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Often times a centrifuge user experiences the loss of process fluid or product during centrifuge processing. The main conduit for product loss is sludge ejection.
This article discusses the various aspects of product loss with centrifuges and ways to mitigate such losses.
During the regular operation of an Alfa Laval disc centrifuge, the high centrifugal force pushes the heavier solid particles outward towards the bowl wall. The particles accumulate in the designed sludge space while the clarified fluid exits the bowl. This accumulation of sediment particles forms the separated sludge.
Alfa Laval self-cleaning centrifuges discharge the separated sludge intermittently during a sludge discharge cycle. During this cycle, the sludge holding space is momentarily open, and the bowl discharges the separated sludge.
The accumulation of sludge fills the sludge and water space with the centrifuge bowl. As this space fills up, the inside perimeter of the sludge moves radially inwards toward the center of the bowl that houses the liquid separation space.
If too much sludge accumulates, the sludge starts to block the flow passage of the separated fluid(s). Therefore, it is essential to discharge the sludge before encroaching on the fluid passages.
Excessive sludge accumulation can lead to the sludge entering the inter-disc space blocking the space between bowl discs. It is very important to avoid this from happening because it causes a complete blockage of the fluid flow, which can cause vibrations and can be a safety hazard.
A hydraulic mechanism executes the sludge discharge from the centrifuge bowl. The sliding piston (shown above) moves downward by releasing the water underneath it. This movement of the sliding piston opens the sludge discharge ports around the bowl periphery.
The high centrifugal force within the bowl causes the accumulated sludge to eject out through the ports instantly. The sliding piston moves back upward after the discharge to close out the sludge ports.
The water harness controls the operating water working this hydraulic mechanism through electric solenoid valves.
Ideally, the centrifuge should discharge only the separated sludge to minimize fluid product loss. However, due to the reasons mentioned below, some process fluid escapes the bowl and the sludge leading to product loss.
The main reasons for the product loss from the centrifuge bowl are explained below.
Self-cleaning disc stack centrifuge bowls have different designs for sludge ejection. The older version of these bowls has a ‘full-shoot’ design wherein the centrifuge ejects the entire bowl contents during the discharge cycle. The bowl typically has 70% of the product within during the process. A full-shoot bowl discharges all the products leading to product loss.
The newer design of self-cleaning centrifuges features a ‘partial-ejection’ design wherein the bowl opens and closes the sludge ports within a short period of time. This short sludge ejection cycle allows the centrifuge to eject only the accumulated sludge while retaining the fluid product. This design reduces product loss significantly.
The discharge cycle time is based on the percentage of solids and the flow rate. However, if there is any error or miscalculation in the cycle time, it is possible to have sludge ejection cycles too frequently.
An ejection during the cycle where the sludge space is not full with sludge implies that fluid product is present in the sludge space. A premature sludge discharge cycle will eject out the product along with the sludge leading to product loss.
To minimize the amount of product loss during discharge, it is necessary to calculate the sludge discharge cycle accurately. The sludge discharge reference chart is a good starting point to estimate the cycle time.
If the bowl is not fully closed or the sliding piston leaks, the sludge can leak out through the sliding piston seal. Once the local sludge has exited, the process fluid follows to escape through the opening leading to product loss.
Read our article on causes and fixes for centrifuge bowl leaking.
Under certain circumstances, the process fluid is displaced with water before the discharge cycle. The water used to displace the product fluid is known as displacement water.
The water outlet from the centrifuge bowl is temporarily blocked, and water is fed into the centrifuge bowl. The incoming water pushes out some of the process fluid through the clean fluid outlet. The displaced water is then discharged with the sludge in the discharge cycle.
The displaced process fluid is recovered from the bowl and not lost with the sludge.
If the displacement water is not used, the displaced product is lost during the sludge ejection cycle.
The implementation of a displacement water step, if feasible, is recommended to alleviate the loss of product fluid under such conditions.
by Sanjay Prabhu MSME
Engineering Manager, Dolphin Centrifuge
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