Design and operating principles can be found in suppliers literature. The decanting centrifuge consists of a rotating horizontal centrifuge bowl with a conveyor turning in the same direction but at a lower speed. The slurry is introduced at the centre of the narrow end of the bowl and travels to the liquid overflow at the opposite end.

The heavier particles (Barites and drilled solids) are thrown to the outer walls, by the centrifugal force, and are transported back to the narrow end by the screw conveyor to be discharged at the underflow. The lighter materials remain in the middle of the flowstream and are discharged at the overflow with the liquid.

Centrifuge operating efficiency is affected by the following parameters, in decreasing order of importance:

1. Feed flow rate
2. Bowl speed
3. Bowl / conveyor differential speed
4. Pool depth

Note:

• The cut point is proportional to the mud viscosity.
• The feed flow rate capacity reduces significantly as mud gradient is increased.
• Gear box torque may rise and become unstable at elevated mud gradients.
• The centrifuge must be fitted with a mechanical torque sensor coupled with an overload protection device for automatic shut down of the main centrifuge motor and the feed pump when tripped by conditions of excessive torque.

Centrifuge Operational Guidelines

• For removal of low gravity drilled solids, centrifuges shall be:
  • run as required in top hole sections.
  • run continuously or as required in 12 1/4” and 8 1/2” hole sections.
  • set for moderate speed/high volume processing.
• Shale shakers shall be dressed with the finest practical screens to ensure optimum operational efficiency of centrifuges.
• The start-up procedure for centrifuges is as follows:
  • Start rotation of bowl/screw conveyor and allow to reach stable operating speed.
  • Start feed pump at low rate and slowly introduce mud to centrifuge bowl.
  • Gradually increase feed rate until the desired rate is achieved, taking into consideration the application (solids removal or gradient reduction), mud gradient and manufacturer’s recommendations.
• Centrifuges can be operated in two modes:
  • Solids discharge (for weighted mud) or
  • Barite recovery for  weighted mud (> 10.2 ppg).
• If the object is to reduce mud gradient, centrifuges shall be set for high speed/ low volume processing.
• System performance must be checked, at least by measuring gradient of overflow and underflow, on a regular basis.
• Centrifuge performance must be judged in terms of maximising the removal of low gravity solids while minimising environmental damage and loss of expensive fluid phase and/or Barites.
• The shut-down procedure for centrifuges is as follows:

1. Reduce feed pump rate to minimum

2. Switch off feed pump.

3. Continue to run centrifuge until solids discharge ceases.

4. If possible, flush through with water, when using water based mud, being careful to direct returns away from the active system.

5. Turn off power to centrifuge.

Centrifuge Set-up

Solids Discharge Mode

• Whole mud is fed to the centrifuge which must be set to rotate at medium RPM.
• The liquid effluent must be routed back to the active mud system and the solids underflow discharged.
• Gradient of the overflow must be checked to ensure it is as close as possible to the gradient of the liquid phase to ensure maximum removal of fine solids.
• Where high fluid viscosities are required, for hole cleaning purposes, dilution fluid (water, brine or base oil) may be added at a known rate directly to the feed mud entering the centrifuge to reduce viscosity and enhance solids separation.

Barites Recovery Mode

• This process involves utilisation of two centrifuges.
• Whole mud is fed at high rate to the first centrifuge which must be set to rotate at a low speed.
• Barites removed by the first centrifuge is returned directly to the active system.
• The Barites-free mud from the first centrifuge overflow must be fed to the second unit of higher capacity which is set to rotate at maximum RPM.
• Low gravity solids removed by the second centrifuge are discarded.
• The stripped liquid phase overflow is returned to the active mud system.
• Liquid overflow from the second unit must be checked to ensure that it is as close as possible in gradient to the unweighted liquid phase (water, brine or base oil) of the mud.