Lets talk orifice coefficients
Lets talk orifice coefficients
(OP)
I'm trying to do some honest research and determine the best orifice coefficient to use in a manufactured grate application.
As I understand it, the orifice coefficient is intended to represent a loss in effective flow area of the orifice due to flow contraction as water moves past the sharp edges of the orifice. This effective flow area reduction drops (therefore the orifice coefficient increases) with two things - how rounded the orifice edges are, and with increasing head on the orifice.
That would tend to make me think that the orifice coefficient for flow through grates, which have multiple parallel sharp edges at fairly low heads, should be reasonably low. Certainly lower than the 0.6 everybody pulls out of their hat as a standard.
I've attached a PDF, that has two pages. One from Sturm's Open Channel Flow, my favorite hydraulics textbook, which shows Table 6-2 taken from FHWA studies on orifice coefficients for inlet headwalls. The table references "Bodhaine 1976," which I have not read. Note that the left hand column of the table is for sharp edged orifices, and the Y axis shows HW/d, which is the ratio of headwater to orifice (in this case culvert) diameter. The highest coefficient it shows is 0.59 - for a head to diameter ratio of 5:1.
The other page is from HEC-22, where they're discussing sag inlets with grates operating in orifice flow. They routinely use an orifice coefficient of 0.67 for the flow through their grates throughout the whole document, even though the relative head on a road inlet is fairly small.
Mixed messages?
Is there a better document to go to, when trying to identify the appropriate orifice coefficient for a grate, or for small diameter orifices? My application is an array of small squares cut into the side of a metal plate, with no edge rounding.
As I understand it, the orifice coefficient is intended to represent a loss in effective flow area of the orifice due to flow contraction as water moves past the sharp edges of the orifice. This effective flow area reduction drops (therefore the orifice coefficient increases) with two things - how rounded the orifice edges are, and with increasing head on the orifice.
That would tend to make me think that the orifice coefficient for flow through grates, which have multiple parallel sharp edges at fairly low heads, should be reasonably low. Certainly lower than the 0.6 everybody pulls out of their hat as a standard.
I've attached a PDF, that has two pages. One from Sturm's Open Channel Flow, my favorite hydraulics textbook, which shows Table 6-2 taken from FHWA studies on orifice coefficients for inlet headwalls. The table references "Bodhaine 1976," which I have not read. Note that the left hand column of the table is for sharp edged orifices, and the Y axis shows HW/d, which is the ratio of headwater to orifice (in this case culvert) diameter. The highest coefficient it shows is 0.59 - for a head to diameter ratio of 5:1.
The other page is from HEC-22, where they're discussing sag inlets with grates operating in orifice flow. They routinely use an orifice coefficient of 0.67 for the flow through their grates throughout the whole document, even though the relative head on a road inlet is fairly small.
Mixed messages?
Is there a better document to go to, when trying to identify the appropriate orifice coefficient for a grate, or for small diameter orifices? My application is an array of small squares cut into the side of a metal plate, with no edge rounding.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com





RE: Lets talk orifice coefficients
RE: Lets talk orifice coefficients
The C for weir flow too anyways (3.0).
Well if it's from tests.
RE: Lets talk orifice coefficients
Ah well...
RE: Lets talk orifice coefficients
length of the orifice (thin plate or longer tube)
upstream conditions (is flow converging, diverging upstream)
edge conditions (is there rounding, knife edge etc on the entrance)
exit conditions (how does the flow expand after leaving the orifice)
so, comparing orifice flow testing related to culverts (long tube, no flow expansion, various types of inlet conditions, head/diameter ratio low) with grates (more like a plate, but flow does not expand well) is comparing apples to oranges. Use the FHWA values which were based on actual tests on grates.
RE: Lets talk orifice coefficients
Once they start to talk about storage routing in chapter 8, they go back to "(0.40 - 0.60)" for the orifice coefficient. That's also how it's shown in their "list of symbols" in the beginning of the document.
They don't explain (at least as far as I can tell) why they use a different one for pavement drainage through grates. Maybe they were doing all their testing on welded grates with rounded bars?
Thanks Canada, that's helpful, and reaffirmed my thinking to just go with 0.60. I'd love to get to the bottom of this HEC-22 mystery though.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Lets talk orifice coefficients
RE: Lets talk orifice coefficients
But using a C = 0.67 instead of C = 0.60 is increasing the capacity by 12% and this should be explained more extensively in a document as such as the Urban Drainage Design Manual.
RE: Lets talk orifice coefficients
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Lets talk orifice coefficients
RE: Lets talk orifice coefficients
I can't seem to remember and or recall the conversion factors from imperial to metric for weir coefficients. I think the following ranges are correct? Does anyone know the conversion factor to get the coefficients to metric?
Broad Crested Weir C=2.6-3.1(imperial)
Sharp Crested Weir C=3.1-3.3(imperial)
Trapezoidal Weir C= 2.7-3.1 (imperial)
Cipoletti Weir C=???
Ogee Weir= 3.2-4.1 (imperial)
Thanks in advance.
RE: Lets talk orifice coefficients
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Lets talk orifice coefficients
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Lets talk orifice coefficients
RE: Lets talk orifice coefficients
C(imperial) * (9.81/32.2)^0.5
So a C (Imperial) of 3.1 = 1.71 in metrics.
Broad Crested Weir C= 1.45 - 1.71
Sharp Crested Weir C= 1.71 - 1.83
Ogee Weir C= 1.83 - 2.26