7.4 Pumping suspensions

Pumping suspensions

Pumping suspensions or liquids where particles of inorganic and organic solid matter are to be found, more or less finely distributed, suspended in liquids either as contaminants or for the purpose of transportation.

Suspensions belong to the non-Newtonian liquids. The characteristics of the liquid, the suspension, depends upon the size of the solid particles, the density, form and hardness and also upon the  concentration of particles in the liquid. These parameters are of decisive importance in the choice of the pump. It is usual in pump catalogs to state the suitability of pumps for various kinds of liquids, e.g. pure, no solid contamination, slightly contaminated, contaminated, slurry, mud, transport of pulps, solids, etc. without closer definition.

For applications involving pumping suspensions or selecting a pump for various suspensions the following parameters needs to be specified:

• Particle size
• Are the particle de-formable or
• Abrasive
• Concentration

Size of particle

Particles of size 1 to 100 mm appear in liquids from sewage draining building excavations, dredging, pumping solids, certain foodstuffs, etc. If particles of this size are allowed to reach the pump, then a non-clogging type of pump will be needed. Particles of size 1 to 10 mm occur in applications similar to the above mentioned. Examples of particles from < 1 mm are shown in Figure 7.4a.

Figure 7.4a Illustration of particle sizes and comparison with clearances and play in pumps.

Pumping deformable particles

Particles of organic matter, soft, fibrous particles which can be compressed or may be subject to damage. For example, dispersion of plastic or natural rubber in water – latex. Latex can coagulate (the process of emulsion polymerization is used to make several commercially important polymers of which many are used as solid materials but if the dispersion is the end product it´s called a latex) under pressure and therefore requires careful pumping.

Wearing, abrasive particles

Particles of inorganic matter, minerals, sand, particles are hard, sharp-edged and so on.

Particle content in suspensions

Content refers to the quantity of particles in suspension (dry substance = DS – content) and is expressed as a percentage by weight, i.e. the particle weight times 100 divided by the total weight of the suspension. The particle weight is calculated for completely dry particles. In practice, however, it is often accepted that air-dry particles have a 10-12% water content.

When the content is approx. < 1 % the choice of pump can usually be the same as for water. When the content is approx. > 1 % then several other factors come into the picture. In the choice of pump, the particle size and the DS-content have to be considered together. A sufficiently small particle size can mean that a normal water pump may be used providing the liquid flows easily, is non-wearing and does not deposit sediments in the pump itself. Where solid material has to be transported in a finely distributed form, water is the commonest form of conveyance.

Table 7.4b Examples of particle when pumping solids.

Calculate density of slurry or suspension

The density of a suspension can be calculated from the formula:

where

ρ = density of the suspension/slurry (kg/m³)
ρ l = density of the liquid (kg/m³)
ρ p = density of particles (kg/m³)
DS = percentage of weight of solid particles (%)

or

ρ = (vol %) * ρ p + 100 – (vol %) * ρ l     (Equ.7.4b)

where

vol % =  proportion of solid material stated as volume percentage

Figure 7.4b shows the density of a suspension or slurry consisting of solid particles suspended in water.

The example shows that a suspension having a particle content of 40% by weight and a particle density of 2700 kg/m³ has a total density of about 1340 kg/m³.