ERDC TN-DOER-T6
September 2004
Sand slurry flow results were more consistent and accurate than those for gravel.
The Doppler meters produced higher values than the control meter at low slurry velocities
but fell off significantly at higher slurry velocities, producing much lower values than the
control.
The most critical element in the use of production meters is the calibration of the individual
components.
A "pipeline" production meter system was temporarily installed on the dustpan dredge Jadwin to
evaluate the reliability and repeatability of the system and components (Pankow 1989) where the
data proved consistent and reasonable. Scott (1992) summarizes the testing and evaluation of a
production meter (electromagnetic flowmeter and nuclear density meter) installed on the hopper
dredge Wheeler while dredging in fine-grained sediment. This study indicated that, provided the
density and flow meters are calibrated and maintained, reliable, accurate density determination in
the pipeline is possible when dredging silts. The study concluded that use of production meters
for calculating production on a load-to-load basis may be limited due to overflow and leakage
through the hopper doors, but for calculating the total production of dredged material through the
pipeline for a given project, production meters can provide a reliable and accurate measurement
of dredge production (Scott 1992).
Rullens (1993) states that an accuracy of 2 to 3 percent can be obtained under ideal conditions
with calibrated instruments (electromagnetic flowmeter and nuclear density meter) and an
inverted u-tube pipeline configuration. However, space restrictions onboard hopper dredges
usually preclude the use of this optimum pipeline configuration. Additional error components
include:
Presence of gas in the slurry.
Flow that falls below critical velocity value for stationary deposition.
Use of an integration method over a period of time that multiplies measurement errors.
Difficulty in calibrating systems.
Very rapid variations in density and velocity in the pipe.
Presence of debris.
Wear and tear.
Rullens concludes that the velocity and density measurement method should only be used to
ensure optimum production in the dredging cycle of a hopper dredge under two conditions:
When in situ sediment density is very difficult to establish.
When overflow is used, but overflow losses are not measured.
Accurate measurement of in situ sediment density is essential for the accurate calculation of
volumetric production (Scott 2000). When the overflow method is used to optimize dredge
production it is impossible to measure absolute production of the dredge; the measurement of
velocity and density only gives relative information about the quantity and quality of the dredged
material (Rullens 1993).
4
Previous Page
