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 sleave/size of rotating assembly)x rpm of motor
2015年10月15日星期四
Decanter centrifuge to Control Mud Viscosity in Weighted Water-based Drilling Mud
The purpose of the 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.
* Add enough dilution water to reduce the effluent viscosity to 35-37 sec/qt marsh funnel viscosity. If the overflow is 37 sec/qt or less without dilution, none is needed.
* Operate the decanter centrifuge at a slower feed rate for longer periods to maximize efficiency and provide a more homogeneous mud system. It is better to run the centrifuge for eight hours processing 10 gpm than for four hours processing 20 gpm.
Weighted Oil-Base Muds
Single centrifuges have typically been run to control viscosity only on weighted water based muds. On an oil base mud solids control system, the cost of discarding the fluid phase and chemicals with the colloidal solids can be prohibitive.
* Add enough dilution water to reduce the effluent viscosity to 35-37 sec/qt marsh funnel viscosity. If the overflow is 37 sec/qt or less without dilution, none is needed.
* Operate the decanter centrifuge at a slower feed rate for longer periods to maximize efficiency and provide a more homogeneous mud system. It is better to run the centrifuge for eight hours processing 10 gpm than for four hours processing 20 gpm.
Weighted Oil-Base Muds
Single centrifuges have typically been run to control viscosity only on weighted water based muds. On an oil base mud solids control system, the cost of discarding the fluid phase and chemicals with the colloidal solids can be prohibitive.
Decanter Centrifuge Operation in Mud System
Decanter
centrifuge is usually installed after the desilter
in mud solids
control system. The drilling mud from the desilter is pumped through the
inlet to the feed chamber and slung from the feed ports into the pool. It is
accelerated again with a high shearing effect due to the rotation of the bowl.
The level of the pool is determined by adjustment of the weirs at the large end
of the bowl (effluent discharge).
As the slurry flows through the pool in the channels between the conveyors flights, the particles settle according to Stocke's law, but at an accelerated rate due to the higher G forces. Sand particles settle almost instantly, then the finer particles settle. The particles that cannot be settled under the existing settings of the centrifuge will discharge out the overflow with the liquid phase (effluent).
The screw conveyor rotates in the same direction as the bowl, but at a slightly slower rate. This difference is determined by the gear box. Typical gear boxes are 80:1, 125:1, 40:1, and 52:1 gear box. 125:1 means that for every 125 revolutions of the bowl, the conveyor turns one less or 124. With this ratio, a centrifuge rotating at 1800 RPM has a conveying speed of 1800/125=14.4 RPM.
The settled solids from a cake on the side of the bowl and are conveyed along the outer edge of the pool towards the solids end (small end) of the rotating assembly and are discharged out the solids discharge ports at high velocity. As the solids travel across the beach, they lose their free liquid film due to centrifugal squeezing and drainage. They are discharged with only the absorbed moisture; provide the centrifuge is operated properly.
This discharge cake is very heavy, and sticky. It will not flow because of the lack of free liquid. It is important to realize this when installing a centrifuge. If possible, install over the place where the solids are to be discharged. If this is not possible and this slide so that it will self clean. If this is not possible, then some sort of mud wash or water wash will be needed to keep the solids from building up and packing off the centrifuge. Another option is a small amount of remix water added within the bowl housing directed at the solids discharge to softed the slurry. Remember to have the solids directed towards an mud agitator to keep the solids from simply dropping to the bottom of the mud tank.
As the slurry flows through the pool in the channels between the conveyors flights, the particles settle according to Stocke's law, but at an accelerated rate due to the higher G forces. Sand particles settle almost instantly, then the finer particles settle. The particles that cannot be settled under the existing settings of the centrifuge will discharge out the overflow with the liquid phase (effluent).
The screw conveyor rotates in the same direction as the bowl, but at a slightly slower rate. This difference is determined by the gear box. Typical gear boxes are 80:1, 125:1, 40:1, and 52:1 gear box. 125:1 means that for every 125 revolutions of the bowl, the conveyor turns one less or 124. With this ratio, a centrifuge rotating at 1800 RPM has a conveying speed of 1800/125=14.4 RPM.
The settled solids from a cake on the side of the bowl and are conveyed along the outer edge of the pool towards the solids end (small end) of the rotating assembly and are discharged out the solids discharge ports at high velocity. As the solids travel across the beach, they lose their free liquid film due to centrifugal squeezing and drainage. They are discharged with only the absorbed moisture; provide the centrifuge is operated properly.
This discharge cake is very heavy, and sticky. It will not flow because of the lack of free liquid. It is important to realize this when installing a centrifuge. If possible, install over the place where the solids are to be discharged. If this is not possible and this slide so that it will self clean. If this is not possible, then some sort of mud wash or water wash will be needed to keep the solids from building up and packing off the centrifuge. Another option is a small amount of remix water added within the bowl housing directed at the solids discharge to softed the slurry. Remember to have the solids directed towards an mud agitator to keep the solids from simply dropping to the bottom of the mud tank.
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