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A self-cleaning disc stack centrifuge ‘cleans’ itself by ejecting the separated sludge from the bowl. The centrifuge bowl has to ‘open’ and ‘close’ to facilitate this ejection step. Therefore, the opening and closing of the bowl are critical steps in the disc centrifuge operation.
A bowl not opening or not closing is a common problem faced by users of self-cleaning centrifuges.
This article will discuss the troubleshooting steps the disc stack centrifuge operator needs to take in cases where the bowl opening and closing functions are not working correctly. We discuss Alfa Laval disc centrifuges, but these principles apply to most disc-stack centrifuges from other manufacturers.
It might be beneficial to the reader to refer to our article about the different parts unique to disc centrifuges. It illustrates critical bowl parts with photographs.
The following illustration shows the cross-section of a ‘self-cleaning’ Alfa Laval centrifuge bowl. We briefly explain the operation of the bowl opening and closing mechanism in the context of this diagram.
The Closing Water entering the Closing Water Chamber pushes the Sliding Piston upwards. The closing water force is higher than that of the process fluid above the sliding piston.
The Springs push up the Operating Slide, which carries the Drain Valve Plug up, closing the Closing Water Drain Channels.
The bowl is now closed, and the process fluid can enter the bowl processing space.
The purpose of the bowl opening sequence is to discharge the accumulated sludge in the sludge space. Therefore the feed to the centrifuge must be shut off before this step.
The bowl opening process starts with allowing the Opening Water into the chamber above the Operating Slide. The bowl Opening Water overcomes the Springs’ upward force pushing the Operating Slide downwards.
This downward movement of the Operating Slide opens the Closing Water Drain Channels. The Closing Water escapes from under the Sliding Piston. The force of the process fluid above the Sliding Piston pushes it downward. This action opens the Sludge Ports, ejecting the accumulated sludge from the bowl.
The system stops the Opening Water supply after the sludge ejection is complete. The water in the Opening Chamber drains through the Opening Water Drain Nozzle, and the Springs under the Operating Slide push it upwards. This movement closes the Closing Water Drain Channels by the Drain Valve Plugs.
At this point, the automatic system supplies the Closing Water. It enters the Closing Water Chamber under the sliding piston through the Closing Water Channels. This water pushes the Sliding Piston upwards. The bowl is now closed, and the sludge discharge cycle is complete.
Specific indications point to the bowl of the centrifuge not being closed.
If the priming water does not flow through the water sight glass after a reasonable amount of time, the bowl has likely not closed. Confirming the open bowl condition is the priming water flowing through the sludge outlet.
The reader should also be aware that this condition can occur if the small lock ring is not tight and leaking.
The other indicator of the bowl being open is a surge in motor current during the prime water supply. The open bowl leads to the prime water exiting the bowl at high velocity through the sludge ports. This escaping water creates resistance to the rotating bowl, exerting more load on the motor, causing the current to rise.
There are multiple possible reasons for the centrifuge bowl’s inability to close. From our experience, it is often related to the operating water supply. We will discuss most operating water-related issues and how they relate to the centrifuge bowl closing issues.
A simple oversight such as shutting the operating water valve will prevent the bowl from closing due to lack of water. The fix is simple – open the operating water valve.
Electric solenoid valves control the bowl closing and opening water flow, and a defective solenoid valve will cause a lack of closing water to the bowl. This shortage of closing water will prevent the bowl from closing.
Manual activation of the solenoid valves and checking for water flow is the simplest way to root cause of this problem. Replacement or repair of defective valves will solve this problem.
As shown in the photograph above, two flexible hoses supply the opening and closing water to the centrifuge. Accidental switching of these two hoses is possible, especially after centrifuge service. After each service, the operator should verify the hose connections to the centrifuge to connect the opening and closing water hoses to the right ports.
Yes, Volume! Not only pressure. All self-cleaning centrifuges have a specific flow rate of bowl closing water needed to close the bowl. We often find the flow rate too low even though the water is under the right pressure.
For a typical Alfa Laval centrifuge, the closing water should flow at a minimum of 3 GPM at the designated pressure. For example, a 30 GPM capacity self-cleaning centrifuge bowl will struggle to close if the closing water supply is through a ½” pipe.
The closing water flow rate is easy to check. The operator can disconnect the closing water hose from the centrifuge and measure the flow rate into a 5-gallon pail.
Closing water pressure is also a critical factor in bowl closing. Most self-cleaning centrifuges require a lower closing water pressure than the opening water.
Higher than designated pressure causes the closing water to overflow into the opening water passage, which can have the opposite effect. High-pressure closing water can instead cause the bowl to open because the operating slide can get pushed down, opening the closing water drain channels.
As shown in the above drawing, the valve plugs prevent the closing water from escaping from the closing water chamber. Worn-out valve plugs fail to create the seal required to hold the closing water in the closing water chamber. Therefore it is essential to check the condition of the valve plugs if the bowl is not closing.
A simple tip is to flip the valve plugs in a bind if one surface is uneven or worn out. In other words, the opposing end of the valve plugs may have a smoother surface, which can temporarily fix the leak. However, it is also essential to check the height of the plugs. Short plugs can cause the same issue.
The distributing ring distributes the incoming closing water into the closing water chamber. In older, used centrifuges, one may find the circular slot crimped or crushed in the distributing ring. Attempted removal of the bowl body without first removing the three bolts holding the distributing ring causes the slot crimping.
Closing the gap in the slot prevents the water from entering the distributing ring. Most often, this condition cannot be repaired and requires the replacement of the distributing ring.
There may be one or two o-rings around the distributing ring. The presence and condition of this o-ring are also crucial for the bowl closing process. The operator should replace this o-ring at the slightest sign of wear or damage. We also recommend coating this o-ring(s) with the sealing lubricant.
For more information on disc stack centrifuge-specific parts, please read our Disc Stack Centrifuge Parts Glossary.
The distributing cover conveys the water to the control paring disc, which jets the closing water into the above distribution ring slot. There is a thin, flat gasket between the distributing cover and the control paring disc.
If this gasket is dry-rotted or torn, it causes the closing water to leak. Therefore some or all of the closing water do not get to the closing water chamber preventing the bowl from closing.
Replacement of this gasket during routine maintenance can avoid this cause of the bowl not closing.
The centrifuge bowl speed issue is only related to centrifuges with a centrifugal clutch in the transmission.
Friction pads mounted on clutch shoes transmit the torque from the motor to the horizontal shaft. Over time these pads tend to wear and should be replaced as part of the regular maintenance. When the friction pads have excessive wear, the clutch cannot transmit the required torque to the bowl. This loss of power slows the bowl speed or RPM.
The closing water uses the centrifugal force generated by the bowl rotation to pressure the upward movement of the sliding piston. Low bowl speed leads to lower pressure, which prevents the bowl from closing. The timely replacement of the friction pads alleviates this problem.
Like the bowl, not closing condition, failure to open is also deducible via observations during the operation.
An audible sound and short vibration spell accompany the bowl opening event. The operator easily observes these indicators. The absence of these signs at the anticipated bowl opening time is the first indication of the bowl failing to shoot. The operator should investigate such occurrences immediately.
The operator can confirm the fail-to-shoot conditions by visually checking the sludge discharge through the sludge outlet during the sludge ejection cycle.
The bowl opening and sludge discharge event causes a momentary spike in the motor current draw. One of the fail-to-close indicators above explains the cause of this current draw increase. The absence of this surge in motor current at the expected time of sludge ejection is another indication of possible fail-to-shoot conditions.
Finally, consistently increasing the vibration level of the centrifuge through multiple discharge cycles is possibly caused by excessive sludge buildup in the bowl. This buildup is a direct result of the bowl being unable to open. The operator should root cause excessive centrifuge vibrations immediately.
As with the issues related to the bowl-not-closing, similar problems can prevent the bowl from opening during the sludge discharge cycle. However, the bowl not opening is more important than the bowl not closing because failure to discharge the sludge can cause excessive sludge accumulation in the bowl.
Un-discharged sludge accumulation can cause severe damage to the centrifuge and possibly lead to catastrophic failure of the centrifuge. The user has to give paramount consideration to the potential for operator injury under such conditions.
All the issues and relevant fixes mentioned above in the ‘Issues with the Closing Water System’ apply to the opening water system.
All the abovementioned factors in the ‘Closing Water Flow-Rate’ section apply to the bowl opening issues.
The opening water pressure is essential for the bowl to open. Maintaining the correct opening water pressure is key to the bowl opening mechanism depending on the centrifuge.
Operating water is always present in the opening water chamber. Over time this water evaporates, which causes mineral deposits around the operating slide. These deposits hamper the free movement of the operating slide needed for the sludge ejection cycle.
A thorough cleaning of the operating slide and spring plate to remove any mineral deposits fixes this issue.
The operating slide has a seal ring (o-ring) around its outer periphery. This seal prevents the incoming opening water from escaping the opening water chamber. Any leaks from this seal ring will prevent the operating slide from moving downward, not allowing the bowl to open.
Replacement of this seal ring will fix this issue. Lubricating the replacement seal with the specific lubricant ensures the longevity of this part.
The above-mentioned telltale signs have physical characteristics (i.e., vibration, sound, current spike, flow or no-flow in passages, pressures, etc.), which are detected utilizing modern instrumentation.
Dolphin Centrifuge’s control systems have built-in features to monitor many of these signs and to alert the operator. Our control system can help the operator identify and fix it with a built-in troubleshooting guide based on the sensed parameters.
This detailed illustrated troubleshooting article should help our present and future customers handle these common, easily surmountable issues while using self-cleaning disc centrifuges.
Also, Dolphin Centrifuge customers can always call us to avail of our lifetime technical assistance.
by Sanjay Prabhu MSME
Engineering Manager, Dolphin Centrifuge