March 4, 2010
Determining The Size Of The Hydraulic Dredge
Needed For A Dredging Project.
By Don Searles
Dredging Specialists
1. Determine
the volume of material to be dredged in cubic yards or cubic meters.
1.1.
English System:
To get cubic yards. Take the length in feet x the width in feet of the
area to be dredged x the depth in feet of the material to be dredged,
divided by 27 to get cubic yards. If you know the number of acres to be
dredged, take acres x 1613 x depth of material in feet to be dredged to
get cubic yards.
1.2.
Metric System:
To
get cubic meters. Take length in meters x width in meters of the area
to be dredged x depth in meters of the material to be dredged. If you
know the hectares of the area to be dredged, take the number of hectares
to be dredged x the depth in meters, of the material to be dredged x
10,000 to get cubic meters.
2. Determine
the time allowed to complete the project. Days, weeks and months.
3. Determine
the hours that you will work. Per day, per week. Will you run 1 shift,
2 shifts or 3 shifts? Could be 3 shifts 24/7.
4. Determine
the type of material to be dredged. Fine sand, medium sand, large sand,
small gravel, large gravel, silt which is mud, clay, cobbles 3 to 10
inches[76 to 254mm], boulders +10 inches [254mm]. When dredging a lake,
the material will most likely be mud.
5. Determine
if you will be dredging sediments or undisturbed material. Undisturbed
material is much more difficult to dredge. Stay away from dredging
undisturbed material unless there is a valid reason like the depth
needed for large ship passage.
6. Determine
the horizontal pumping distance. Maximum and minimum.
7. Determine
the vertical pumping distance called static head. This is the vertical
distance from the water surface to the discharge point.
8. Determine
the altitude above sea level where the project is located.
9. Determine
the maximum digging depth. This is the distance from the water surface
to the lowest point of dredging.
Only after the above 9 items have been determined is it
possible to determine the size of the dredge, the inside diameter of the
pipeline and the horsepower of the engines needed to do the job.
Dredge Sizes:
The size of a hydraulic dredge is determined by the inside diameter of
the discharge flange on the dredge pump. Dredges come in sizes from 8
inch [203mm] to 36 inch [914mm]. An 8 inch [203mm] dredge will
typically pump 2,500 gpm [gallons per minute] [9,464 lpm] with an engine
horsepower of around 300 [223 kw] and a weight of 25 tons. A 36 inch
dredge [914 mm] will pump up to 70,000 gpm [265 cmm] with up to 10,000
horsepower [7,456 kw] and a weight of several thousand tons. With a
given size dredge the production can vary widely and is determined by
the outside diameter of the impeller in the dredge pump, the speed of
the impeller, the horsepower of the engine driving the dredge pump, and
the friction and static head losses in the pipeline.
Using booster pumps, pumping distances of 20 miles [57 km] have been
attained.
When items 1 through 9 are known, Dredging Specialists
can design the dredging system and recommend the dredge needed,
determine if booster pumps are needed, the size of pipe and many other
factors that are needed to complete a dredging project.
The
following chart will provide a ball
park estimate of dredge production.
1Dredge
Size 
2Production in Cubic Units
Per Pumping Hour
When Pumping Mud 
3Production Per Working
Hour 70% Of Pumping
Hour Production
Shown in Column 2. 
8 inch [203 mm] 
113 cu yd [86 cm] 
79 cu yd [60 cm] 
10 inch [254
mm] 
160 cu yd [122 cm] 
112 cu yd [84 cm] 
12 inch [305
mm] 
250 cu yd [191 cm] 
175 cu yd[134 cm] 
14 inch [356
mm] 
350 cu yd[268 cm] 
245 cu yd [188 cm] 
16 inch [406
mm] 
440 cu yd [336 cm] 
308 cu yd [235 cm] 
18 inch [457
mm] 
540 cu yd [382 cm] 
378 cu yd [267 cm] 
20 inch [508
mm] 
712 cu yd [544 cm] 
498 cu yd [381 cm] 
24 inch [610
mm] 
1,000 cu yd [765 cm] 
700 cu yd [536 cm] 
8
and 10 inch [203 to 254 mm] dredges can typically be hauled on one
truck. Larger dredges must be disassembled and hauled on several
trucks. For 810 and 12 inch [203254305 mm] dredges a large crane is
needed to launch them. Larger dredges are typically assembled at the
dredge site on shipways made of timbers, and pushed into the water with
bulldozers. Most dredges, 20 inch [508 mm] and larger, are not portable
and must be floated to the job.
