You are correct Joe. Even though I do considerable research into the many
different empirical equations for pressure loss that exist today and have
existed in the past, I still see many misconceptions. It is as if it were
folk lore.
One of the broad misconceptions is that the HW C-factor is a measure of or
is used for the roughness of the pipe. That is only a small part of what
it represents. In the HW formula for pressure loss, the C-factor
encompasses absolute roughness, relative roughness (absolute / diameter),
Reynolds number, viscosity at different temperatures, and viscosity at
different diameters. This is quite a spread. While it is true that the HW
equation operates in a small range of the Moody diagram, it has shown to be
effective for sprinkler systems. Large water works systems are a different
matter. This is where the C-factor used can lead to serious errors.
To research the difference between the HW and the DW equation, I suggest
the book "Experiments upon the Flow of Water in Pipes and Pipe Fittings" by
John R. Freeman. In this book are hundreds of extremely accurate
measurements of water flow through different size and types of pipe, new
and old. Even though the losses were measured using the DW equation, you
can back engineer the losses to the HW equation and even calculate the
Darcy friction factor and the absolute roughness. One of the more
interesting tidbits is that new pipe has a C-factor that is closer to 130
than 140. You will also notice that the C-factor fluctuates due to
different flows, Reynolds numbers, and diameters.
I also did not see anyone relate that Hazen and Williams were actually
looking at sewage and water works systems and that they did not consider
that a C-factor below 100 was valid.
Thank You
Rahe Loftin, P.E.
Region 7 - GSA
Office - 817-978-7299
Fax - 817-978-8644
Cell - 817-371-3102
"Joe Hankins"
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Further to what Roland says, any difference between H-W and D-W friction
loss calculations are completely overshadowed by the error introduced by
arbitrarily assigned C-factor/roughness coefficient. As I have said many
times, don't waste time measuring with a micrometer when you're cutting
with a chain saw!
Joe
Roland Huggins wrote:
> Although the following statement was previously made and repetition
> becomes truth (like the need for roof vents in sprinklered buildings)
> allow me to say, lets not repeat it anymore. It just isn't true.
>
> Actually a head to head comparison between Hazen -Williams and Darcy-
> Weisbach, HW will under estimates by a small amount that gets larger
> as the velocity goes up. Now this is for black steel (it reverses
> for copper). For instance a 1 in. sch 40 at 32 ft/s (86 gpm): HW -
> 1.46 psi/ft and DW - 1.83 psi/ft representing a 20% difference. At
> 10ft/s there is an 11% difference. As the pipe size increases, the
> amount of difference shrinks with 2 in. showing a 15% difference.
> This is not linear since 4 in. pipe shows a slightly larger
> difference than a 3 in. one.
>
> If we really want to discuss reality (verses the applied method since
> sprinkler discharge does NOT reflect a science based reality), the
> use of total pressure verses velocity pressure presents a very
> conservative factor. Most get hung up on the second sprinkler
> discharging slightly less (and miss that the second ranch line also
> discharges less) but the overall system demand is greater using total
> pressure.
>
> Now if someone would show me a sprinkler that discharges all its
> water in a 10 X 13 rectangle and the density is uniform, THEN I MIGHT
> BE INTERESTED IN SPLITTING HAIRS
>
> Roland
>
> On Jan 16, 2007, at 5:50 AM, Paul Sincaglia wrote:
>
>> However, as the forum has
>> been kicking about for days now...Hazen-Williams overestimates friction
>> losses at higher pipe velocities. For our normal demand calculations
>
>
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