8/10/2019 Pipe Sizing ASHRAE
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33.4
Allowances for expected decreases in capacity are sometimes
treated as a specific amount (percentage) . Dawson and
Bowman
(1933)
added an
allowance
of 150Jo
friction loss to new pipe
(equivalent to an 8 decrease in capacity). Hennington,
Durham,
and Richardson 1981) increased the friction Joss by
15
to 20 for
closed piping systems
and 75
to 90 for open systems. Carr ier
(1960) indicates a factor
of
approximately
1.75
between friction
factors for closed
and
open systems.
Obrecht and Pourbaix (1 7) differentiated between the
corrosive
potential
of
different metals in potable water systems and concluded
that iron is the most severely attacked, then galvanized steel,
lead,
copper, and finally copper alloys (i.e. brass). Hunter
1941)
and
Feeman
1941)
showed the same trend. After four years
of
cold and
hot water use, copper pipe ha d a capacity loss of
25
to 65 . Aged
ferrous pipe has a capacity lo
ss
of
40to
80 . Smith 1983) recom
mended increasing the design dischange by 1.55 for uncoated
cast
iron,
1.08 for iron and steel, and 1.06 for cement or concrete.
The Plastic Pipe Institute
1971)
found
that
corrosion is not a
problem in plastic pipe, the capacity
of
plastic pipe used in Europe
and the United States remaining essentially the same after 30
years
in use.
Extensive age-related flow
data
are available for use with the
Hazen-Williams empirical equation. Difficulties arise in its
ap
plication, however, because the original Hazen-Williams
rough
ness coefficients are valid only for the specific pipe
diameters,
water velocities, and water viscosities used in the original
ex
periments. Th
us,
when the Cs are extended to different diameters,
velocities, and/or water viscosities, errors
of
up to about 50 in
pipe capacity can occur (Williams
and Hazen 1933, Sanks 1978).
Water Hammer
When any moving fluid (not just water) is abrupt ly stopped
as
when a valve closes suddenly, large pressures can develop.
While
detailed analysis requires knowledge
of
the elastic properties of
the pipe and the flow-time history, the limiting case of rigid
pipe
and instantaneous closure is simple to calculate. Under
these
condjtions,
(9)
where
Ph = pressure ri
se
caused by water hammer, lbr/ft
2
e = fluid densi
ty,
lbn/ ft
3
c
=
velocity
of
sound in the fluid, ft / s
V
=
fluid flow velocity, ft/s
c for water is 4720 ft / s, although the elasticity
of
the pipe
reduces the effective value.
30
20
:::
0
10
.-
8
-
:::
6
u
I)
4
0
3
J
7
I
I li
l/ 1
I 11
I
Cl
2
(
w
I
0.5
0.5
2 3 4 6 810 20 30 40 60 8 KlO
1989 Fundamentals Handbook
Example 3. What
is
the maximum pressure
ri
se if water flowing at
10
ft/s is stopped instantaneously?
Solution
Ph
= 62.4 x 4720 x 10/ 32.2 =
91468 lb
/ ft
2
= 635
psi
Other Considerations
Not discussed in detail in this chapter,
but of
potentially great
importance are a number
of
physical and chemical considerations:
pipe
and
fitting design, materials,
and
joining methods must be
appropriate for working pressures and temperatures
encountered,
as well as ~ i n g suitably resistant to chemical attack by the
fluid.
Other Piping Materials and Fluids
For fluids not included in this chapter
or
for piping materials
of
different dimensions, manufacturer's literature frequently
sup
plies pressure drop charts. The Darcy-Weisbach equation and
the
Moody chart
or
the Colebrook equation can
be
used as an alter
native to pressure drop charts
or
tables.
HOI
AND CHILLED WATER
PIPE
SIZING
The Darcy-Weisbach equation with friction factors from the
Moody chart or Colebrook equation (or, alternatively, the
Hazen
Williams equation)
is
fundamental to calculating pre
ss
ure drop
in hot and chilled water piping; however, charts calculated
from
these equations (such as Figures I,
2, and
3) provide easy deter
mination
of
pressure drops for specific fluids and pipe standards.
In addition, tables
of pressure drops can be found in Hydraulic
Institute (1979)
and
Crane Co. (1976).
Most tables and charts for water are calculated for
properties
at
60 F. Using these for hot water introduces some error,
although
the answers are conservative;
i
cold water calculations overstate
the pressure drop for hot water. Using 60 F water charts for 200
F
water should
not
result in errors in
J.p
exceeding 20 .
Range of Usage of Pressure Drop Charts
General Design Range. The general ran ge
of
pipe frict ion
Joss
used for design
of
hydronic systems
is
between I
and
4 ft/
100
ft.
A value
of
2.5 ft/100 ft represents the
mean
to which most systems
are designed. Wider ranges may be used in specific designs, if
cer
tain precautions are taken.
Piping Noise. Closed loop hydronic system pi ping is
generally
sized below certain arbi trary upper limits, such as a velocity
lim it
of 4 fps for 2-in. pipe and under, and a pressure drop limit of
4 ft
I
....
....
T J
1/
' I Ill l I I r .....
200
3 4
r oo m
2000 300l 10000 40000
100
00
FLOW
RATE
U S
gal/min
Fig. 1 Friction Loss for Water in Commercial Stee Pipe (Schedule
40)
