Environmental Engineering Reference
In-Depth Information
TABLE 9.8. Measures of Line Plumes in Stratified
Environments
h
l
z
l
z
l
x
l
n
n
m
n
Reference
N
N
N
N
Stagnant:
1.5 1.7
2.5
2.3 Daviero and
Roberts
(2006)
1.8 1.7
-
-
Roberts et al.
(1989a)
Flowing:
2.2 1.4
F
1/6
(
F
> 0.3)
2.4
F
1/6
(
F
> 0.3)
-
Tian et al.
(2006)
-
1.7 (
F
< 0.1)
3.2 (
F
< 0.1)
-
Tian et al.
(2006)
-
1.5
F
1/6
(
F
> 0.3)
-
-
Roberts et al.
(1989b)
Figure 9.11.
Diffuser ports.
Source
: Red Valve Co., Inc.
velocities through the discharge ports are sufficient to
prevent seawater intrusion into the diffuser, (3) the port
diameters are large enough to prevent clogging, and (4)
the velocities in the diffuser pipe are sufficient to prevent
deposition of suspended material within the pipe.
The discharge from each port,
Q
0
(lT
−3
), can be esti-
mated using the orifice equation:
equal regardless of the total discharge. However, in cases
where the diffuser is laid on a slope, the distribution of
port discharges depends on the total effluent discharge,
and is not the same for all flows.
For a diffuser to flow full, the ratio of the total port
area downstream of any pipe section to the area of the
pipe section should not exceed 0.7 and should ideally
be in the range of 0.3-0.7 (Fischer et al., 1979). This
criterion usually requires that the diameter of the dif-
fuser pipe be reduced at discrete points along the
diffuser.
Seawater intrusion into the diffuser can be prevented
by keeping the port Froude number,
F
p
, much greater
than unity (Wood et al., 1993), where
F
p
is defined by
∆
(9.49)
Q C A
=
2
g h
0
D p
where
C
D
is a discharge coefficient (dimensionless),
A
p
is the port area (l
2
),
g
is the acceleration due to gravity
(lT
−2
), and Δ
h
is the difference in total head across the
port (l). For ports cast directly into the wall of a dif-
fuser,
C
D
can be estimated by (Fischer et al., 1979)
u
g D
e
F
=
(9.52)
p
′
2
V
g h
V
g h
d
(
)
0 975 1
.
−
rounded entrance
(9.50)
2
∆
where
u
e
is the port discharge velocity (lT
−1
),
g
′ is the
effective gravity (lT
−2
), and
D
is the port diameter (l).
Ports must be smooth, bellmouthed, large enough to
prevent clogging, and made of material resistant to
mussel and weed growth. There is much debate concern-
ing minimum port sizes, with typical recommended sizes
of 200 mm (8 in) for unscreened effluent (Brown, 1988),
65 mm (2.5 in) for secondary-treated effluent, and
50 mm (2 in) for tertiary-treated effluent (Wilkinson,
1990). In some cases, the diffuser port is covered by a
rubber valve that collapses to prevent seawater flow
into the diffuser. Such a rubber port valve is shown in
Figure 9.11, where a diffuser port without the valve is
also shown.
The diffuser diameter must be such that the velocity
in the diffuser exceeds the critical velocity required to
C
=
D
2
d
(
)
0 63 0 58
2
.
−
.
s
harp entrance
(9.51)
∆
where
V
d
is the velocity in the diffuser pipe (lT
−1
). The
head outside a diffuser typically remains constant along
the diffuser and equal to the elevation of the ocean
surface, and frictional head losses within the diffuser
cause the head difference, Δ
h
, across the ports to decrease
with distance along the diffuser. According to Equation
(9.49), to maintain the same discharge through all ports,
the port area,
A
p
, must be increased along the diffuser to
compensate for the decrease in the head difference, Δ
h
.
Head losses along the diffuser can be calculated using
the Darcy-Weisbach equation. For horizontal diffusers,
the ports can be sized such that the port discharges are
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