Dredging Experience – Discover Dredging

Another Fascinating Exhibit To Help You See Through The Dust

New settling and sedimentation exhibit at the Damen Dredging Experience

Sometimes, explaining a part of the dredging process can be as simple as seeing through the mixture. In this exhibit we can demonstrate what happens beneath the surface of the cargo in the hopper1,2,3. Along the way, we explain some quirky behaviour in other phases of the dredging process, also. The exhibit consists of five tubes in a frame, that can rotate around a horizontal axis. In the tubes are various types of soils. Each with their own settling behaviour. The exhibit was recently added to the Damen Dredging Experience. One more reason to highlight it here.

Samples in the settling and sedimentation exhibit

One major part in the dredging process is the hydraulic transport of particles in a carrier fluid. Pickup and transport have been touched upon in previous posts4,5. Here we concentrate on the end of the process: settling and deposition. This can be either in a hopper or on the discharge area. In both cases you will only see the fluid surface during the process and at best the top of the deposited sediment. How the material came there, was deposited and stacked up can’t be readily seen. As the tubes allow these processes to be observed from the side, we can follow the events.

The exhibit can be started by upending the frame with the tubes. The material that sat in the lower end gets now on the top end. They all are released at the same time and we see immediately see the differences in settling velocity for the different particle sizes6. The gravel falls down within ten seconds. The sand is slower and the clay even has problems getting started. One nice observation is the mixture of soils. Against the height of the tube, the fractions in the sample are released simultaneously. Still, the fractions separate over the fall height and stack up again in their original order. This not only happens in the tube. In the hopper or the discharge area, a widely graded sediment will sort itself to the various fractions.

Table of irregular shapes (Source: Wikipedia)

Although for all the samples the particles are released simultaneously, you can still see a slight difference in settling velocity within each sample. This can be either due to slight variations in size that are possible within each mesh size used for sieving. Another cause for the differences might be the differences in shape. A perfectly spherical particle will have a faster settling velocity than an oddly shaped potato7.

Explanation of terminal velocity and hindered settling

And even then, the initial particles that fall down have a greater velocity than the particles in the bulk of the sample, even when having the same particle size and shape. This is due to the water flowing up around the particle. The upward flow is slowing down an adjacent particle. This interaction is called ‘hindered settling’. At high concentrations this can contribute to the efficiency of pipe line transport8. But for the settling it is really hindering the loading time.
At the very end of the settling, the particle gets deposited at the bottom, or on top of another. The water that is caught in between has to escape. This causes one last puff of fluid to flow upward and take the very find dust present upward. This happens with each particle that settles and causes the layer of dust to lift to the surface of the deposited sediment. So even when loading a cargo of gravel, you will always end up with a layer of dust on top. So, don’t judge the quality of the cargo just by the dust you see on top. Take a deeper sample or base your evaluation on the signals from the sensors from the screening tower.

Full cargo load of gravel, covered with dust. And the seagulls know that the dust layer also collects all the snacks

References

2021: Muddling Along To A Healthy Year In Dredging

Calendar picture of a sunny dredging site

Dear readers, I sincerely wish you all the best for this new year. As things are looking positive on the vaccine front, we should keep testing negative this year. There are lots of items and events to look forward. Last year was not quite what we’ve hoped for and wished each other.

I do hope all of you are OK and are still together with your loved ones. I am fortunately in that respect. So, next to the Covid situation, there were a lot of other noteworthy items. Most of them have been featured here on Discover Dredging. My trip to Bangladesh, just before all the lockdown measures. The graduation of Carsten1 and Omar2 that were completely over Teams. Some attention to our Damen Dredging Experience3. And some other items. Did you enjoy the Donald Duck review?4

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

One sad event was, the last issue of Dredging and Port Construction5. We’ve had to say goodbye to our trusted DPC. Although by now, there has been a kind of digital revival on the CEDA Website6. They opened up a section for Industry News, edited by Namrata Nadkarni. It stands out from the other online media covering dredging as there has been some attention to smaller topics, that would normally not get covered. As I think it is a good initiative, I urge you to subscribe and also participate by sending them any noteworthy news items for publication. Not specifically the usual company press releases, but also the more thought provocative opinions and perspectives on the activities of our dredging community. Active participation could make this CEDA Industry News flourish this year.

Announcement of the CEDA Industry News section (Credit:CEDA)

Further personal activities within CEDA are the Dredging Management Commission7 and I am looking forward to the exciting new concept for the CEDA Dredging Days8. Will you participate? Also, I already took the course on ‘Dredging for Sustainable Infrastructure’9, but you should keep an eye out on the next instalment, coming soon10.

A continuation from last year on my Discover Dredging will be some more articles on our Damen Dredging Experience. There are still some exhibit left to review and teaser:… There are some new exhibits arriving! Hope to have that covered when the pandemic will fizzle out we can show you around through our museum. And I hope to continue on pump and dredge technology.

Overview of the Damen Dredging Experience

Currently, there are some students working on their graduation thesis. Their very interesting topics will be highlighted here as well. Some others will start soon. When you are looking for a fun place to have your internship our your graduation, you might consider a visit to our career page11 or contacting Frank Bosman for any opportunities. Although most positions are filled at the moment, later this year we would welcome fresh brains again.

As our offices will be closed for regular work, I will be working from home most of the time. Probably most of you will do likewise. To keep track of the passage of time over the days, you should have a calendar. As a special service to my audience, I was granted the use of a set of beautiful pictures to create a calendar. You can download the file here and with some DIY skills you should have some original wall decoration for this year.

Stay healthy and stay safe. Hope to see you later this year.

References

Increase Your Dredging Knowledge At The End Of The Discharge 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

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.

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

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

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)

Tag: Dredging Experience

Another Fascinating Exhibit To Help You See Through The Dust

References

See also

2021: Muddling Along To A Healthy Year In Dredging

References

See also

Increase Your Dredging Knowledge At The End Of The Discharge Line

References

See also