A decanter centrifuge operates on the principle of accelerated settling. Particles will settle in a viscous media according to stoke’s low, which in simple form is written as follows:
If a sphere settles in a liquid of known density and viscosity, and the settling rate is measured, then by stokes low the density of the ball or sphere can be calculated if the diameter is known. Or. The diameter can be calculated assuming the density is known.
Principles of the decanting centrifuge:
A decanter centrifuge is a device that creates a centrifugal force, and imparts that centrifugal force to its components by rotation.
The centrifugal force created by the centrifugal is dependent on the diameter and RPM of the bowl. To calculate the g force, use the flowing formula:
“g” force for centrifuge
G’s = RPM2(0.0000142)(Diameter of bowl, inches)
Example: 24” bowl, 1530 rpm
G’s = (1530)2(0.0000142)(24)
The performance of a centrifuge for a given mud is dependent on the following factors:
1. The g’s exerted on the fluid
2. The retention time in the centrifuge (the longer the mud is in the centrifuge the smaller the particle that can be separated)
3. Conveyor differential (the faster the differential, the wetter the solids and the more solids are discharged)
All of the above are factors which are considered in different designs. Some designs perform better for different applications. Designs often vary in bowl diameter, length, RPM operating range, gear ratio, pool depth. Etc.
The items which can typically be manipulated in a centrifuge operation are the feed rate and dilution (if any), the gear ratio (by changing gear box), the pool depth (by changing the weir settings), and the rpm (by changing the sheaves), which changes the g force.
To calculate the RPM of a centrifuge, use the following method:
RPM of a centrifuge
RPM=(size of motor’s sleeve/size of rotating assembly)x rpm of motor
The purpose of oilfield decanter centrifuge as applied to a weighted water-base drilling mud is to control viscosity. This is accomplished by concentrating the colloidal-size and clay-size solids fraction into the effluent (liquid) steam and disposing of this phase. The economic justification of using the centrifuge on this type of system is the simultaneous return of weight material. Using a centrifuge on weighted water-based muds eliminates the expense of throwing away (jetting) whole mud to control viscosity. Water dilution and excess waste volume are minimized.