Caclulating Head Loss and change in Velocity 1

[phllp581]

After using the Darcy Weishbach equation to calculate head losst through a pipe, can I then take that head loss result and apply Bernoulli equation to calculate the new velocity given the head loss? If I do it this way I seem to get velocity in=velocity out, but this doesn't seem to be right, shouldn't the velocity drop?

 

[bimr]

There is a chart in this link that converts k factors to equivalent lengths for gas services.

http://www2.iccsafe.org/states/oreg...dix A_Sizing and Capacities of Gas Piping.pdf

 

[katmar]

Very few texts even begin to discuss how to include the pressure drop through fittings into the compressible flow equations. I agree that using equivalent lengths is a convenient way to do it, and it is easily incorporated into the isothermal equation. The relationship between the K-values and equivalent length ratios (i.e. L/D) is given as Equation 2-4 in the Crane manual. This is K = fL/D. The Crane manual does not highlight the fact, but the form in which it presents the K-value data actually gives the L/D ratio directly. For example Crane gives (on page A-29) the K-value for a 2D 90 degree bend as 12f_T. By comparing this relationship with the Equ 2-4 above it is clear that the value 12 is the equivalent length ratio.

There is a lot of confusion over the role of f_T, but fortunately it is not really a problem when we confine ourselves to turbulent flow - which gas flow usually is.

Another factor that sometimes concerns people is whether it is reasonable to assume a constant friction factor over the length of a gas flow pipeline. This concern generally arises from the knowledge that the friction factor is dependent on the Reynolds number which in turn seems to depend on the velocity - and we know the velocity increases along the pipeline. If the Reynolds number is written in terms of the mass flow rate rather than the velocity we get the relationship Re = 4M/(3.14[μ]D). Since M, the mass flow rate, is constant over the length of the pipe Re varies with [μ] only, which in an isothermal system is effectively constant - making Re and therefore the friction factor constant for the length of the pipe.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[zdas04]

Katmar,
I read your post before I had breakfast, sat there thinking about it for 45 minutes and only occasionally remembering to eat. Then I got in the shower and turned a normal 10 minute exercise into an hour thinking about the places that I've recalculated friction factors inside programs that I didn't need to. I've seen Reynolds Numbers presented in mass flow rate terms before, but never really paid much attention. Thanks a lot for screwing up my schedule this morning. A paradigm shift is always a good way to start your day, thank you.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist