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Though there are several advantages to disc-stack centrifuge, there are some equally important disadvantages. The following are some of the main disadvantages of disc-stack centrifuges and corresponding workarounds.
Particle Size in Process Liquid
The stack of conical discs (disc-stack) has a preset spacing between the discs. Typically this is in the range of 500 to 1,500 microns. This spacing defines the largest particle size the centrifuge can handle.
Particles bigger than the inter-disc space cannot pass through the discs and get stuck. These stuck particles cause a blockage in the flow-path, causing the centrifuge to malfunction.
A ‘plugged disc-stack’ requires stoppage and manual cleaning of individual discs, a laborious and time-consuming process.
Pre-straining the process fluid to a mesh size smaller than the disc spacing is the most common way to avoid disc centrifuge blockage issues due to large particles.
Maximum Solids Load
Unlike a decanter centrifuge, a disc-stack centrifuge cannot handle a high percentage of solids in the process fluid. Whereas a decanter centrifuge can handle up to 50% solids by volume, a disc-stack centrifuge is limited to about 8% solids. This limits the application of disc centrifuges to fluids with a low sludge content. The reason for this limitation follows.
A decanter centrifuge ‘plows’ out the separated sludge continuously. A self-cleaning disc-stack centrifuge ejects the separated sludge intermittently. Process liquids that have a high sludge volume require frequent sludge discharge cycles.
Frequent sludge discharge cycles degrade the efficiency of the centrifuge. They can also cause the drive motor to get overloaded due to the current surge accompanying each discharge cycle.
In process fluids with high sludge volume comprising large and small particles, we recommend a decanter centrifuge followed by a downstream disc-stack centrifuge. This configuration allows the decanter to separate the large bulk solids before the disc-stack centrifuge polishes the remaining small particles.
Intermittent Sludge Discharge
As described above, self-cleaning centrifuges eject the separated sludge during a sludge discharge process. This cycle is automatically triggered by the centrifuge controller, based on a timer. Therefore, sludge ejection is an intermittent occurrence.
In cases of applications that require a continuous discharge of sludge, the user might view this intermittent discharge as a disadvantage.
Also, in the case of ‘full-discharge’ centrifuges, the process flow needs to be stopped during the sludge ejection process. This flow-stoppage is not suitable for applications where the centrifuge is in the primary process flow-path. It is advisable to install the centrifuge in a kidney-loop on the main fluid supply tank in such applications.
Sludge Buildup in Centrifuge Bowl
Unlike a decanter centrifuge that uses a scroll to push out the separated sludge, a disc-stack centrifuge uses the high centrifugal force to eject the accumulated sludge from the bowl. In rare cases, some sludge may stick to the bowl wall during the ejection process. This sludge can build up in the bowl to cause an imbalance or prevent subsequent sludge ejections.
In cases of sticky-sludge applications, a bowl rinse or cleaning cycle may be introduced periodically to washout any buildup sludge. For example, in crude oil applications with paraffin, diesel fuel is sometimes used as a solvent to remove any paraffin buildup inside the bowl periodically in a flush cycle.
Moving Parts in Sludge Discharge Mechanism
As seen in the image above, some internal moving parts within the bowl underbody use water to actuate. This mechanism is sensitive to mineral deposits due to water evaporation from these cavities over time.
These buildups and part’s wear related to regular use can cause malfunctions of the self-cleaning disc-stack centrifuge bowl’s sludge ejection mechanism. Referring to the diagram below, the sliding piston and the operating slide operation are affected by mineral deposits.
Therefore, preventative maintenance of disc-stack centrifuges is more important than that of a decanter or other types of centrifuges.
Auxiliary Operating Water Requirement
A self-cleaning disc-stack centrifuge requires clean water for the operation of its sludge discharge mechanism. This operating water operates the hydraulic mechanism within the underside of the rotating bowl.
This requirement of clean, soft, filtered water is a disadvantage, especially in remote field locations where clean water may not be available. Though operation can recycle the operating water under certain conditions, it is not always possible to do so.
In summary, the above-mentioned disadvantages of the disc-stack centrifuge should be considered along with the advantages before centrifuge selection. The workarounds to some of these limitations are equally important and can mitigate some of the disadvantages.