Novel Density Measurement By A Little Dredging Engineer

 

Design of an experiment to test a novel density measurement.
Design of an experiment to test a novel density measurement.

Innovative ideas need an open mind, not hindered by trodden paths of thought that limit one’s ability to be creative. And whose mind is open and flexible enough to trust such a task? Children! In order to keep me sharp, I sometimes discuss tricky problems at the dinner table with my daughter. Lately I presented her with the problem of density measurement in a slurry pipeline. There is a well-known technology: nuclear radiation dissipation detection. But there are already two words in that concept that we are asked to replace with a safer and more environmentally friendly alternative1. Her solution was to measure how much particles would stick on a glue lathered rod held in the mixture flow. During one of my daughter’s holidays from school, we took the opportunity to test her novel concept.

Preparations for the density measurement experiment.
Preparations for the density measurement experiment.

If this is going to be a novel measuring technique, we should prove, that there is a relation between the amount of particles sticking to the glue and the density of the mixture. This had to be tested in a controlled experimental environment: a bucket. We filled it with a known amount of water and by adding sand to it, we gradually increased the density. There were several types of glue to be tested, which we applied to short pieces of PVC tube. Glues we considered were: hobby glue, mounting kit, wall paper glue, contact adhesive and duct tape. There was one reference tube with no glue. We kept the samples in the mixture for 30 seconds, while vigorously stirring the slurry with a mixing drill.

Density measurement in progress.
Density measurement in progress.

Before and after the sample was dipped in the slurry, we measured the mass of the sample on a scale. The difference in mass should represent the amount of particles that stuck to the glue. These differences were recorded and plotted in a graph. Surprise! There is a distinctive trend in the data points that would support our hypothesis on the relation between mixture density and particle capture.

Results of the density measurement.
Results of the density measurement.

Some remarks on these results. There is a wide variation in trend lines. Some of the samples became lighter than they started. Probably, because they dissolved in the mixture. The measurements for the 600 and 1200 grams of diluted sand is notable. This is probably related to an observer variability. The strongest relation between mixture density and grain capture is with contact adhesive and duct tape. Interestingly, the best predictor for the average capture over the glue types was the reference tube. Probably, the water clinging to the tube already contained enough particles to result in a measurable signal.

It was fun to do this exercise and the results are interesting. But, I doubt we can make a viable product out of this concept. In the dredging industry, we prefer a non-invasive technology, the sample rod would be knocked out in seconds. Several manufacturers are already developing alternatives2,3. In our ¡VAMOS! project, we tested one concept, that seems very promising and you will definitely hear more about it in due time. It is time to say good bye to the nuclear density sensor and adopt technologies that are future proof. Our children will be grateful if we leave them a better world.

Non-radioactive density measurement (Credit: ¡VAMOS!).
Non-radioactive density measurement (Credit: ¡VAMOS!).

References

  1. Regeling bekendmaking rechtvaardiging gebruik van ioniserende straling, Staatcourant NL
  2. Magnetic resonance multiphase flowmeters, Krohne
  3. ITS exhibiting at Europort and CEDA Dredging Days 2017, ITS

See also

Production management, Damen

Discussion at LinkedIn post

The Dredge That Refused to Work on Monday Morning

Cutter wheel dredge ‘Sylvia’ at work on the TIWAG Langkampfen power dam reservoir

One of the first commissioning jobs I had to do for my previous employer, was on the dredge ‘Sylvia’, as she refused to work on Monday morning. And no one knew why. She was purchased by the TIWAG, as they recognised that power dams are a blocking the natural sediment transport and could potentially damage the turbines in the power dam1. To prevent anything serious happening, the power dams were fitted with ‘silt traps’. TIWAG ordered a dredge to clean up the silt trap and make some money on the excavated sand, classified by a dewatering bucket wheel on shore2.

Overview of the TIWAG dredge ‘Sylvia’ and dewatering bucket wheel at Kufstein

At a power dam you would expect, that electric power wouldn’t be a problem. Well, those power dams generate power, but they don’t like to let you plug in your dredge. So, ‘Sylvia’ had to have her own power generation with a diesel engine. As power dams are usually high in the mountains, the dredge pump should preferably be placed on the ladder. The natural setup would then be an electric drive. On ‘Sylvia’ the classic and reliable electric shaft principle was chosen. The diesel engine speed is transferred to the dredge pump by the variable frequency. Depending on the generator speed, the field coil is excitated according to a controller, that can act as some sort of electrical clutch.

Explanation of the electric shaft arrangement on dredge ‘Sylvia’

One thing to be aware of with an electric shaft, is the starting current of the electric motor. The electrical engineer that designed the system just worked with the nominal operating point. I wasn’t involved in the choices, but nowadays I will warn people to about this misconception.

If the pump is started on an empty pipe, even with a moderate speed, the dredge pump receives no resistance and the capacity through the pump increases enormously. The power requirements increase to a level above the nominal operating point! After several blown fuses and an incinerated generator the system was slightly modified. Also, to restrict the power surge, a discharge valve was installed to ramp up against. At zero capacity, the pump requires virtually no power.

Start-up procedures plotted in pump curves

The difficulty with ‘Sylvia’, was, that there was no possibility to hook up the discharge valve to the hydraulic system. Instead, the valve was connected to the working air compressor. It primarily served as an air source for the bubble point to measure the operating depth of the dredge wheel.

Retrofitting a pneumatic operated gate valve to the pneumatic system on board

After installation, this worked fine. No blown fuses anymore. Should be fine. Until next Monday morning. Fuses tripping at every start-up. Only after several hours she would run normally. It turned out, the gate did not close completely under pressure. It required the full pneumatic operating pressure to close all the way down.

Aha! As the compressed air vessel was leaking air through the bubble point on the ladder over the weekend, there was not enough pressure to close the gate valve completely. As the mixture only needs a small opening to already draw a lot of capacity, the fuses tripped every Monday morning. Once the compressor was able to top off the vessel later in the morning, the gate valve did close completely and the dredge pump could start. A simple ball valve to close the bubble point remedied the leak and no more problems on that system again.

Characteristics of an opening gate valve

References

  1. Kraftwerk Langkampfen, TIWAG
  2. Several million tons of gravel extraction with a suction dredger, Stichweh

See also

A Reservoir of Dredging Opportunities