifice and the finish or tapering or beveling of the edge and walls.

CVG is correct, the orifice coefficient is the product of the velocity coefficient and the contraction coefficient. The contraction is the major factor causing the reduction of flow. Supress the contraction and achieve maximum flow.

A final word - In stormwater applications we usually like to drain our rainfall runoff storage facilities at the maximum permissible rate. Using an low coefficient, like 0.6, potentially allows the storage facilities to drain faster, if the conveyance is available. This allows greater than design storms to be handled before failure occurs. (There will always be a large enough storm to cause failure or flooding, like a 1,000 year rainfall event) So if the orifice is finished well, nicely rounded or beveled edges, the actual orifice coefficient will probably be much higher than 0.6 and your storage system will drain much faster. I always specify well rounded or beveled edges when possible for orifices and pipes/box inlet/headwall. It is a inexpensive way to maximize flow.

Best of luck...

Clifford H Laubstein
FL PE 58662

 

[cimmeron]

The best reference would be Brater and King, Handbook of Hydraulics published by McGraw-Hill Co. Refer to Chapter 4. Using a 0.7 coefficient is a bit high, a standard default factor used by many agencies is 0.6. The coeffecient varies a little bit depending on the height of water over the weir and other factors. The book has several tables explaining this variance.
Cimmeron