Moving Solids in a Pipeline Part 2 – Slurry Categories

In part 1 we discussed solids settling in a pipeline and how laminar and turbulent flow affect settling.  It may have left you with the impression turbulent flow is always the way to go but it does come at a cost. Wear is increased and energy/ HP  consumed and required pump output are both higher. (larger pumps)

Today we will therefore look at types of slurries and how they react to flow rates below the velocities associated with turbulent flow.

Given time any suspended solid with a specific gravity of greater than one will settle out. In a pipeline, however, the slurry is kept moving and if kept moving fast enough settling will not occur.  A slurry that doesn’t settle at all at a given flow is said to be pseudo-homogeneous.  The illustration below shows how the solids are equally dispersed across the whole pipe section.

Although settling slurries in full turbulent flow can have even particle distribution across the pipe, the term pseudo-homogeneous is normally used in association with slurries in laminar flow.

An example of just such a slurry is MFT (mature fine tailings) in the Alberta tar sands.

When the particles within a slurry are fully suspended but not uniformly distributed (non-homogeneous) the slurry is said to be heterogeneous.

This can occur in laminar flow when the velocity is close to the critical settling velocity but still sufficient to stop the formation of a” bed” in the pipe. (see below for info on “beds”)

This is most often associated with transitional flow where the particles are settling quickly but turbulence close to the bottom of the pipe section constantly re-suspends the solids.

In applications where the size and density of the solid is substantial heterogeneous flow is possible in full turbulent flow.

When the particles are mostly suspended, a bed may form at the bottom of the pipe. If the particles continue to move (roll or sluff) it is said to be a moving bed application.

This can occur in laminar flow when the velocity is close to, but not quite high enough to match the critical settling velocity.

Transporting solids in this format requires a well-graded pipeline, as the solids will tend to sluff into low spots, possibly plugging the pipeline with little warning.

When particles fall out of suspension, and form a bed that does not move, it is said to be a stationary bed application.

This can occur in laminar flow when the velocity is below the critical settling velocity.

Assuming this is a properly graded pipe and there is a fairly homogeneous slurry, stationary bed applications are quite viable and illustrations such as the one to the left do not have to be a prelude to a plugged pipe.

With a slurry of all fine particles, the pipe section will reduce as solids settle.  As the pipe section reduces, velocity will increase to a velocity exceeding the critical settling velocity and stable transport of solids will continue.

Before I leave today I must caution you that all the info above is predicated on slurries that are fairly small and uniform. A larger random solid that enters a pipeline will not react as discussed above and can form the basis of a plug and the beginning of bad things to come.

Cheers

RJ