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All self-cleaning disc-stack centrifuges have a hydraulic mechanism that operates the sludge ejection system. The centrifuge uses water as the medium of operation of this system, known as centrifuge operating water.
The operating water serves two primary functions in the sludge ejecting mechanism of the disc-stack centrifuge. First, it accumulates under the sliding piston of the centrifuge bowl. The centrifugal forces caused by the rotation of the bowl create pressure in this water.
This pressure pushes the sliding piston upward and seals it against the bowl hood. The movement of the sliding piston seals the process chamber of the centrifuge bowl.
The centrifuge can now process and separate the incoming process fluid in this enclosed chamber.
For the sludge ejection, operating water enters the chamber above the operating slide, which pushes the slide downward. This operating slide’s movement opens the ports below the closing water chamber, allowing the closing water to escape the closing chamber.
With no pressure from the closing water, the sliding piston moves downward. This action opens the sludge discharge ports along the bowl periphery. The high centrifugal force inside the bowl ejects the accumulated sludge instantly.
After the sludge ejection, the closing water is reintroduced to the closing water chamber. This water then pushes the sliding piston upwards to form a seal with the bowl hood. The process-fluid flow starts after the bowl closure, and the separating process continues.
Centrifuges manufactured by different manufacturers have specific requirements for the operating water. Most centrifuges need operating water at a specific pressure and flow rate.
Alfa Laval full-shoot centrifuges use high and low-pressure water for bowl opening and closing, respectively.
A specially designed water handling system is an integral part of any self-cleaning centrifuge. This sub-assembly is known as the Water Harness or Manifold.
The water-harness of a self-cleaning centrifuge is an assembly of solenoid valves with pressure reducers to regulate the water pressure. This assembly connects to the centrifuge through flexible hoses.
There are multiple water outlets on this manifold that connect to the corresponding ports on the centrifuge frame. The water pressure regulators allow for adjustment of the water pressure. The harness also includes ball valves to regulate the water flow rate.
The centrifuge controller triggers the solenoid valves of the water-harness based on a sequence of operations. For example, on startup, the bowl-close solenoid opens, allowing closing water to enter the closing water chamber. This action closes the bowl enabling it to process the fluid.
When it is time for sludge ejection, based on an internal controller timer, the bowl open solenoid opens to inject a high-pressure pulse of water into the centrifuge bowl operating water space. This water operates the pushes the operating slide downwards to cause the bowl to open and the sludge ejection.
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The amount of operating water used by a centrifuge is a function of the bowl size and centrifuge type.
For example, a typical Alfa Laval centrifuge of medium processing capacity uses approximately 1~2 gallons of water to close the centrifuge bowl. The same centrifuge uses about 1 gallon of water to discharge the sludge.
The operating water is an essential component of the trouble-free operation of an industrial centrifuge. Specific parameters of the operating water are essential from the operational perspective. The following is a list of parameters to consider and monitor.
The quality of the operating water refers to the cleanliness and dissolved minerals in the water. This cleanliness parameter is further sub-categorized into the following two parameters.
Operating water hardness is a measure of the presence of dissolved minerals in the water. Ground or well water typically carries a lot of dissolved solids. This water is known as hard water and is not suitable for the operation of self-cleaning centrifuges.
Undissolved sediments are often present in unfiltered water. These particles make the water impure and not suitable for centrifuge operation.
The operating water stays in the bowl operating chamber. Over time some of this water evaporates from the chamber. If the operating water has dissolved minerals, these impurities are left behind as crystallized deposits in the bowl operating space.
The accumulation of these deposits causes the bowl to malfunction, as they can block the operating nozzles.
Similarly, sediment from unfiltered water deposits in the operating water system prevents the operating slide from moving freely. These sediment deposits can cause the bowl opening and closing mechanism to malfunction.
The centrifuge bowl requires a preset amount of water to close. It is important to note that a provision within the operating chamber allows some of this water to escape.
If the volume of incoming water is not sufficient, the water cannot build up in the bowl to cause the sliding piston to rise and close the bowl.
Insufficient operating water volume prevents the centrifuge bowl from closing. The bowl’s failure to close leads to the process fluid exiting the bowl through the sludge outlet, which is undesirable.
Like the water flow rate, the operating water pressure is equally essential for the self-cleaning centrifuge bowl’s functioning.
If the bowl closing water pressure is below the required pressure, the bowl will not close properly. This failure to close will result in the process fluid exiting the centrifuge through the sludge outlet.
If the closing water pressure is too high, the water can inadvertently enter the opening water chamber and cause the bowl to open. This opening of the bowl is unintentional and will cause process fluid loss through the sludge outlet.
Given that the operating water is close to the process fluid chamber, the operating water temperature affects the process fluid directly. The effects of operating water temperature are primarily related to the heat loss of the process fluid from cold operating water.
In rare cases, if the process fluid is at a low temperature, hotter operating water can indirectly heat the process fluid affecting the process efficiency.
For the operation of the hydraulic mechanism of the sludge ejection system, the operating water needs to be in a liquid state.
The maximum operating water temperature cannot be above the boiling point of water. If the operating water boils, the steam generated will cause the centrifuge to malfunction since steam cannot operate the hydraulic mechanism.
If the operating water temperature falls below 0 C, the water will freeze. The freezing of this water will prevent it from flowing and working the hydraulic system.
Therefore, exposure of the centrifuge to sub-zero temperature will cause the centrifuge to malfunction.
Partially frozen operating water can cause an imbalance in the bowl that can lead to excessive centrifuge vibration. In some cases, this can lead to a catastrophic failure of the centrifuge and possible injury to the operator.
Water may not be suitable for specific applications. Customers often ask if they can use other fluids instead of water to operate the centrifuge bowl.
The Alfa Laval and Westfalia centrifuges are specially designed to use water as the operating medium. Considering the extremely high centrifugal force experienced by the bowl, it is crucial to use the correct fluid for the system's operation.
Air or other gases do not have the density to generate the required pressure to operate the hydraulic mechanism. Therefore gases cannot work for the operating system.
The operating water (opening and closing) intermittently exits the centrifuge through a specific frame drain. The water drains out under gravity and does not have any inherent pressure. Therefore, the operating water drain should always connect to a hose or pipe with a downward slope.
Connecting the operating water drain to an upward-sloping hose or pipe will cause the operating water to back-flow into the centrifuge bowl frame. The water will collect in the bowl casing and cause a drag on the rotating bowl, slowing the bowl and leading to many operating issues.
The collected water will also enter the operating water space causing malfunctioning of the sludge discharge mechanism.
The discharged water is typically contaminant-free if the centrifuge is functioning normally. However, in case of a centrifuge failure or malfunction, the discharged operating water may carry the process fluid, often oil. This discharge water is contaminated and needs appropriate handling.
Following the above discussion, a clean operating water discharge is fit for disposal in the regular drain. However, if oil or some other fluid contaminates the exiting operating water, then it cannot be disposed of in the drain and should be disposed of properly.
Fresh, clean water is not always available in abundance. In remote field locations, it is hard to find clean, potable water. In such cases, reuse of the used operating water is desirable.
Again, as discussed above, if the centrifuge is in good working condition, the operator can reuse the exiting operating water. If the operating water is contaminated with the process fluid, the exiting water may be collected and decanted for reuse.
by Sanjay Prabhu MSME
Engineering Manager, Dolphin Centrifuge