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

Do You Have Wear Parts For Spare?

Severely worn impeller. Though still with acceptable discharge head.

A dredge company makes its profit by economically transporting sand by mixing it with water. Unfortunately, this happens to be the best combination to literally ‘sand blast’ steel. Every effort should be made to reduce the wear and tear on the dredging components and especially the dredge pump. If not tenderly cared for, your dredge pump may erode away. Performance and profit will follow down the drain, also.

The background of the wear on the dredge components, is scratching. The small hard particles in the sediment are blasted against the surface of the wear parts. As the sand is usually harder than the steel, the steel gets scratched. Enough scratches on top of each other makes the wear. The principle of scratching different materials against each other was scientifically explored by Friedrich Mohs. Although the effects were already known by the ancient Greeks1. Mohs proposed a hardness scale, that is very practical and will give you a first estimate of the hardness2.

Mohs hardness scale in relation to tool material (Credit: National Parks Service)

On the left side are the classical Mohs minerals, that we also sometimes encounter in dredging. On the right side there is also a suggestion of tool material that is of comparable hardness. If you need to scratch on the mineral on the left, you need at least a tool of the corresponding hardness on the right. Quartz is a main component of sand. And from the scale, you can see, that a normal steel nail will not be tough enough to make a scratch. And that is exactly what we see in dredging. Wear parts for handling the soil are usually made of sophisticated alloys to be harder than sand. The wear rate reduces significantly beyond the hardness of sand. In selecting wear parts material3, we usually discuss the wear index. This is the factor in which a certain material lasts longer than normal construction steel under the same conditions.

Wear part material hardness in relation to wear index

There is a trend: harder material lasts longer. Off course it is very attractive to select the hardest material. But, there are two considerations:

  1. Hard material tends to be very brittle. For an uniformly distributed sediment with no heavy lumps (read: rocks) this might be fine. As soon as rocks and debris are involved, the wear part might crack due to impact from stones etc.
  2. Hard material tends to be expensive. It requires exotic elements to cast and extensive treatment and machining to finally reach the required hardness.

Still, the harder material might be your choice. If the best material at 10 times the wear index, is three times as expensive as the softer cheap material, your will end up with a three times better wear rate per Euro (or Dollar). And it is not an investment. Wear parts are consumables.

Speaking of money, wear parts do involve some financial planning. At best, the contractor has his own stock. In case of a worn wear part, the part is immediately available. Though this requires some investment indeed. We do have a stock for emergency deliveries, but transport costs time also. Let alone, if the part has to be cast. Casting is a laborious process that can take 16 to 20 weeks. Even with all the modern progress, we are still limited by the physical processes involved. But the casting process itself sure looks quite spectacular!

Casting molten steel in a sand mold

References

  1. Mohs scale of mineral hardness, Wikipedia
  2. Mohs Hardness Scale, National Parks Service
  3. Product Specification Sheet (only in print), Van Voorden Foundry

See also