The removal efficiency is also a function of the viscosity and density of the mud. Since the settling rate of a particle is decreased by increased viscosity and density of the mud, the percent of particles of any given size which reach the underflow is decreased. In other words, an increasing amount of the larger particles go to the overflow and are returned to the mud system. This means that desilting should be started before the drilled solids build up in a mud and increase the viscosity and density.
Solids that are not removed from the system at the first opportunity tend to be reduced in size as they are re-circulated. As they reduce in size, they not only become too small to be removed by a desilter, but they also quickly build viscosity which reduces the efficiency of the desilter in removing the larger particles. For these reasons, the desilter should be considered as a means of preventing solids buildup rather than a means of reducing solids content.
When water is being added to the mud, it should be added upstream from the desilter in order to reduce the mud viscosity and improve the efficiency of the desilter. Addition of chemical, bentonite, or other mud additives should be added to the system downstream from the desilter so that none is discarded by the desilter.
Desander or fine-screen shaker should be used ahead of the desilter in order to remove the larger particles that can clog or overload the desilter. Clogged discharge on the underflow of a cyclone probably accounts for more inefficient desilter performance than all other problems.