Orifice Cd 1

[nzo]

Hi there, can anyone tell me what you use for Cd when calculating the flow through a pipe with some head on it? I know 0.6 is used typically for a sharp edged orifice restriction and 0.86 or so for a thick plate but if you don't have an orifice restriction at the end of the pipe and just want to calculate the flow through a pipe running under some pressure, what coefficient would you use? I'm thinking atleast a 0.8. Thanks a lot for any suggestions.

 

[psmart]

If you know the entrance energy loss coefficient Ke, you can calculate the Cd as follows:

Cd = Cc / (1+Ke)

Where Cc is a contraction coefficient normally taken as 0.9. Ke ranges from 0 (no entrance loss) to 1 (max entrance loss). For example, an average Ke of 0.5 (for a square-edged flush inlet) will yield Cd=0.6

Of course, this all assumes that the culvert is operating under orifice/inlet control. Otherwise a complete culvert analysis is needed.

 

[JoeTank]

Cd varies within a range that hovers near 0.6 for the case you cite.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[bltseattle]

Lindeburg's Civil Engineering Ref Manual 6th Edition p. 3-24 has a robust table of orifice coefficients......

Just a reminder, whenever one designs for a completely submerged, non-vented, pipe inlet you need to consider whether you are creating a siphon, and whether that poses a problem in your system. This issue does not apply if the outlet is non-submerged.