Plotting William Hazen and Friction loss equations

[chicopee]

I don't know if this is the right post for this question.
I have used the Q to the 1.85 exponent semi-log graph since the 1970's to plot as straight lines the above equations when checking water supply and fire sprinkler adequacy as it is the acceptable method by municipalities and insurance companies.
Yet, I never questionned, until now, the use of the semi log graph paper instead of the log-log graphing paper which should be the choice for these two equations. Can any one explain why the semi log paper is acceptable?

 

[BRIS]

The equation is (Ploss = k * Q ^ 1.85) which plots as a straight line on log-log paper. If you plot as a straight line on semi-log paper you will not get the right answers. Although within the limits of flow and pressure loss that you are working over for any single pipe it will not be far out.

But why bother - in the 1970's when working with a slide rule it made sense to plot two points and draw a straight line. Now the simple solution is to stick the equation into a spread sheet and plot the curve to whatever scale you want ??

 

[chicopee]

Bris, so you are inferring that with spread sheets, the plot will be Log-Log. Are engineering firms currently using that approach?

 

[BRIS]

I am saying that you can easily use a spread sheet to plot log - log which will give a straight line. I don't know what others are using - we only use Colbrook White. I am not in the US so I cannot comment on what is accepatble in the US.

 

[RWF7437]

The Darcy-Weisbach equation with the Moody diagram is considered to be the most accurate model for estimating frictional head loss in steady pipe flow. Since this approach requires a not so efficient trial and error solution, an alternative empirical head loss calculation that does not require the trial and error solutions, such as the Hazen-Williams equation, may be preferred by some.

The Darcy – Weisbach Equation is usually stated as :

EPANet2 provides the following Roughness Coefficients for New Pipe

Material H-W C(unitless) D-W e(millifeet) Manning's n(unitless)

Cast Iron 130 - 140 0.85 0.012 - 0.015
Concrete 120 - 140 1.0 - 0.012 - 0.017
Galvanized Iron 120 0.5 0.015 - 0.017
Plastic 140 - 150 0.005 0.011 - 0.015
Steel 140 - 150 0.15 0.015 - 0.017
Vitrified Clay 0.013 - 0.015



“

 

[chicopee]

RWF7437,I agree with your info,howvever it still does not answer why a semilog graph instead of a log-log graph. You see the ordinate(pressure or head) is plotted linearly when in fact it s/b on a log scale.
I believe that the error is not significant and the graph is simplified. I'll have to work out some past flow tests to see if this is right.