# Settling Slurries

Settling slurries comprise a carrier fluid (sometimes referred to as the vehicle) conveying solid particles. The accepted approach in determining the friction loss of the slurry mixture is to calculate the 'solids effect' - the excess friction loss caused by the solids content over that for an equivalent flow of the carrier fluid. A number of interrelated factors influence the solids effect. The three main factors are the solids' particle characteristics, the settling velocity of the particles and the velocity of flow of the mixture, but pipe inclination is also important. If all or a fraction of the solids are relatively fine, they can be held in suspension and create a 'homogeneous' carrier fluid, the friction characteristics of which needs to be determined. In these circumstances the carrier fluid friction loss then becomes proportional to that of the pure fluid. This type of slurry flow represents one extreme of slurry flow. At the other extreme, with coarser particles moving only as a contact load, (as a sliding bed with none in suspension) the solids effect is considerable and the pressure loss much greater than for the carrier fluid alone. This is referred to as fully stratified flow. In between these extremes are two regimes, pseudo-homogeneous and heterogeneous. Pseudo-homogeneous flow contains particles between about 0.4mm and 1.5mm and displays a variation of solids concentration with height across the pipe diameter, depending on velocity. Again the pressure drop is proportional to that obtained for an equal discharge of carrier fluid. Heterogenous flow - particle size around 1.5mm to 4mm and lower velocities - exhibits a greater non-uniformity of solids concentration. The solids are supported partly by fluid suspension and partly by inter-particle contact. The settling velocity of the solid particles and the percent by volume in the mixture are important factors. Real-life slurries may contain a wide particle size distribution and consequently all four regimes of flow - a homogeneous carrier fluid, and pseudo-homogeneous, heterogenous and fully stratified fractions - may be present at the same time. The technique is to estimate the excess pressure drop separately for each of the four regimes and then sum them for the total. Design methods are highly empirical. Eight approaches are available with the Slurry module: ▪Durand ▪Wasp ▪Wilson, Addie, Sellgren and Clift (WASC) ▪Sellgren, Wilson Four-Component Model ▪Liu Dezhong ▪Vsm ▪V50 ▪4CM