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Decanter centrifuge performance optimization involves tuning or adjusting specific operating parameters to improve the separation efficiency of the decanter.
In this article, we discuss the 5 most effective and straightforward ways to achieve optimal decanter operation.
We will address the following typical questions related to the optimization process.
The pond depth is the radial distance between the internal bowl wall and the inner surface of the concentric liquid layer. The high centrifugal force causes the liquid to form a concentric layer within the bowl.
It is easy to adjust the pond depth by changing the weir plates with different radii to the decanter bowl axis of rotation. The following is an image of a decanter weir plate.
The adjustment of the pond depth has the maximum effect on decanter centrifuge performance. In this section, we discuss these effects.
Weir plates with shorter radii allow a deeper pond formation within the decanter bowl. This shorter radius increases the settling space available for the sedimentation of sludge particles. Therefore, a deeper pond facilitates better sedimentation and sludge separation. Removal of more solids means clearer liquid.
However, the deeper pond depth also implies more liquid will cover the taper section (beach) of the bowl leading to reduced beach length, as shown in the diagram below. The reduced beach length impedes the liquid removal from the sludge as it travels up the beach toward the solids' outlet ports. Therefore, a deeper pond causes wetter solids.
The decanter auger pushes the sedimented solids towards the solid ejection ports along the tapered section of the bowl (beach). A deeper pond exposes a shorter section of the beach and reduces the torque required to push the solids out.
In summary, a deeper pond leads to:
In contrast to a deeper pond, in this section, we discuss the effects of a shallow pond on decanter centrifuge performance.
Weir plates with longer radii cause the depth of the liquid layer to decrease, leading to a shallow pond depth. A liquid layer with reduced depth reduces the space available for solid settling, leading to reduced solids removal. Therefore, a shallower pond depth causes more solids in the liquid, leading to a murkier liquid.
As shown in the diagram above, a shallow pond exposes more of the taper section of the decanter bowl, i.e., increased beach length. The more extended beach enhances the separation of solids from the liquid as the auger pushes the sludge up the beach under the high centrifugal force.
Therefore, a shallower pond causes drier solids.
A more extended beach also implies that the auger has to push the sludge over a longer distance, thereby increasing the torque on the auger.
In summary, a deeper pond leads to:
The rotational velocity of the bowl is known as the bowl speed or bowl RPM. This speed is directly proportional to the centrifugal force generated by the bowl. Therefore, a higher bowl speed indicates greater centrifugal force.
The decanter bowl is directly connected to the drive motor. A slower motor speed reduces the bowl speed, and conversely, a higher motor speed leads to higher bowl RPM.
An increase in the bowl speed implies a higher g-force acting on the solids in the fluid. An increase in sedimentation is a direct consequence of higher centrifugal force. More sedimentation causes more solids to be separated from the liquid and pushed by the conveyor (auger), leading to higher torque on the auger.
Also, increased sedimentation means lesser solids in the existing fluid leading to clearer liquid and more solids removal.
A higher bowl RPM generates a higher centrifugal force acting on the solids being pushed up the tapered beach section of the bowl, which squeezes out more liquid from the solids causing drier solids.
As explained above, a higher bowl speed requires the decanter motor to operate at a higher RPM leading to more energy consumption.
In summary, a higher bowl speed leads to:
Reduced bowl speed has the opposite effect on the decanter centrifuge performance due to increased bowl speed, as explained above.
A reduction in bowl speed leads to a reduction in the decanter bowl’s centrifugal force, leading to decreased sedimentation and, consequently, reduced solids removal.
The reduction in solids removal means lesser solids that need to be pushed out, leading to reduced load and torque on the conveyor.
Following the above, a lower bowl speed causes reduced solids removal and more solids in the separated liquid, leading to a murkier liquid.
Lower bowl RPM implies the motor is turning at a lower speed leading to reduced energy consumption.
In summary, lower decanter bowl speed leads to:
The auger speed refers to the differential speed between the decanter bowl and the auger. This speed indicates how quickly or slowly the auger rotates inside the bowl.
The decanter bowl connects to the auger through the gearbox. The auger speed changes with the gearbox sun-wheel shaft speed. In other words, the user can adjust the auger speed by changing the sun-wheel shaft rotation speed.
If the auger is rotating faster, it can move more solids to the solids discharge ports per rotation, allowing the decanter higher solids handling capability. Therefore, it is desirable to have a higher auger speed for high solid applications.
A higher rotational speed of the auger also implies the solids are pushed out quickly before they accumulate and cause more resistance to the auger, thereby reducing the torque required.
Following the above considerations, a higher speed of the auger causes the solids not to have enough settling time and therefore retain more liquid. This quicker discharge leads to wetter solids.
Summarizing the effects of higher auger speed:
A slower auger RPM causes lesser solids movement reducing the solids handling capacity of the decanter. This reduced capacity means that a slower auger speed is better suited for low-solid applications.
Also, a lower auger speed means the solids are pushed out slower, giving more time for the solids to settle and accumulate. Higher solids accumulation leads to higher torque on the conveyor.
Similarly, a lower auger speed allows more time for the solids to settle out, thereby removing more water from the solids and ejecting drier solids.
Summarizing the effects of lower auger speed:
The auger pitch is the distance between the flights on the decanter auger. An auger with flights closer to each other is known as a fine-pitch auger. On the other hand, if the flights are farther from each other, it is known as a coarse pitch auger.
The pitch of the auger is a preset mechanical feature of the decanter. The user has the option to order the decanter with different auger pitches. However, once the decanter is manufactured, the only way to change the pitch is to replace the auger with a different pitch.
A fine pitch of flights on the auger has similar performance effects as increasing the auger speed. So, a fine pitch leads to lower solid moving capacity.
The quick movement of solids also leads to higher auger torque.
As explained above, a coarse pitch auger has similar effects on the decanter performance as a slow auger speed.
Therefore, a coarse flight pitch moves more solids per rotation which exerts lesser torque on the auger and gearbox.
Effects of Auger Pitch on decanter performance are as follows:
|Coarse Pitch||Fine Pitch|
|Solids Handling Capacity||High||Low|
|Torque On Auger||Low||High|
The fluid process temperature is significant when processing viscous fluid through a decanter centrifuge. Higher viscosity fluids such as crude oil resist the separation of solids. As the temperature of the fluid increases, the viscosity reduces, leading to better separation.
The fluid temperature does not affect the decanter performance in cases of low viscosity fluids such as water.
The 5 adjustments mentioned above help optimize decanter centrifuge performance. Needless to say, each application has a different set of parameters. However, following the guidelines above, the user can optimize the decanter centrifuge under set operating conditions.
by Sanjay Prabhu MSME
Engineering Manager, Dolphin Centrifuge