Moody Diagram and Equations

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Main Question or Discussion Point

What is the usability of the Colebrook, Haaland, and Swamee equations in engineering work and the need or not to tailor the equation selection i.e. will Haaland always work or in some circumstance should a different equation be used? What about the iterative Colebrook vs. the explicit Haaland? Is it better to iterate or just get a direct albeit more inaccurate (maybe?) friction factor from Haaland? etc.
 

Answers and Replies

  • #2
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The Colebrook equation will give you the most accurate solution but it can be difficult to solve for because it requires iteration. Because of this, the Haaland and Swamee-Jain equations were created and are approximations of the Colebrook equation which are much easier to solve. As they are only approximations they are also less accurate. Sine this isn't 1963 and just about everyone has a computer with Excel on it, it's best just to use the Colebrook equation for the most accurate solution.
 
  • #3
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Not to disagree with Topher, but beware the 'need for accuracy' in calculations using the Moody chart. It irritates me to see calcs with the friction factor determined to 6 significant figures; irritating because it projects an image of precision not inherent in the method. By which I mean, if you are designing a system, you should consider that the actual losses will be as calculated, plus or minus 10 or 20 %. So, dont sweat over the friction factor to any higher 'accuracy' than that.

The only reason I can see for using a formula (vs manual reference to the actual diagram or chart) is because people develop automated methods that they wish to use in unknown future situations. If you know the conditions are fully turbulent (ie, 99% of real-world calcs) then all you need is the fT vs pipe size table (eg, Crane page A-26).

Sorry, it's a pet peeve.
 
  • #4
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Not to disagree with Topher, but beware the 'need for accuracy' in calculations using the Moody chart. It irritates me to see calcs with the friction factor determined to 6 significant figures; irritating because it projects an image of precision not inherent in the method. By which I mean, if you are designing a system, you should consider that the actual losses will be as calculated, plus or minus 10 or 20 %. So, dont sweat over the friction factor to any higher 'accuracy' than that.

The only reason I can see for using a formula (vs manual reference to the actual diagram or chart) is because people develop automated methods that they wish to use in unknown future situations. If you know the conditions are fully turbulent (ie, 99% of real-world calcs) then all you need is the fT vs pipe size table (eg, Crane page A-26).

Sorry, it's a pet peeve.
I disagree that "99% of real-world calcs" are in the fully turbulent region on a Moody Diagram. Low pressure steam, water, compressed air and ducted air under normal conditions fall in "Transition Zone" of Moody's original 1944 diagram or the "Rough with Re Dependence" in more current versions of Moody's. Does anybody know why the Moody diagram has been modified?
 
  • #5
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I disagree that "99% of real-world calcs" are in the fully turbulent region on a Moody Diagram.
Yeah, you're probably right. In my world, if the flow isn't turbulent, then the pipe is too big. I must have forgotten that many people live in other worlds.

I still think, though, that some go too far in seeking accuracy in calcs like this.
 

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