Geoscience Reference
In-Depth Information
grain-size distribution is of the sand to be used. A particle size distribution curve can
determine the average diameter of the sand.
DDDDD
DDDD
D
++++
DD
+
DD
+
(5.1)
50
80
90
D
10
20
30
40
60
70
=
mf
9
where:
D
mf
=
average grain diameter [m].
Once the average grain diameter is established, the critical pump speed (
v
cr
) for
an appropriate pipe diameter (
∅
pipe
) can be determined. Führboter among others [49]
developed experimental formulae to determine
v
cr
given different values of
D
mf
and
∅
pipe
. Table 5.1 below shows the variation in
v
cr
for different pipe diameters
∅
pipe
and
average sand grain diameters
D
mf
.
Because the critical pump speed is the speed where sand deposition in the pressure
pipe just does not occur; in practice a minimum pump speed
0.5 m/s is used.
Higher speeds are not advisable since the required capacity (and thus energy consump-
tion) and wear and tear of the pump increases by the square of the pump speed.
Various techniques are used to feed the sand-water mixture into the sand pump:
=
v
cr
+
The sand may be supplied from barges or ships and can be gathered from the bot-
tom using a suction dredger. Another option is to use a sand pump: In this case a
crane unloads the barges and then deposits the sand into the sand pump.
The sand is extracted using a cutter-suction dredger or suction dredger and
pumped directly into the sand pump. This method generally applies only to small-
scale work using a suction dredger of limited capacity. Normally, the capacity of
modern dredgers is too high to fill the tubes directly.
Table 5.1
Critical pump speed
v
cr
as a function of sand quality and pipe diameter.
∅
pipe
in m
0.30
D
mf
sand
in
μ
m
Sand type
0.35
0.40
0.45
0.50
100
Fine
2.40 m/s
2.59 m/s
2.77 m/s
2.94 m/s
3.10 m/s
200
Moderately fine
3.09 m/s
3.34 m/s
3.57 m/s
3.78 m/s
3.99 m/s
300
Moderately coarse
3.43 m/s
3.71 m/s
3.96 m/s
4.20 m/s
4.43 m/s
Table 5.2
Sand production
Q
[m
3
/hr] as a function of the sand quality, the critical pump speed (based
on
v
cr
+
0.5 m/s from Table 5.1) and a sand/water mixture ratio of 20%.
∅
pipe
in m
0.30
D
mf
sand
in mm
Sand type
0.35
0.40
0.45
0.50
0.100
Fine
148 m
3
/hr
214 m
3
/hr
296 m
3
/hr
394 m
3
/hr
509 m
3
/hr
0.200
Moderately fine
183 m
3
/hr
266 m
3
/hr
368 m
3
/hr
490 m
3
/hr
634 m
3
/hr
0.300
Moderately coarse
200 m
3
/hr
291 m
3
/hr
403 m
3
/hr
538 m
3
/hr
697 m
3
/hr
