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.
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.
There is a trend: harder material lasts longer. Off course it is very attractive to select the hardest material. But, there are two considerations:
- 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.
- 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!
- Mohs scale of mineral hardness, Wikipedia
- Mohs Hardness Scale, National Parks Service
- Product Specification Sheet (only in print), Van Voorden Foundry