A slurry system has been created and the solids data defined at the system inlet using PSD data. The system has been modeled and all pipes show a warning message indicating that the slurry may in fact be non-settling as the d50 particle size calculated based on the user-defined PSD defined is less than 75 micron (0.075 mm). The warning message appears as follows:
The reference value of 75 micron (0.075 mm) is a useful guide and is quoted in reference literature as a means of helping define a slurry flow characteristic.
If we check the Rosin-Rammler (R-R) graph plot on the Chart tab for the inlet boundary, we can see the d50 particle size is estimated to be approximately 0.02093 mm (20.93 micron) which of course is noticeably lower than the guide value of 75 micron and therefore, likely to be non-settling. For reference purposes, the d85 value is estimated to be circa. 0.17442 mm (174.42 micron). The Rosin-Rammler graph plot appears as follows:
At this stage, it is worth further checking the slurry characteristics in reality or checking any available Rheology data.
Lets consider a second approach to the same system only in this case, the user has defined a d50 of 0.028 mm (28 micron) and d85 value of 0.177 mm (177 micron). The model is firstly solved using the Durand equation which only uses the d50 value in the solution. Based on this solution, the Durand method is also highlighting that the slurry may in fact be a non-settling type.
The system is then solved using the WASC method which uses both the d50 and d85 and returns the same set of warnings. Again, this indicates that the slurry may in fact be non-settling.
We can cross-check the slurry using other means. If we refer to the image below, we can see that it indicates that at a d50 particle size of 0.028 mm (28 microns) and an operating velocity of circa. 1.5 m/s and above, the slurry is likely to be a homogeneous suspension. This therefore serves as a useful means of predicting the likely slurry flow conditions.
Flow Regime Boundaries for Settling Slurries.
The chart below allows us to estimate the flow conditions based on particle size and specific gravity. A d50 of 28 microns (0.028 mm) and a solids specific gravity of 2.65 (solids density of 2650 kg/m3) returns a slurry characteristic which is likely to be homogeneous.
Slurry Flow Regime as a function of Particle Size & Specific Gravity.
Note, other literature sources refer to slurries exhibiting non-settling flow characteristics when the typical particle size is circa. sub 40 micron (0.040 mm). The particles are basically held in suspension within the "carrier" fluid.
The following illustration also serves as a useful guideline which can be used as a basis of predicting the likely flow conditions.
In general, the recommendation would be to seek reliable slurry/solids information and information on the system operating conditions. Ideally, Rheology data should be sourced for the slurry which can then be used as design inputs for a model.
