Synopsis
Though a disc stack centrifuge has several advantages, there are some equally important disadvantages. The following are some of the main disadvantages of disc-stack centrifuges and corresponding workarounds.
The bowls described here are the Alfa Laval MAB, MOPX, and WHPX disc stack designs that Dolphin Centrifuge remanufactures. Dolphin is the independent specialist in Alfa Laval centrifuges, so these notes come from rebuilding and running those machines rather than from a brochure.
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 clogged 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 Content
Unlike a decanter centrifuge, a disc-stack centrifuge cannot handle a high percentage of solids in the process fluid. 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
Though all disc-stack centrifuges feature continuous flow or flow-through designs,
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 applications requiring 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 must be stopped during the sludge ejection process. This flow-stoppage is unsuitable for applications where the centrifuge is in the primary process flow path.
Therefore, a kidney loop installation on the fluid supply or storage tank is advisable for such applications.
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Sludge Buildup in Centrifuge Bowl
Unlike a decanter centrifuge that uses a scroll to push out the separated sludge, a disc-stack centrifuge uses high centrifugal force to eject the accumulated sludge from the bowl.
In rare cases, some sludge may stick to the bowl wall during ejection. This sludge can build up in the bowl to cause an imbalance or prevent subsequent sludge ejections.
In sticky-sludge applications, the user may periodically introduce a bowl rinse or cleaning cycle to wash out any buildup sludge.
For example, diesel fuel is sometimes used as a solvent to periodically remove any paraffin buildup inside the bowl in a flush cycle in crude oil applications with paraffin.
Moving Parts in Sludge Discharge Mechanism
The image above shows that 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 wear related to regular use can cause malfunctioning 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 to operate its sludge discharge mechanism. This operating water operates the hydraulic mechanism within the underside of the rotating bowl.
The requirement for clean, soft, filtered water is a disadvantage, especially in remote field locations where clean water may not be available.
Though the operation can recycle the operating water under certain conditions, it is impossible to do so.
Summary
In summary, the 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.
Dolphin Centrifuge is the independent specialist in Alfa Laval centrifuges: remanufactured machines, turnkey systems, parts, and service, built on 40+ years of Alfa Laval experience. Talk it through with Sanjay Prabhu, MSME, before you buy, and we will tell you honestly whether a disc stack is the right machine for your fluid.
Dolphin Centrifuge is an independent remanufacturer and supplier of Alfa Laval centrifuges. It is not affiliated with, authorized by, or a distributor for Alfa Laval.