Why Particle Distribution Matters

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Pipe

Previously we have spent some time discussing how particle size can be described in terms of different standards. It’s now time to look at how the distribution of particles is important for people in the pump/dredge world.

“Particle Distribution” is simply a term used to describe how much of a specific slurry or deposit is of any given particle size. It is the key to determining how the slurry will react when we pump it and, in the case of a deposit, how it will re-suspend.

Let us first look at how distribution affects production when we are using a pump to re-suspend a solid.  Maximum production is obtained when a pump is continually in contact with the subject material. This is obtained when the material comes or flows to the pump.  If the operator continually has to move the pump to keep it supplied with solids, it is far harder to maintain a consistently high rate of production. Furthermore, if operated in a sweeping motion the pump only accesses material from one side, again reducing solids production.

In short, we want the material to move or flow to the pump. Sand at the tide line on a beach is an example of a material that flows well. As the person in the photo below found out, it is impossible to dig a hole with her feet, as the sand just flows into the hole she is trying to create.

wet sand

In terms of dredging , wet beach sand  is said to have an angle of repose of close to zero. Deposits  with  low or slight angles of repose result in a constant feed of material to the pump and high production rates . In contrast materials with  high angles of repose such as virgin deposits of aggregate , require the pump operator to constantly move the pump resulting in down time and lack of efficiency.

When dredging or relocating any deposit, we hope the material deposit has a low angle of repose, but which materials yield slight angles? Well, any deposit that has fairly uniform particle size that is not ultra-fine is a good candidate. Having all the particles of a fairly consistent size means there are no smaller particles that could fill the gaps within the deposit. Gaps result in a deposit that is difficult to compact and allow the particles to roll over each other as they flow toward the pump.

Geological deposit of blue clay

You will notice that above, I stated a qualifier of “uniform particle size that is not ultra-fine”. Materials that have a uniform ultrafine particle distribution have virtually no gaps between the particles and, as such, form a very consolidated deposit that will not flow. Blue Georgia clay, or for that matter any consolidated clay, will resist movement of any type.

Deposits with a uniform particle distribution that is ultra-fine also result in an issue when we look at re-suspending or “slurrying“ a deposit.   Let’s look at clay again.  You can shoot water at it, and it just deforms, or you can agitate it, and the surface particles wash off, but it stays fairly uniform.  Clay and other similar deposits are said to be “plastic” and virtually impossible to sufficiently agitate the solids to form a slurry.  (Note: Cutter head dredges can pump plastic materials, but they are, in effect, pumping chunks of material, not a slurry).

Clay

Particle size distribution not only effects things before the material enters the pump system it also effects the situation within the pump in two key areas.

The first area is wear. Deposits that have all large particles wear pumps far more quickly than deposits that have a mixture of particle sizes.  If there is a mixture of particle size, the smaller particles can effectively get in the way of the larger particles and stop them from directly impacting on the wear components in the pump. The same holds true for the discharge pipe.

The second concerns settling velocity.  Although the subject of settling velocities is a discussion in itself, particle distribution does affect settling velocity.  In short, the heavier or higher the viscosity of the carrier liquid, the more energy it can impose on large articles to keep them moving through a pipeline.  Hence, slurries that contain large particles, as well as smaller particles, are less likely to have to plug problems than slurries that contain just the large particles when pumped at the same flow rates/velocities.

While on the subject of settling velocities, it should be noted that slurries composed entirely of ultra-fine particles will not settle out in pipes. This allows for very low flow velocities and provides the associated benefit of low friction losses within our piping system.

Pipe

Now that we understand the importance of knowing particle distribution how do we express it. First data must be obtained on the makeup of the deposit. Relating  back to previous  lessons  we must identify percentages of particles that pass through standard mesh sizes. This info can then be summarized on a ‘particle distribution graph.

The sample graph below plots the total percent of the sample that will pass any given screen size.

Graphic

 

In this sample illustration the plotted horizontal and vertical lines are there as the source for what is referred to as the D60 and D10 numbers.

The “D number”  is just  an abbreviation to represent the percentage of product that will pass a given screen size. In the example above the D60 is approx 0.6 mm, which means that 60 % of the sample is smaller that 0.6 mm. The D10 would be approx. 0.02 mm.

I personally find this representation of particle distribution somewhat difficult to visualize. The graph below is, in my opinion, easier to visualize than the graph shown in the previous slide. However, having to calculate the area under the curve makes establishing “D numbers “ far more difficult.

I hope this brief discussion helps everyone to better understand the importance of determining and accurately communicating particle distribution.  If someone refers to a “D” number and you are having questions about a deposit, gather as much info as you can and then call our very competent application engineering department at Hevvy/Toyo Pumps.

Cheers

RJ

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