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A decanter centrifuge, like any other rotating equipment, vibrates during operation. Also, amplified vibration is typical during the startup and shut-down of the decanter. These are amplified vibrations when the bowl is rotating (while ramping up or slowing down) at the decanter’s resonance frequency.
These phases of amplified vibration are normal and should not cause concern as long as they are within set limits. However, if the decanter vibrates beyond its vibration limits, it is essential to stop it and investigate the cause of excessive vibration.
This article discusses the unusual vibrations of a decanter centrifuge, its causes, and individual fixes. Decanter vibrations can occur suddenly or increase gradually over time. We categorize and discuss sudden and gradual vibration issues in separate sections.
The following list summarizes the causes of decanter vibration, which we will discuss in detail in the subsequent sections.
Gradually increasing vibration
Suddenly occurring vibration
During regular operation, a decanter centrifuge's moving and consumable components are subject to wear and tear. Over time this wear causes the vibration level of the decanter to increase. It is a good idea to monitor the centrifuge’s vibration level to keep track of the gradual increase in vibration, indicating wear on the decanter’s specific components.
The two main rotating parts of the decanter centrifuge are the bowl and the auger (aka conveyor). The auger pushes the sludge along the bowl wall, as shown in the diagram above. Abrasive particles in the sludge can cause wear on both the bowl and the auger.
However, a sludge layer, known as a ‘cake wall’ (shown above), builds between the conveyor and the bowl wall. This ‘cake wall’ is stationary with respect to the bowl wall and acts as a protective barrier to prevent erosion of the bowl from within.
The auger pushes the separated sludge along the bowl wall and up the conical section (aka beach). This contact causes wear on the sludge-facing side and the top edge of the scroll flights. The photograph below shows such wear on a typical decanter conveyor.
The pillow-block main bearings facilitate the entire decanter rotating assembly rotation. These are heavy-duty, specially designed bearings for this application. As with all friction components, the bearing has a set lifespan, and its replacement at periodic intervals is critical.
Over time, as the bearings wear out, they tend to develop some ‘play’ outside of the manufacturing tolerance. This play causes a gradual increase in the vibration of the decanter bowl. Periodic checks or control system logging of the decanter vibration level is one way to monitor vibration levels.
New decanter machines from the OEM have vibration levels between 2 mm/s and 4 mm/second. Used decanter centrifuges typically operate within an 8 mm/s vibration level. If a decanter centrifuge’s vibration level is above 10 mm/s levels, it is time for a thorough inspection and bearing replacement.
As mentioned above, all rotating equipment has some level of inherent vibration. In decanters, vibration originating from the Rotating Assembly (RA) travels through the decanter frame and all components attached to it.
Incorrect assembly of the rotating assembly is often related to the pillow block's alignment to the bowl’s rotating axis.OEM and established remanufacturers should provide a rotating assembly alignment procedure. Incorrect (skewed) alignment of the bowl can cause the bearing races to be misaligned and generate excessive torque on the drive motor. This misalignment is also the reason for vibrations in many cases.
Decanter components such as belt guards, belts, motor mount plates, motors, collecting vessels, etc., are all bolted to the frame. Loosening of any fasteners will cause the vibration of the component. This improper assembly can cause the entire decanter assembly to resonate with excessive vibrations.
Therefore, it is essential to check and tighten all fasteners on the decanter frame during scheduled maintenance or in case of excess vibration.
Some vibrations occur suddenly without any pre-indication. These vibrations could occur while the decanter operates or when it starts after a shutdown. We discuss the possible causes of such vibration below.
As discussed above, the auger or conveyor is exposed to maximum wear and tear within the decanter. The conveyor often has weld-on hard-surfacing or welded tiles for erosion protection of the flights.
Operational damage to this protective layer or added tiles causes an immediate imbalance, which leads to excessive vibration of the decanter. A loud noise often accompanies this vibration, which is unexplained by external visual inspection.
The image below shows a conveyor with a broker (missing) tile. The ejected sludge likely carried away the broken tile fragments.
Similarly, a missing chunk of the weld-on hard-surfacing can cause excessive vibrations. This missing mass causes an immediate imbalance in the auger leading to excessive vibration.
Again, this vibration cause will not be evident from external inspection of the rotating assembly. Dismantling of the bowl and extraction of the auger reveals the damage.
Therefore, replacing any missing or broken tiles or damage to the weld-on hard surfacing is critical to the decanter’s vibration-free operation.
During regular operation, the separated sludge collects on the decanter bowl wall. The sludge stays on the bowl wall due to the bowl rotation’s high centrifugal force.
During the decanter’s shut-down, the centrifugal force is no longer present due to the absence of bowl rotation. If the sludge is not compacted enough, it may dislodge from the bowl wall's upper (top) side and fall due to gravity (as shown above).
This transfer of separated sludge to the bowl bottom causes an imbalance in the bowl. Upon startup, this temporary imbalance causes excessive vibration until the conveyor is above to push out the sludge reinstating the bowl balance.
Therefore, starting the conveyor rotation before the bowl during the decanter startup is always good. This pre-start allows the conveyor to push out any of this ‘fall-over’ sludge before the bowl starts to rotate.
Though it is quite rare for a bearing to fail during a decanter centrifuge operation, such failure can cause catastrophic damage to the equipment and cause personal injury. The first and obvious indication of a bearing failure is the sudden, excessive vibration of the decanter. A loud grinding noise typically accompanies a bearing loss.
In practice, however, unanticipated bearing failure is quite rare. Under regular operation, telltale signs such as increasing vibration and higher motor current draw toward a bearing nearing its useful span.
Therefore it is crucial to monitor the vibration levels of a decanter and maintain a data log. This performance data can help the operator spot a gradual increase in vibration levels, indicating deteriorating bearings or other wear within the decanter. Dolphin Centrifuge control systems offer optional decanter centrifuge bearing vibration, temperature monitoring, and login functionality.
In summary, there are several causes of the vibration of decanter centrifuges. Some of these causes are to be expected due to the normal wear caused by the usage of the decanter centrifuge. However, other vibrations can occur suddenly and can be of catastrophic nature. It is critical to the root cause and eliminates the cause of such vibration for the safety and longevity of the equipment and operating personnel.
by Sanjay Prabhu MSME
Engineering Manager, Dolphin Centrifuge