What!? My impeller is already worn?

Equipment on board wears. In dredging that is a fact of life. Often the reason why something wears faster or slower is not well understood.

Today I want to talk with you about wear due to the soil and other main factors that increase the wear rate. Especially the wear on pump parts and lines.

I have met several companies that looked at wear in a time manner. Meaning they expected an impeller to last 6 months and pipelines for 12 months, for example. Today I will show why this is not only the wrong way to look at it, but it can hurt your operation when you take this approach.

Let’s first start with the factors that have the biggest impact on how fast things wear.

Soil grain size

The first factor is the size of the individual particles of the soil. The bigger the particles the faster it wears. So gravel wears faster than sand and that wears faster than silt. Rock usually wears the fast as it has the biggest “particles”.

Soil grain sharpness

Next one is the sharpness of the particles. A complete round particle wears a lot less than a particle with a lot of sharp edges.

This is another reason rock wears faster than gravel. But sometimes it can be that fine sand wears very fast, because it is so sharp.

Mixture velocity

Velocity of the mixture is the biggest factor of wear that we can control. Lower velocity results in lower wear. In some cases this relation is to the third power. Meaning if you double the velocity, the wear rate goes up eight times! So keep mixture velocity as low as possible.

Volume of material

Amount of mixture moving through the pump or pipe directly impacts how long the part will last measured in time. If you double the volume, then the lifetime goes in half.

Lifetime

When talking about lifetime of wearparts in relation to soil we can look at it from two angles. The first is that we look at the lifetime measured in time. So a pipe lasts 12 months and an impeller for 6 months and so on. This is useful for making a planning of when to buy spares and planning scheduled maintenance.

Source: Jan De Nul

But the second, often overlooked one, is the lifetime in volume of material. So an impeller lasts 1 Mill. m­­­3 and pipes for 2,5 Mill. m3 for example.

An impeller wears because of contact with soil, so when you put more soil through the impeller it will wear faster.

Consider a TSHD on a dumping project with a production of 300.000 m3 per week. An impeller here lasts for 4,8 Mill. m3. In this case the duration that the impeller lasts 16 weeks.

If this same TSHD now does shore discharging with all other factors being equal the lifetime of that same impeller now is only 8 weeks. That weekly production is moving through the pump twice, once when loading and once when unloading, so that is 600,000 m3 per week.

So when you are used to look at lifetime in time only then suddenly you see the lifetime cut in half. This unexpected change will play havoc with your spares planning, maintenance planning and of course the budget you allocated for the spares as you now have to buy twice the number of impellers to finish the project.

How to increase the accuracy of your wear estimates?

Start with measuring the wear of impellers in m3. How much volume has passed through it before it had to be replaced? What was the grainsize? What is the sharpness of the material? What mixture velocity did the dredger operate at?

When planning for a project, take all these aspects into consideration. Look at the expected soil type to be dredged and the expected mixture velocity. Based on the previous collected data make an estimate.

Continue doing over the years and over time the accuracy of your estimates will become better and better, making sure you get less surprises when you are dredging.\

 

Did you have ever had unexpected fast wearing? Tell us about it in the below comment field. What were the circumstances? What did you do to react to the wear?

 

To your  long term great results,
Timon Vinke

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