MEASUREMENT IN THE PIPELINE

ERDC TN-DOER-T6

September 2004

dense) and the final deposition manner. Cohesive soils tend to increase in volume when removed

from their in situ position. Hydraulic dredges usually bulk up sediment more than mechanical

dredges due to water entrainment. New work material tends to have higher initial bulking in the

placement area than maintenance material because it is usually in a more consolidated in situ

state. A general rule of thumb is the larger the grain size, the lower the bulking factor (for

relative approximations, sand can bulk up 1.0 to 1.2, silt 1.2 to 1.8, and clay 1.5 to 3.0 (USACE

Dredging Fundamentals PROSPECT Course, 1998)).

Measurement in the pipeline consists of an inline production meter that measures the slurry flow

velocity and density before it enters the hopper. Measurement in the means of conveyance

involves determining the volume and/or weight of the hopper load. Several American dredges

measure one or the other of these parameters to quantify production, but most U.S. hopper

dredges measure both to calculate the load's average density or some derivative thereof, i.e., tons

dry solids (Welp and Rosati 2000). Various methods (described later in this technical note) are

used to measure the hopper load weight and volume. Determining barge production by

"measurement in the bin" usually involves quantification of bin load volume. Bin load weight is

also used as a production parameter on barges, but on a more limited basis. This technical note

describes these different measurement methods and discusses their respective advantages and

disadvantages, capabilities, and limitations.

MEASUREMENT IN THE PIPELINE: A "measurement in the pipeline" dredge production

meter is a system that determines slurry velocity and density, and uses these two values to

calculate dredge production. The system usually consists of a density meter, flow meter, data

recorder, and an output display that indicates production in units of mass or volume per unit

time. The major types of flowmeters used on hopper dredges include electromagnetic and

Doppler acoustic devices, while slurry density is usually measured by a nuclear density device.

The electromagnetic flow meter works on the principle of electromagnetic induction and is

designed to measure the flow of conductive liquids in a pipe. Two electromagnetic coils

surround a pipe made of anti-magnetic materials and produce a magnetic field at right angles to

the flow direction. As a conductive liquid passes the metering section, the lines of force from the

magnetic field are cut, producing a low-level voltage at the stainless steel pick-up electrodes. The

electrodes measure the potential difference, which is proportional to the flow rate and

independent of the solids concentration (Herbich et al. 1992). The Doppler flow meter uses the

theory of the 'Doppler effect'; i.e. there is an apparent change in the frequency of sound, light, or

radiowaves as a function of motion. These meters consist of a piezoelectric crystal transducer, a

Doppler frequency receiver, and a transmitter. The transmitter sends a continuous ultrasonic

signal at an angle to the direction of flow through the pipe wall and into the liquid stream. The

sound waves are reflected back to the receiver by particles, bubbles, or other discontinuities in

the liquid. The difference between the transmitted and the reflected frequencies, called the

`Doppler shift,' is analyzed and the flow rate of the slurry is displayed (Herbich et al. 1992). The

nuclear density gauge (Figure 1) measures density using the energy-absorption method. A

radioactive source emits gamma-ray energy through the discharge pipe. The rays are absorbed in

proportion to the density of the slurry, and a detector handles the gamma ray energy. The

transmitted energy is finally converted into a linearized output, which indicates density changes.