Pressure drop calc over a sloping pipe 2

[isaivani]

I have a sloped pipe that gravity drains to a collection pot.How do I go about calculating the pressure drop over this line?
Do I first use Manning's equation to get the velocity and then darcy to calculate the pressure drop?
Thanks

 

[katmar]

If the line is not full it is meaningless to talk of a pressure drop. If your Manning calculation shows that the line will cope with the desired flow at the given slope without being 100% full, then that is all you need to know.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[isaivani]

Thanks, but if the pipe is running full what then?

 

[katmar]

If the pipe is full then yes, Darcy is the correct method to use to calculate the pressure drop due to friction. Drain lines are not designed to run full because with a fixed elevation difference between the inlet and outlet there is only one flowrate that will have everything in equilibrium. If the potential flowrate is higher than this value there will be a backup at your inlet, and if there is less flow available the pipe will not run full. At equilibrium the friction head will exactly match the elevation head.

Also, if the pipe is full and the flowrate is known (because there is only one flowrate that meets all criteria) then you know the velocity without using Manning.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Artisi]

A very clear and concise explanation - worth a star.

Phitsanulok
Thailand