# Darcy or Hazen-Williams?



## mattsffrd

how do you know when its safe to use the hazen-williams equation instead of the darcy equation to find head loss due to friction? i know hazen-williams is only for turbulant flow, but it seems like when working through problems, they just automatically know when to use it and when to go through solving the darcy equation. by the time you go through and figure out whether something is turbulant or laminar you might as well just use the darcy equation. is there an easy way to tell?


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## Chucktown PE

mattsffrd said:


> how do you know when its safe to use the hazen-williams equation instead of the darcy equation to find head loss due to friction? i know hazen-williams is only for turbulant flow, but it seems like when working through problems, they just automatically know when to use it and when to go through solving the darcy equation. by the time you go through and figure out whether something is turbulant or laminar you might as well just use the darcy equation. is there an easy way to tell?



I used Hazen-Williams for everything. If the velocities are very low or you're working with a more viscous fluid you may need Darcy but for the Civil exam you're safe using HW. Darcy would become very complicated for iterative solutions as well.


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## mattsffrd

sounds good...i'll just stick with HW unless something is blatantly laminar then (unless the actual question is simply asking for head loss due to friction...then i'll probably solve it out with darcy)


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## IlPadrino

mattsffrd said:


> how do you know when its safe to use the hazen-williams equation instead of the darcy equation to find head loss due to friction? i know hazen-williams is only for turbulant flow, but it seems like when working through problems, they just automatically know when to use it and when to go through solving the darcy equation. by the time you go through and figure out whether something is turbulant or laminar you might as well just use the darcy equation. is there an easy way to tell?


I think you've already answered your question... Using the Darcy-Weisbach equation requires you to know or calculate the Darcy friction factor (f) which is a function of the fluid properties (Re) and pipe material (e/D). If you're not given anything to help you calculate f then there's a good chance you should be using the Hazen-Williams equation which requires you to know the Hazen-Williams coefficient © that just depends only on the pipe material.

Remember that both Darcy-Weisbach and Hazen-Williams are a derivative of the "original" by the French engineer Antoine de Chezy who calculated friction loss by using a coefficient reflecting the roughness of the pipe, the internal hydraulic radius of the pipe and the hydraulic slope. Hazen-Willaims is an emperical relationship that is only valid for a certain range of conditions... so when it's used, it's certainly much simpler. For Darcy-Weisbach, there are many ways of finding f and with the Swamee-Jain equation, you can avoid charts, tables, and equations for different types of flow - so it's always going to be an as good or better answer than Hazen-Williams.

Bottom line: For water flowing at ordinary temperatures (maybe 40-75F) and at turbulent flow, you can use Hazen-Williams. At other temperatures, in non-turbulent flow, or for liquids with different viscosities, use Darcy-Weisbach.

If you're really curious from an engineering (vice exam) perspective, start googling about the two equations... start here for one such discussion.


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## mattsffrd

great post, thanks! yeah i see what you mean, basically if you have the tools to figure out the friction factor, use darcy. i probably should have realized that lol. i'm assuming for the morning portion it will be pretty obvious which one to use anyway, and i'm doing transportation in the afternoon.


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## Polysloman

If you're really curious from an engineering (vice exam) perspective, start googling about the two equations... start here for one such discussion.


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## IlPadrino

Polysloman said:


> what about Manning for Trapezoids. is there an equation that works best/tailored for open channel that I can keep in my HP 35S for the breath questions. The review Teacher always use the general eq. it works but it takes more time when applied to a trapezoid channel. First get the area, then wetted perimeter...
> Help!
> 
> Thanks.


I'd stick with the tables in the CERM (Appendix 19.E in the 10th Ed.)... quick and simple - less chance for a calculation mistake.


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## mattsffrd

yeah some of those appendix tables are a godsend...the wetted pipes section (16.A) is extremely useful as well


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## djm

Darcy!


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## djm

Hazen-Williams!


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## djm

No seriously; Hazen-Williams


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## IlPadrino

djm said:


> No seriously; Hazen-Williams


Too much Miller Lite?


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## djm

Grain Belt


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