Sensible Flow Sensing Stories, The Correct Capacity Measurement

Example of a flow meter on a Cutter Suction Dredge
Example of a flow meter on a Cutter Suction Dredge

My last posting was a nice story about how they measured mixture velocity in the old days1. Luckily, we have a much better solution nowadays: the electromagnetic flux flow meter2. It is real time and can be viewed from the convenience of the operating cabin. This device can be part of the production measurement for a CSD, TSHD or sometimes in a separate production measurement unit.

Combined velocity and density indicator in the operating cabin
Combined velocity and density indicator in the operating cabin

The working principle of an electromagnetic flux flow meter is based on the Faraday laws of induction3. When a conductor moves through a magnetic field, a current will flow in the third direction perpendicular to both. Due to the resistance of the water, the resulting potential can be picked up by electrodes that are in contact with the mixture.

Explanation of the electromagnetic flux flow meter
Explanation of the electromagnetic flux flow meter

For the principle to work, the electrodes and the mixture have to be isolated from the housing. This is why you always have some isolating liner in this type of flow meters. Off course, the isolation material will wear down due to the abrasion of the mixture. Usually when working in relatively soft sediments, the isolation liner is made of durable polyurethane rubber. The electrodes are flush with the surface of the liner and are not much exposed to wear. When the liner is worn down, it can be easily replaced by the supplier. When working in a more abrasive environment, a more durable isolation liner can be chosen. e.g. Ceramic tiles embedded in a soft adhesive layer.

Arrangement of an electromagnetic flux flow meter
Arrangement of an electromagnetic flux flow meter

The measured voltage gets processed by an amplifier that has to be placed close by. The outgoing signal is mostly the usual 4-20mA and can be transmitted directly to a velocity indicator or a production indicator. Sure, it is good to have a high velocity, as that represents a good production. But it is also indicating a high power consumption. One is more sensible to increase the mixture density and decrease the velocity for an efficient production. To monitor the dredge process, both signals can be combined in a single indicator to present the production to the operator.

Example of different executions of production indicators
Example of different executions of production indicators

The left example is the classic ‘mechanical’ cross-needle indicator. Where the needles intersect, the production can be read on the lines between the scales. On the right, the rotating needles have been replaced by digital linear scales. The velocity is represented horizontally and the density vertically. Consequently, the production lines are also modified. Instead of a high production vertically in the centre, the highest production is now in the upper right corner.

These flow sensors are quite accurate and are reliably indicating the correct value. Still, it is good practice to check the indicated flows, after installation. This calibration can be done for one or two points. The easiest check point is with static water. The other point will be with some known flow. If the installation is on a TSHD, it is straightforward to fill the hopper. Be aware, that the flow has to be integrated over the filling time. For a CSD type application we may have to resort to the described end of pipe indicator from previous post. And if the values are off, erratic or otherwise not making sense, you might have to check whether the housing of the sensor is correctly grounded to both other flanges.

Ungrounded and correctly grounded housing of a flow sensor
Ungrounded and correctly grounded housing of a flow sensor

References

  1. Increase Your Dredging Knowledge At The End Of The Discharge Line, Discover Dredging
  2. Magnetic flow meter, Wikipedia
  3. Faraday’s law of induction, Wikipedia

See also

Increase Your Dredging Knowledge At The End Of The Discharge Line

Keeping watch at the end of the discharge pipe line
Keeping watch at the end of the discharge pipe line

Solving something at the end of the pipe is usually a less desired approach. However, in dredging, it is the place where the valuable stuff is delivered, it might be a good place to start monitoring your process. Let me explain this to you by going back to latest discussed exhibit at the Damen Dredging Experience1.

Pump power exhibit at the Damen Dredging Experience
Pump power exhibit at the Damen Dredging Experience

You might have observed in the pictures of the pump power exhibit, that the velocity of the water flow is indicated by the parabolas of the trajectory. The arc of water is bound by gravity and obeys this trajectory always; independent of the density of the mixture. The two equations of motion can be combined, where the time parameter falls away and the height for a certain distance is only depending on the initial horizontal velocity2. As such, it is fairly accurate indication of the pipe flow. The calculation is universally applicable on earth and the results can be presented in a very simple graph to take with you. Every parabola is labelled with the corresponding horizontal velocity.

Nomogram to find end of pipe velocity
Nomogram to find end of pipe velocity

The above example is a straightforward method to measure the mixture velocity. The US Geological Survey even extended this approach as a standard method to measure the production of wells3. The resulting nomogram has a slightly different layout, as it is intended for finding the production instead of the velocity. For production planning, this will be useful. For monitoring your dredging process, the velocity might be more important. Both approaches of this elegant method do have the benefit, that there is no obstruction needed as in the case of an orifice measurement4.

Nomogram to find the end of pipe production
Nomogram to find the end of pipe production

There is an unconfirmed anecdote that my old professor de Koning started his career as a velocity measurer. In the old days, when he was working as a twelve year old boy with the dredging company of his father. He was assigned to keep watch at the end of the pipe and monitor the mixture pouring out. He had a simple beam with a plumb bob. The beam was moved along the top of the pipe, until the plumb bob was touching the arc of mixture. On the beam were two markings. When the beam was moved in and passed the first mark, the mixture velocity was too low and a red warning flag had to be displayed. If the beam had to move out and the mixture velocity was too high at the second mark, a green flag had to be flown. There was also another white flag, in case only water came on the reclamation area. With this very simple setup, the dredge master could check through his binoculars what the state of the dredging process was.

Working principle and explanation of end of pipe meter
Working principle and explanation of end of pipe meter

They were clever in those days. But the physics still apply. So, even today, one might have a situation, where there is no electronic velocity measurement available (broken, not supplied, not (yet) purchased) and you have to push the limits of the operating envelope of the dredging process. Then, there is probably always somebody around that might be appointed volunteer to be head of the velocity measurement crew. Who knows, he might have a bright future in the dredging academia.

Professor de Koning of the dredging chair at the TU Delft (1981-1993)
Professor de Koning of the dredging chair at the TU Delft (1981-1993)

References

  1. Presenting Pump Power Peculiarities, Playing With Pumps And Pipes, Discover Dredging
  2. Projectile motion, Wikipedia
  3. Estimating discharge from a pumped well by use of the trajectory free-fall or jet-flow method, US Geological Survey
  4. ISO 5167 Measurement of fluid flow by means of pressure differential devices inserted in circular cross-section conduits running full, ISO

See also

Innovations In The New MAD Series To Increase Uptime And Reduce Fuel Consumption

Innovative Marine Aggregate Dredge for gravel dredging
Innovative Marine Aggregate Dredge for gravel dredging

This week, there will be a lot of interesting presentations at the CEDA Dredging Days 2019 in Rotterdam1. I would like to draw your attention to one particular presentation that I was involved with at certain moments in the project, though not in writing the paper. Kudos to my colleagues Frank & Frank to write the interesting manuscript2.

The topic of the presentation will be the change of perspective for the concept of marine aggregate dredging. Historically, the marine aggregate dredging takes place relatively close to shore. With the depletion of the deposits and the increase in demand, other locations further out at sea are coming into focus. As Damen, with a heritage in the design of offshore operating vessels, it was a natural choice to cross breed the offshore supply vessels with the marine aggregate dredges. The resulting offspring: the MAD series of hopper dredges3. Frank de Hoogh will elaborate on the seakeeping abilities of this innovative design.

Other dredging related innovations are the suction tube and ancillary equipment, the screening towers and the process sensors. Of those, I have some personal anecdotes on the screening towers and the density sensor. For all other interesting stories, you’ll have to attend the presentation.

The screening towers are fundamental to the efficiency of the process. If the screening is improved, shorter dredge cycles are possible and a better product can be landed onshore. Also, if the requirement for the product change, the screens have to be adapted to the new specifications. Ideally, this changing has to be done at one unloading phase, otherwise you miss a complete dredge cycle. A lot of effort has been done to optimise the design. But the real test was to actually build, modify and test the complete screening tower, before it was even installed on the vessel. So, we had this construction right here at our doorstep for a thorough evaluation.

Screening towers for fit and fat testing at our yard
Screening towers for fit and fat testing at our yard

One other component, that I was even more involved with, was the non-radioactive density sensor. There are regulations in place to phase out nuclear density sensors4 and a lot of alternatives are available. Back in the !VAMOS! project5, we had the opportunity to test a unit of an electro tomography system. The results indicated a good reliability and a worthy replacement for the traditional nuclear sensor6. Because of the tomography picture, there was an additional benefit: we received an early warning on the slurry behaviour. We could actually see when we were too close to the deposition limit in the pipe line. This enabled us to work with higher densities at lower velocities, resulting in better efficiency and less wear. As the rough process conditions in the mining pit were similar to the marine aggregate dredging industry, we proposed to use this on this MAD also. How we further developed and tested this system is for you to hear and see at the presentation.

Testing the non-radioactive density sensor
Testing the non-radioactive density sensor

Due to the physical processes involved in slurry transport, the mixture does not behave like a normal Newtonian fluid. It is some non-linear viscous substance. At high speeds and low concentrations, it is similar to the carrier water. Slowing down, there is a certain critical speed, where there is a minimum hydraulic gradient. At that flow condition, the specific power consumption of moving a cubic meter of soil is the lowest. So, although working at critical speed is dangerous, it has its advantages: low fuel consumption and less wear. Actually seeing the mixture approaching this critical speed from the deposition is an interesting feature of this new density measuring sensor.

Explanation on slurry flow conditions, critical speed and specific power consumption
Explanation on slurry flow conditions, critical speed and specific power consumption

References

  1. CEDA Dredging Days 2019, CEDA
  2. Next generation marine aggregate dredger as platform for innovation and basis for fleet renewal, CEDA
  3. Damen unveils Marine Aggregate Dredger, Damen
  4. Regeling bekendmaking rechtvaardiging gebruik van ioniserende straling, Staatscourant
  5. Project ¡VAMOS! Let’s Go Real!
  6. Real time production efficiency based on combination of non-nuclear density and magnetic flow instrumentation, WEDA

See also