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One of the essential features of a self-cleaning disc centrifuge is to eject the separated solids while operating. The frequency of this ejection process is crucial to the process efficiency and centrifuge wellbeing.
This article discusses the various aspects of the sludge ejection cycle time, from calculating the cycle time to the results of incorrect cycle time.
As explained in our Self-Cleaning centrifuge description, a disc centrifuge separates the fluids from the solids. While the bowl discharges the fluids continuously, the solids accumulate in the bowl.
The centrifuge bowl has a set sludge-holding space around the internal periphery. This space gradually fills up with the separated solids. As the sludge builds up, the sludge accumulation’s inner wall grows radially inwards towards the disc stack.
The interstitial space between the stack discs is an essential passageway for the process fluid. The sludge cannot be allowed to reach the disc-stack and block this fluid flow path.
This need to keep the disc stack flow path open necessitates the built-up sludge’s evacuation periodically through the sludge ejection process.
Self-cleaning disc centrifuges eject the solids through a hydraulic mechanism that has to be triggered by a timer or a PLC. The hydraulic mechanism uses water which is known as operating water.
It is important to note that the operating water does not contact the process fluid. This water is in a separate chamber below the process fluid chamber within the bowl.
As shown in the diagram below, a high-pressure operating water pulse pushes the operating slide downwards. The operating slide has three valve plugs on its upper surface.
When the operating slide moves down, the valve plugs move away (down) from the closing water chamber’s drain ports below the sliding piston.
The opening of the drain ports allows the closing water to escape through the drain water passage. The sliding piston no longer has the upward pressure from the centrifugal force of the closing water. As a result, the sliding piston moves down due to the process fluid’s internal centrifugal pressure.
The downward movement of the sliding piston opens up the sludge ports around the bowl wall. See the image above showing bowl sludge ports. The high centrifugal force within the rotating bowl ejects the accumulated sludge out through these ports.
Other disc-centrifuge articles of interest......
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Disadvantages of a Disc-Stack Centrifuge | Illustrated Guide
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The time interval between sludge ejection cycles is known as the Sludge Ejection Cycle Time. In other words, it is the period it takes for the sludge space within the bowl to get full.
It is clear from the above discussion that the three factors affecting the time for sludge buildup are the rate at which the sludge is entering the bowl and the flow rate. The amount of sludge carried by the incoming process fluid is the second factor: the solids percentage.
Knowing the rate at which the solids are coming in, the final factor is the amount of space for these solids to collect.
We discuss these three factors in detail in the following sections. We also point out the ways to quantify each of these parameters.
Referring to the bowl cross-section diagram above, the bowl sludge space is the volume available for the separated sludge to accumulate in the bowl. Larger bowls have larger sludge space compared to smaller bowls. The available sludge space is proportional to the bowl size.
In technical terms, the sludge space is the bowl’s volume between the bowl hood, the sliding piston, and the disc stack's outer surface.
The bowl sludge space is directly proportional to the sludge ejection cycle time for a set fluid with a set sludge load. Large sludge space allows for longer sludge ejection cycle time.
Conversely, smaller sludge space leads to shorter sludge discharge cycle times or frequent sludge discharge cycles.
Though the manufacturer does not mention the actual bowl’s sludge-space volume, it is possible to get the volume from a knowledgeable supplier.
One can estimate the bowl sludge space by taking some basic measurements of the bowl components. Some simple geometric calculations will provide a reasonable estimate of the sludge space volume.
The volume of process fluid fed to the centrifuge in a given period is the flow rate. The optimum flow rate for a given process should be carefully regulated based on the liquid type, solid load, and final product requirements.
If the sludge space and percent solids in the process fluid do not vary for a given centrifuge, higher flow rates are inversely proportional to the sludge discharge cycle time. This shorter cycle time is due to the higher flow rate carrying solids quickly to the bowl.
The shorter retention time associated with higher flow rates reduces separation efficiency, thereby reducing the amount of separated sludge.
However, it is better to be conservative and use a shorter cycle time to reduce the risk of undesirable results of longer than required cycle times.
The simplest way to measure the flow rate is with an electronic flow meter installed on the centrifuge inlet line. This device provides a constant flow rate measurement that a control system can utilize to regulate the flow and adjust the cycle time.
A mechanical flow meter is also useful for monitoring the flow rate and adjusting the flow or cycle time.
A positive displacement feed pump operated by a VFD (variable frequency drive) also functions as a flow meter. The direct correlation between the pump speed and the pump’s flow indicates the flow rate.
The percentage of solids in the process fluid is the third factor determining the sludge discharge cycle time. Also known as the solid load, the percentage of solids is inversely proportional to the sludge discharge cycle time.
For a given flow rate and size of the centrifuge bowl, the higher the solid load, the shorter the sludge discharge cycle time. In other words, a higher proportion of solids fills up the sludge space faster, which requires a shorter time between sludge discharges.
The primary way to determine the percentage of solids in the process fluid is to run a fluid sample through a benchtop centrifuge. Specially designed tube-centrifuges have graduated tubes that provide an accurate reading of the separated solids.
Mass flow meters are another method of determining the percentage of solids in the process fluid. These devices measure the solid load in a liquid by electronic means and provide a precise and continuous measurement of the solids’ percentage.
The centrifuge controller can use this digital reading to calculate the sludge cycle discharge time automatically.
The actual calculation of the sludge discharge cycle time is quite simple. We are trying to calculate the time it will take for the bowl’s sludge space to fill up. In other words, we are converting the percent solids in the fluid into the period to fill the sludge volume in the bowl.
We can convert the calculation described above into a simple formula shown below.
It is crucial to ensure that the measurement units are consistent to get an accurate and reliable time.
For example, for US customers, the measurement units are:
|Solids Percentage||As a Percentage (for 2% input 2 not 0.02)|
not to be confused with a decimal number
|Bowl Sludge Space||Gallons|
The sample below is an excerpt of a larger table that helps determine the sludge cycle time for a specific centrifuge for different flow rates and percent solids.
The following table shows the discharge time in minutes for varying flow rates and % solids for the Alfa Laval MOPX-207 Self-Cleaning centrifuge.
|Flow GPM ↓ % Solids →||0.1||0.2||0.3||0.4||0.5|
It is essential to understand that incorrect cycle times cause product losses and possible detrimental effects on the centrifuge.
However, the operator should NEVER OVERESTIMATE the sludge discharge cycle time. Overestimation of the cycle time can lead to catastrophic failure of the centrifuge and possible injury or death. Irreparable damage to the centrifuge can also result from an overestimated cycle time.
Longer than optimum cycle time will lead to over-accumulation of sludge in the centrifuge bowl. This overestimation has several detrimental effects on the centrifuge itself and its performance.
Once the sludge space is filled up, and the sludge and a longer cycle time prevent the sludge ejection, the sludge buildup continues. The excess sludge then enters the interstitial space between the discs. These solids fill up the disc stack restricting the fluid flow, which stops the separation process.
In the worst-case scenario, the solids can accumulate in the upper discharge chamber and create a bridge between the centrifuge's rotating and stationary parts. This bridging can impart a sudden braking effect on the rotating bowl assembly.
The rotating bowl’s momentum has tremendous energy, which can dissipate in a destructive way damaging the bowl or even dismantling the centrifuge. This rare event is known to cause catastrophic damage to and physical disintegration of the operating centrifuge.
The first indication of too much sludge buildup due to a long cycle time is solids’ appearance in the liquid phases. Once the sludge space is full, the separated solids have no room and will flow out with the separated fluid.
A sudden appearance of solid in the otherwise clear centrate indicates the sludge space is full due to a longer than optimal cycle time.
The accumulation of too much sludge can be irregular inside the bowl. This excess sludge causes an imbalance in the bowl leading to excessive vibrations. This sudden increase in the vibration level is another indication of too long a sludge discharge time.
If the sludge enters the space between the discs and restricts the fluid flow, it will reduce centrifuge fluid discharge. The excess incoming fluid will overflow and come out through the frame or other unusual path.
This fluid exit through an unusual outlet indicates a longer than required discharge cycle time.
Shorter than optimal sludge discharge cycle times cause the sludge ejection before the sludge space is full. In this case, the loss of some of the process fluid occurs with every sludge discharge. This loss is not desirable from the operator’s perspective.
The frequency of sludge discharges increases due to unnecessarily short cycle times. This increase causes the sludge discharge mechanism’s frequent operation and has a detrimental effect on the centrifuge’s service life.
The centrifuge motor current spikes during the sludge discharge process. If the sludge discharge cycle time is too short, the motor’s frequent current spikes can cause the motor to overheat, leading to premature motor failure.
The easiest way to detect shorter than optimal discharge cycle time is to monitor the discharge from the sludge outlet. The presence of large amounts of process fluid could be due to the short cycle time. Therefore it is essential to sample the sludge discharge periodically and quantify the proportion of sludge, and process fluid therein.
To summarize the above, the sludge discharge cycle time is crucial for operating a self-cleaning disc-stack centrifuge.
With the calculator and formula provided in this article, a centrifuge operator should determine the optimum cycle time between sludge discharges and get the maximum efficiency out of the centrifuge.
by Sanjay Prabhu MSME
Engineering Manager, Dolphin Centrifuge