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

What Dutch Dikes are Really Made of

Placing protecting basalt blocks on a dike by a stone setter (Credit: van den Herik-Sliedrecht)

The Netherlands coastline is, to a great extent, defined by unnatural sharp lines: dikes. Even when you zoom into a more human scale, the dikes are decorated by geometric patterns: hexagons. Just like the dikes are not a natural component in the environment, the hexagons are an invasive specie: basalt blocks imported from more rocky countries, as Germany, England and Norway. They are carefully placed by hand to protect these structure from waves and tides. This armoured lining is usually the final stage of a dredging project: protecting what just has been constructed.

My personal experience with these blocks, is that we used to play with them, when we were just little boys. We sailed to the newly constructed Pampushaven, where there was a stockpile of these basalt blocks leftover from the reclamation of the Flevopolder. We puzzled them together like a real dike, or built forts of them. Even after forty years, the stock pile is still there!

Playground stock pile of basalt blocks for dike and shore protection (Credit: Google street view)

What I remember was, that they are extremely heavy. And their perfect prismatic geometry intrigued me intensely. I thought they were made that way. By now they feel less heavy and I have learned that they are some kind of volcanic rock called ‘basalt’. But it puzzled me how molten basalt can solidify into these perfect jigsaw pieces. Until last week, I’ve read an article in the Guardian1 about a geological feature called ‘Giants Causeway’.

Recent article about the origin of the basalt blocks at the Giant’s Causeway (Credit: Guardian News & Media Ltd)

As it happens, I also visited this Giant’s Causeway. And it really feels like standing on a dike at home. But this ‘dike’ was laid down by mother nature. Molten lava flowed over chalk beds. As it was clamped between other layers during cooling, the internal stresses caused the cracks to be distributed evenly in a nice geometric pattern. So, Finn MacCool2 wasn’t involved after all!

The internal structure of dikes, dams, jetties and groynes is a bit different and purposefully designed for the intended application. Usually there is a body of sand, that is designed to take the load of the tide and waves. Next an internal lining with appropriate permeability. Either watertight for keeping the water out, or open structure for draining the wave run-up. Height, width and slopes depending on the requirements.

Example of an internal structure of typical dike

Evaluating all these requirement choices in a well-balanced design is an art by itself. Don’t cut corners, it’s all about safety of the people living behind the dike. It is best left to specialised companies that are familiar with the design and construction of these civil works. In the Netherlands, the trade was often handed over from father to son and whole families became intertwined in these specialist dredging companies.

So, now we know the origin of the stones for the dikes and how they are used. But the real resource used to protect our dikes and the land behind it are: all those unnamed men that have literally put their back into placing those stones. We owe them our land!

‘The stone setter’ by Ineke van Dijk placed in 1982, on the occasion of the 50th anniversary of the Afsluitdijk (Credit: Wikipedia)

References

  1. Scientists solve mystery of how Giant’s Causeway was formed
  2. Finn MacCool

See also

A Sample of Soil Samples

Soil sample exhibit at the Damen Dredging Experience

‘Welcome Sir, very nice you would like to buy our dredge. What do you want to dredge with it?’
‘Sand.’
‘Ah Sir, very well. Our dredges are very capable of dredging sand. What kind of sand are you going to dredge?’
‘Grey sand!…’

Sounds familiar? Well to me it does. Sometimes, even our most esteemed customers lack a basic knowledge of their primary process. Often, I’ve been called into a meeting with the customer, to explain about sand and its physical properties. Nowadays, we can show them an exhibit in the Damen Dredging Experience1 to discuss their particular case. Eventually, we can tease out the information we need, to inform the customer the estimated production of their dredge. Educated customers do know about their operation and can make the estimation themselves.

There are several parameters that influence the performance of the dredge. Particle sizes, grain forms, densities, mineral types, cohesion and many more. And we are very happy when the customer already has his own soil report of his particular operation. This would comprise bore logs, particle size distribution and cone penetration tests. If this is not possible, a sample of the concerned soil will do. We do have our own small soil mechanics lab, in which we can measure the required properties.

Elementary soil mechanics laboratory at our company

The most useful property is the particle size distribution or PSD. This can be done in a sieve tower. (red circle) The soil sample is placed in the top sieve and the sieves are vibrated to separate the various fractions. The contents of the sieves are weighed and plotted in a logarithmic graph. This resulting PSD can then be used in production estimations.

Procedure to establish a particle size distribution

For a good measurement, we need a sample the size of a 1.5L coke bottle. About two thirds of soil and one third of water. We need the water to capture the fines in the sample. Do not drain it! And the bottle is a good container for transport and widely available. Stash it in your check-in luggage. Otherwise, you will run into trouble with the airport security about carrying liquids in your hand luggage.

Examples of sample containers encountered for estimation

All these variations in soil properties have a major influence on the performance of the dredge. So, this is why we would like to know exactly what the customer is going to do with our dredge. Otherwise, we might end up in an analogous situation when somebody wants to buy a truck. You can explain all about the installed power and the cargo capacity and then you get this question:
‘Sir, can you tell me how much paper I can transport with this truck?’
‘Ah well, we are not in the transport business ourselves, but as you are a good customer, we can make an estimate for your convenience. What kind of paper? Kite paper? Cardstock? In blocks, boxes or roles?’
‘Oh, I don’t know. Just paper’.

Model dump truck from a customer2, typically used at dredging projects

References

  1. Kommer Damen opening the ‘Damen Dredging Experience’, DredgingToday
  2. 2. 35 ton dumper, Martens en Van Oord

See also