Using reynold number vs C

[ketanco]

in question 17-2 of practice problems for the civil engineering pe exam by michael lindendburg, 13th edition, there is a pipe that connects points A and B. it gives pipe length, diameter, pipe material, temperature of water and the flow rate Q. B is 60 feet above Point A. it asks the pressure at point A if the pressure B is 50..

when i did this, i used C from the tables, coefficient of friction for the steel pipe in order to calculate the head loss. i didnt use the temperature at all. but they used the temperature information and did it by calculating reynolds number. how do we know when we use C or reynolds number when calculating the head loss hf? why didnt they use the formula with the L, C, V, D to calculate the head loss? I used that one and still was able to calculate something. although it was wrong. they used the Reynolds number. I didnt understand why

 

[bradlelf]

First question you need to ask yourself is ... what is the flow type? Turbulent or Laminar?

Hazen-Williams should be used for Turbulent; Darcy for Laminar

Without seeing the actual problem is it a bit hard to help with the solution though, as I do not have the 13th addition.

 

[K19]

when i did this, i used C from the tables, coefficient of friction for the steel pipe in order to calculate the head loss. i didnt use the temperature at all. but they used the temperature information and did it by calculating reynolds number. how do we know when we use C or reynolds number when calculating the head loss hf? why didnt they use the formula with the L, C, V, D to calculate the head loss? I used that one and still was able to calculate something. although it was wrong. they used the Reynolds number. I didnt understand why

This is a weakness of the Lindeburg practice problems... when the NCEES test makers are looking to see your knowledge of friction loss & energy balance, they're definitely not going to put you through the comical rigamarole of looking up & calculating viscosity, specific/relative roughness, flow velocity (from Q/A), Reynolds Number... etc. just arrive at the friction factor f. For an actual PE-exam question, you'll either be told which equation to use - Darcy or Hazen-Williams - or it will be evident by what information is given in the problem (f or C).

FWIW Darcy equation is also valid for turbulent flow! I only have the 12th ed of the CERM, but in it there's a pair of example problems intended to illustrate how the calculated head loss varies depending on which equation you use.

 

[bradlelf]

I can tell you that as a practical application in pump design/sewer i used C factor.

I found on the exam that if a temp was given use Re with the dynamic viscosity; if not I always just the C and dynamic head equation of 17.31 in CERM