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The paring disc (aka impeller) is the centripetal pump built into the disc stack centrifuge bowl. The dense phase (often water) paring disc gets blocked with solids (plugged) in high sludge applications. This article discusses the cause of this problem and possible solutions for Alfa Laval and similar disc-stack centrifuges.
The heavy phase paring disc impeller’s location is above the gravity disc inside the disc centrifuge bowl. It is in a chamber within the upper chamber cover or small lock ring. The disc centrifuge bowl section below shows the exact location of this impeller.
As indicated by arrows in the drawing, the separated water accumulates in the water space in the bowl. The water gets pushed by incoming, separated water to travel through space between the top disc and bowl hood into the upper chamber.
In the water chamber, the rotating water enters the paring disc pump (impeller). The vanes within the impeller pressurize the water as it passes through. This pressurized water then exits the bowl through the upper water housing.
As detailed in the centrifuge bowl cross-section above, the disc bowl’s water space is between the light phase (oil) and the sludge space. If the separated water carries sludge particles with it, they pass through the upper water chamber.
There are crevices and ribs within the chamber cover. The sludge particles tend to get stuck in these crevices and potentially start building up in the upper chamber. The vanes within the impeller have sharp corners where similar sludge particles can buildup.
Thus the sludge particles carried by the separated water phase can buildup and block the paring chamber cover and the water paring disc.
Suppose the process fluid contains microparticles that are below the separation capacity of the centrifuge. In that case, the suspended solids tend to travel with the water and subsequently build up in the chamber cover and paring disc impeller.
The other possible reason for the escaping water to carry sludge particles is too much sludge accumulation between the bowl discharge cycles. In other words, infrequent sludge discharge cycles can allow the sludge to start traveling with the water as described above. Under these conditions, the entire bowl cavity, including all fluid chambers, will get clogged with solids.
Unexpected water in the clean oil phase is the first symptom of a blocked water paring disc. If the centrifuge was working as desired and the operator notices a sudden increase of water in the clean oil, then it is a sign of a restricted water paring disc.
This excess water in the separated oil phase is due to the clogged water paring disc’s inability to pump out all the separated liquid. The water then builds up in the water chamber and starts to escape with the separated oil.
There are mechanical symptoms of the upper chamber cover being clogged with sludge. The primary indication is a longer startup time and a noticeable higher current drawn by the centrifuge motor, which is a clear sign of some resistance to the bowl rotation.
This resistance could be due to the packed rotating chamber cover coming in contact with the stationary water paring disc through the built-up sludge. This phenomenon is known as ‘bridging.’
If the operator notices any such indications, it is imperative to investigate the cause of the sign urgently. Mechanical symptoms may be an indication of severe accumulation of sludge and need immediate attention. If left unattended, they can cause severe damage to the centrifuge, as detailed below.
As indicated in the symptoms above, a blocked water paring disc will prevent the separated water from exiting the bowl through its designed path. The water then starts coming out with the separated oil phase. The water contaminates the clean oil phase.
A much more severe issue is ‘bridging.’ If bridging occurs during the centrifuge operation, the instant contact between the rotating and stationary parts will lead to a sharp resistance to the rotating bowl. We have heard of the catastrophic failure of centrifuges under such circumstances.
It is conceivable for the entire transmission of the centrifuge to break apart during an extreme reverse torque event on the gear train caused by ‘bridging.’ Therefore the operator needs to inspect the upper chamber cover periodically and consider changes to the process to eliminate the possibility of ‘bridging’ under any circumstances.
To clear a blocked paring disc, the operator needs to stop the centrifuge and wait for it to come to a complete stop. A proper lock-out procedure should prevent accidental starting of the centrifuge.
The operator then needs to open the upper chamber cover and extract the water paring disc. It is best to use a solvent or other dissolving agent to clean both these parts. Verify the cleanliness of the paring disc by blowing compressed air through it.
A thorough visual inspection to look for scratches due to surface contact is also highly recommended.
The presence of colloidal particles suspended in the water phase is a common cause of water paring disc blockage. In applications where such solids are present, routine checking of the water phase coming out of the centrifuge is essential. The presence of suspended solids in the water is indicative of a possible issue.
Likely, the particles do not separate from the water in the centrifuge bowl due to low residence time. A reduced flow-rate will increase the residence time in the centrifuge bowl affecting the better separation of the fine solids.
The introduction of chemicals can help flocculation of colloidal solids. This remedy is worth considering if the process water has significant amounts of suspended solids.
Based on the above discussion, we can summarize that the water paring disc blockage due to sludge is an important issue and should be handled by the centrifuge operator on a timely basis. Ignoring this problem can lead to severe damage to the centrifuge and could harm the operator in case of a catastrophic failure of the centrifuge.