When to go from Partial Full Flow to Full Flow Analysis

[rlondeen]

When doing partially full analysis on closed conduits, if a pipe is surpasses full flow capacity do you switch over to 100% capacity analysis or do you continue analyzing it under partial flow to 93% capacity (real max pipe capacity)?

hypothetical example:
I have 3 pipes with a capacity of 10 cfs each. 2 are expected to carrying 9 cfs and 1 is expected to carry 10.5 cfs. Do I analyze the the first 2 pipes under partial flow analysis and then jump to full flow analysis for the 3rd pipe?

 

[gbam]

rlondeen it all depends on your energy slope and tailwater condition:

If your energy slope is greater than pipe slope the system move to full flow, if not the system will remain open channel flow.

For low tailwater condition verify flow regime. If super no losses transfer upstream. If sub use back water computations to evaluate for HGL from downstream to upstream.

If you have high tailwater (above soffit) then analyze full flow accounting for all minor losses untile your system unseals. Also, check the flow regime within your system if any conduit is super critical be aware that in order to have the system full it must be from downstream constraint.

 

[rlondeen]

gbam,

I see. So it ultimately depends on the HGL. Thanks for the clarification.

Is there a formula for critical flow depth of a circular pipe?

 

[gbam]

Remember your specific energy charts from school? critical depth occurs when Fr = 1.0 = V/(g*h)^0.5; where v -> velocity, g -> gravity const. and h -> hydraulic depth (A/B); A -> flow area & B top width. For a circular section/ pipe diameter, there are charts for critical depth or you can use Manning's equation and bang through the math.

 

[rlondeen]

I was just trying to avoid banging through the math, but it's no big deal. Thanks for your help.