Graduation Of Carsten Markus: Designing And Casting Of Impellers

Impeller under operating load.
Impeller under operating load.

Last week, Carsten Markus graduated on his assignment with our research department Damen Dredging Equipment. He investigated the alloys that are used to cast our impellers and the responses of the materials under operating conditions. We are always improving our dredge pumps and Carsten’s work has been a great contribution for our development.

For every dredging application, the material for the dredge pump parts can be carefully selected according the specific requirements on the sediment handled. Off course, one would like to have the hardest, most erosion resistant material available. Less erosion is less wear and a longer lifetime1.

Wear part material hardness in relation to wear index

However, there is a downside to choosing very hard material: it will be very brittle also2. Basically, the ultimate tensile stress coincides with the yield stress. There is no reserve for the load. When the stress surpasses the yield strength, it just snaps. Conversely, a tough material has a lot of reserve. After deformation beyond the yield stress, the load can still increase without a catastrophic failure. Usually, the stresses would not be that high, the thickness of the wear parts is dimensioned for erosion reserves and thick parts have low stresses. The high stresses can come from concentrations. Either when the wear reserves are eroded, or when a heavy load is concentrated on a very small area. Usually stones. Stones are a very common problem in a dredging project. So, next to the hardness of an alloy, also the toughness will be a very important characteristic. And toughness is related to the yield elongation after failure.

For the various materials you would like to know how the base load relates to the tensile stress. This would be an indication whether there is reserve in the elastic region to accommodate the impact of an occasional stone. This base load depends on many factors in the pumping process. Mainly the pressure generation in the mixture over the blades. Most known literature is about the force distribution in the volute of the pump, as this is directly related to the radial forces and consequently the bearing and shaft calculations. The CFD simulations in this graduation project revealed the skewed load on the volute and consequently the load variations on the impeller.

Transient CFD simulation of a dredge pump. The rotation appears wrong , but is correct (Wagon-wheel effect).

As emphasised before, it is very beneficial to operate your dredge system around the Best Efficiency Point (BEP). Not only the shaft and bearings suffer less, also the impact of the stresses and their variations in the impeller are less. If the operating point differs from the required capacity, the BEP can be moved by changing the dredge pump speed. As long as head requirements permit the adjustment.

Radial load as a function of pump characteristics.
Radial load as a function of pump characteristics.

As a result of Carsten’s research, we were able to improve our operating load models for the whole dredge pump and gained insight into the material responses to these. Thank you Carsten.

Due to the measures taken for the containment of the Corona virus, the graduation itself and the party to celebrate it, where done remotely over internet. Let’s enjoy the real beer later, after all this commotion is over. Stay home, stay safe.

Carsten’s graduation defence session under Covid-19 measures.
Carsten’s graduation defence session under Covid-19 measures.

References

  1. Do You Have Wear Parts For Spare?, Discover Dredging
  2. Brittleness, Wikipedia

See also

Which Teeth Will Survive The Cut? Adapting Your Selection

Me, explaining about our cutter systems
Me, explaining about our cutter systems

After my last post1, I received some comments and questions about the actual products we are applying in our cutter systems for our CSD’s2. Indeed, from a pure physical perspective, last post cuts to the heart of the processes, but does not explain our design of the working tool that makes a cutter suction dredge do its work.

Over the years, there has been a lot of development in this tool. Originally, suction dredges were plain suction dredges, working in non-cohesive sand. When the soil was more cohesive than could be dug with the standard suction dredges, attaching a mechanical device for loosening the ground enabled the suction dredge to work in this environment. From this original concept, the cutter head was already recognisable as a crown with teeth on a back ring and a suction mouth in the centre. From there, a lot of experimentation was done, but ultimately it all came back to this concept. Although modern cutter heads have a vastly improved performance and lifetime.

The cutting process in a modern cutter head is a combination of the rotation of the head and the swing of the dredge. The teeth describe a compound path of translation and rotation and each individual tooth has its own set of cutting parameters for depth and angle varying over time. Moreover, the combination of teeth on the different arms, allow for a staggering of the teeth that each tooth cuts fresh material and optimising the use of the teeth and spreading the wear. This results in a complicated geometry of the arms and a intricate pattern of the teeth.

Teeth system with adapters (left) teeth system direct on arm (right)
Teeth system with adapters (left) teeth system direct on arm (right)

Once a cutter design has been chosen, there is still some tuning possible. Normally, the teeth are fitted on adapters and there are several teeth types available for a certain adapter. Pick points, Chisels and Flares. Most productivity can be expected from the wider teeth. However, the penetration of the wide teeth is less. So, for harder material you want to select narrower teeth.

Adapter system (left), teeth range with adapter (top), teeth range direct on arm (bottom)
Adapter system (left), teeth range with adapter (top), teeth range direct on arm (bottom)

Wear is also an issue3. And as the teeth are in direct contact with the fresh material, the wear rates can be severe. The disadvantage of a teeth and adapter system is that that are quite big. So, less teeth fit on an arm, reducing production on average. As most of our CSD’s are working in more gentle sands we selected a cutter system, that provides more teeth to engage in the action, increasing production. As these teeth are fitted directly on the arms, there are no adapters that wear also. Consequently having no adapters, simpler arms and dirt cheap teeth result in a low investment low OPEX cutter system. Although you might have to check the state of your teeth more often, in the end you spend less money on a cubic meter produced.

Teeth in various stages of degradation
Teeth in various stages of degradation

Teeth can be worn down to the root. Also they are not wearing evenly. Usually, they last longer on the outside, near the back ring. You might consider using different tooth forms over the arm. Experience and practice, will guide you in selecting the best combination. In line with the previous post, the analogy will be on the table. Just as you select different teeth for your fork, you can select different teeth on your cutter depending on the dish being served.

Different teeth selection for tableware
Different teeth selection for tableware

References

  1. Experiencing The Cutting Edge Of Dredging Technology, Discover Dredging
  2. Cutter Suction Dredger, Damen
  3. Wear of Rock Cutting Tools, Peter Verhoef

See also

Don’t Play Games With Your Wear Part Planning

Board game for wear part manufacturing planning
Board game for wear part manufacturing planning

Last week we had another of our training courses for service engineers and field service engineers1. The interaction with people actually working with our products is quite refreshing and every time I understand their issues better. One of those issues is that they have to discuss with the client are spares for the wear parts. In a planned maintenance context, wear parts are a little odd. Sometimes, they are worn away or break down unexpectedly. And that is the moment customers call for spares. We do have a lot of spares on stock, but sometimes even we run out of stock or we advise to use a special execution of the concerned part for the specific operation of the client. And then we have to inform the service people and the client that there is a long lead time. Several times, they are filled with disbelief and under such circumstances it is very difficult to explain the reasons behind it. So, that is why I developed this little game to experience the waiting time for special wear parts.

Layout of the board for the wear part game
Layout of the board for the wear part game (Download pdf version here)

It is based on the old board game of ‘Snakes and Ladders’2. All it takes are the board, one dice and as much tokens as players. All start at the first position. The places are all phases in the manufacturing of the wear parts and each have their specific issues.

  1. Each pattern is used multiple times and wears down, itself. Also, some patterns have to be configured for the specific application, execution or material of the wear part.
  2. Moulding: the pattern is placed in a casting box and filled with sand.
  3. Sometimes there are more casting boxes needed and they have to be stacked carefully. Depending on the configuration, this step might be very short. Then you go directly from 2 to 4.
  4. Part of the casting system is already in the sand box with the pattern, but it has to be finished as the last part of the preparation.
  5. The material is melted in the furnace. This can take some time, depending on the size of the cast.
  6. The actual casting is done in minutes. Fifteen, at the most.
  7. But the cooling in the casting box takes weeks. Wait one turn.
  8. Sometimes the casting has not gone properly and the cast have to be done again. Back to square one.
  9. Satisfied with the cast, then it has to be touched up at the fettling station.
  10. A special heat treatment brings the final hardness and toughness to the product.
  11. The fitting surfaces of the wear parts have to be machined.
  12. Rotating parts have to be balanced. For non-rotating parts, this can be skipped.
  13. Then there is the bottle neck: quality control. If there is a deviation that can n ot be mitigated, you have to go back to square one.
  14. Depending on the location, transport can take weeks.
  15. Don’t start me about customs handling. Your anticipated spares are in bonded storage and customs is missing a document, wait some weeks or skip a turn.
  16. Finally, you’ve made it! Installation on the dredge.

Message of the game: keep your warehouse well stocked with wear parts3,4, or your dredge will be idle for months, before you can work again. Have fun!

Spare parts on stock
Spare parts on stock

References

  1. A well-trained team makes all the difference, Damen
  2. Snakes and Ladders, Wikipedia
  3. Do You Have Wear Parts For Spare?, Discover Dredging
  4. Options for Repairing Parts That Ought to be Replaced, Discover Dredging

See also