User Guide

42 l uponorengineering.com
Comparing Darcy-Weisbach and Hazen-Williams
NSF performed testing to calculate the friction loss of Uponor PEX pipe and Uponor ProPEX ttings. The testing allowed Uponor to
analyze empirical test data and compare it with the Darcy-Weisbach and Hazen-Williams methods.
The following graph illustrates the comparison.
The Y axis represents the percent error from the test data; the X axis represents nominal pipe size. The graph shows
the average error when using the Darcy-Weisbach method is less than 1 percent when compared to the test data.
The Hazen-Williams method, however, yields an 18 percent average error compared to the test data.
Note: The test was performed with 70°F (21.1°C), 100 percent water.
Friction loss of ttings
There are two commonly
accepted methods when
analyzing pressure loss or
head loss of ttings. The rst
and preferred method uses C
v
values to precisely calculate
tting losses. The C
v
value
represents how many gpm can
ow through a tting at 1 psi
pressure drop.
For example, a tting with a C
v
of 5.0 would ow 5.0 gpm at a
1 psi pressure drop across the
tting. Since C
v
is a function
of ow rate versus pressure
drop, it yields an accurate
representation of tting friction
loss. The second method
uses equivalent lengths.
Uponor has created charts
for both equivalent length and
C
v
. The equivalent lengths
were developed using a ow
velocity of 8 fps.
-10%
-5%
0%
5%
10%
15%
20%
25%
30%
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Pipe size — nominal
Percent error from test data
Darcy-Weisbach % difference from test
Darcy-Weisbach average error from test
Hazen-Williams % difference from test
Hazen-Williams average error from test
Figure 4-8: Darcy-Weisbach and Hazen-Williams comparison