Graduation of Arend van Roon: Detecting Flow Regime And Optimising Transport Efficiency

Arend van Roon defending his graduation thesis
Arend van Roon defending his graduation thesis

Our first happy event this year is the graduation of Arend van Roon. He recently graduated on a project in the slurry test circuit at our Research and Development department at Damen Dredging Equipment1. His research was an interesting investigation in the detection of flow regimes. It gives some insight in the processes involved in the transport of water-solids mixtures. Let me share some details from the background with you, as I think this might be helpful for your own operation also.

Overview of the Damen Dredging Equipment slurry pumping test circuit
Overview of the Damen Dredging Equipment slurry pumping test circuit

At first sight, it is hard to imagine, how something heavier than water, the grains, can be lifted when the fluid is moving. Sape Miedema has written the standard on mixture transport in his book ‘Slurry Transport’, explaining his approach with the ‘Delft Head Loss & Limit Deposit Velocity Framework2’. Without going into the academic details, I will try to help you grasp the gist of the phenomena.

DHLLDV book (Credit: Sape Miedema)
Slurry Transport text book cover (Credit: Sape Miedema)

First the grains have to be picked up. When they are lying on the bottom of the pipe, they are fully immersed, surrounded by the fluid on all sides. The free fluid on top and the pore water between the grains under and on the side of the grains. Now comes Bernoulli’s trick. When the fluid in the pipe starts moving, he says that the local dynamic pressure decreases, while the static fluid in the pores remains at the same pressure. The pressure difference between the pressure in the pores and in the moving fluid, pushes the grains out of the bed into the fluid.

Grain pickup and suspension process explained
Grain pickup and suspension process explained

Once the particles are in the fluid are in the fluid, they should stay suspended, or they fall back into the bed. The driving force here is the turbulence in the fluid. Usually dredging slurry mixtures are turbulent. This turbulence causes the fluid to flow in eddies. These are little vortices that generally move in the direction of the flow, but in a moving frame of reference tumble in all directions. Mmh, as they rotate in all directions, why don’t they cancel each other out? Now, imagine being a particle yourself, surfing on those eddies. When it is in a fluid, it tends to sink with a certain settling velocity. Independent of the local movement of the fluid. This means, that on the downward side of the eddy, the particle has a higher total velocity than on the upward side. As the eddy is sort of symmetric, the particle dwells longer in the upward draft than on the downward fall. In this infinitesimal time difference, the eddy transfers some extra kinetic energy from the fluid to the potential energy of the particle. As this loss of kinetic energy is compensated by an increase in pressure (remember Bernoulli) carrying grains in a fluid increases the pressure loss in the slurry transport.

Flow regimes and excess hydraulic gradient requirements in dredging slurry transport (Credit: Sape Miedema)
Flow regimes and excess hydraulic gradient requirements in dredging slurry transport (Credit: Sape Miedema)

This turbulence is in short the background of suspension in the slurry transport process. Depending on al the various governing parameters: densities, viscosity, diameters, velocities etc, the equilibrium of forces result in several different regimes in the slurry flow. Ranging from homogeneous, through stratified to ultimately a static bed. Each with their own particular pressure losses. And that is what we are interested in. On our dredges, we want to transport as much material to the least amount of energy3. We are constantly looking for improvements in our equipment and sensors to assist the operator in visualising and controlling the actual state of his process4. Thanks to Arend’s project and the promising results, we can set the next step in our product development.

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

References

  1. Innovation, Damen
  2. Slurry Transport: Fundamentals, A Historical Overview & The Delft Head Loss & Limit Deposit Velocity Framework 2nd Edition, TU Delft
  3. Innovations In The New MAD Series To Increase Uptime And Reduce Fuel Consumption, Discover Dredging
  4. Dredging Instrumentation, Damen

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

Book Review: Donald Duck A Muddy Fine Business; Artistic Equipment Design

Front page of Penny Pincher magazine with Donald Duck as dredge master (Credit: Disney)
Front page of Penny Pincher magazine with Donald Duck as dredge master (Credit: Disney)

Donald Duck is a Jack of all trades, that he eventually he would end up on a dredge was inevitable. The story was already published in 1977 in a Donald Duck comic magazine. It is written by Freddy Milton1 and drawn by Daan Jippes2. At that time, I read it and already liked it very much. Later, I had it in a comic album3, but lost it moving to a new house. I wanted to review this story here already for a long time. Eventually, I consulted ‘Bul Super’ in Delft4, he advised me to search for Daan Jippes. That helped to find the story back on the internet.5

Opening scene of ‘Muddy Fine Business’ or ‘Success Test’ (Credit: Disney)
Opening scene of ‘Muddy Fine Business’ or ‘Success Test’ (Credit: Disney)

The story revolves around the endless feud between Donald Duck and Gladstone Gander. This time they have to compete in a success test by operating two different vessels: a ferry, ‘Seagull’ and a dredge ‘Aristoteles’. They have varying degrees of success and the outcome is unexpected.

The best known illustrator of Donald Duck is Carl Barks6. But Daan grew into his footsteps and eventually his stories are at the same level as the original master. He was with the Disney studios in the USA, but was working mostly in the Netherlands. In this edition, Jippes drew one of the most Dutch professions: dredging. Jippes had a keen eye for the elements that make up a dredge, tough the execution lacks some reality. Here I want to highlight some shortcomings that will help to illustrate the tricks that should have made it work and explain how real life dredging equipment functions.

Various details of the ‘Aristoteles’ (Modified from Disney)
Various details of the ‘Aristoteles’ (Modified from Disney)

The ‘Aristoteles’ is a truly multi-functional dredging vessel. It features both a grab and a bucket chain and the sediment can be loaded unto the vessel itself like a hopper or into a barge in tow. It is also self-propelled, although there seems to be a magical power generation as there is no visible exhaust pipe. Maybe Donald is again ahead of his time and running fully electric already?

Than the dredging equipment; the grab is suspended from a gantry, but it does not seem to be able to swing. Loading the tow barge would be difficult, as he has to reposition either the dredge or the barge. Apparently Donald should know about the technology of a rotating crane, as the picture on the front page has such a crane. And how would the material end up in the hopper?

Discharging buckets with reception carriage on a bucket ladder dredge
Discharging buckets with reception carriage on a bucket ladder dredge

Maybe with the other dredging tool: the obvious bucket chain? Though it is not supported on a ladder. Maybe that makes sense, as the buckets seem to be positioned on port or starboard in various panels. The material falling from the buckets might end up in the hopper. The vertical orientation presents some difficulty, as the material will fall onto the previous bucket and eventually through the well. In a normal operation, this is controlled by moving a carriage receiving the load and bringing it to the chutes. With a real vertical orientation, this would not be helpful anymore.

I once saw a solution for working with a vertical bucket chain on an exhibit in the National Dredging Museum7. A manual operated drawer-like slide was moved between every passing bucket to catch the load. It seems very labour intensive and prone to accidents.

Even after breaking down these operational details in the design of the ‘Aristoteles’, the vessel serves its purpose in the story: it is a really useful dredge for dredge master Donald. Well done Daan.

Exhibit with vertical ladder at the national dredging museum
Exhibit with vertical ladder at the national dredging museum

Call to the audience

The exhibit is still there, but unfortunately, it is broken. The mechanism has to be repaired, any model building fanatics are invited to help the museum restore it. There is a special event for new volunteers, now!

References

  1. Freddy Milton, Wikipedia
  2. Daan Jippes, Wikipedia
  3. Oom Dagobert En De Ondergrondse Kluis, nr34, Disney
  4. Stripboekhandel Bul Super
  5. Read online Walt Disney’s Comics Penny Pincher comic – Issue #4, ZipComic
  6. Carl Barks, Wikipedia
  7. Nationaal Baggermuesum

See also