A disc stack centrifuge uses the centrifugal force it generates to separate the liquid from solid and liquid from liquid. The centrifugal force exerted is a direct function of the bowl’s rotating speed.
A disc centrifuge cannot reach operating speed due to several reasons. In this article, we will explore the causes and relevant fixes to these problems.
The design of a disc centrifuge assumes a specific bowl rotating speed (RPM) to perform its separating function. Other aspects of the disc centrifuge operation also depend on the bowl operating speed.
An incorrect or slow operating speed is detrimental to centrifuge performance issues. The low operating speed reduces separation efficiency and also prevents the proper functioning of the bowl mechanism.
The operating speed of the centrifuge bowl primarily determines the centrifugal force needed for the efficient separation of the fluids. Therefore, any degradation of the bowl speed has a detrimental effect on the centrifuge’s separation efficiency.
In self-cleaning centrifuges, the bowl operating speed is essential for keeping the bowl from opening during operation. The bowl RPM is also critical for the bowl's opening during the sludge ejection process.
Alfa Laval disc centrifuges have a revolution counter (rotating disc) on the front of the lower frame, as shown in the image above.
To check the speed of the disc centrifuge, the operator counts the rotation on the revolution counter and compares it to the revolution counter speed specified for the particular centrifuge type on the centrifuge nameplate. The revolution counter rotates between 72 and 150 RPM, depending on the centrifuge size.
If the revolution counter is not rotating at the specified speed after the elapse of the ramp-up time, then the disc centrifuge is not operating at the design operating speed.
Use the following 10 steps to check and verify the disc centrifuge operating speed.
Several mechanical and some electrical issues can cause a disc centrifuge bowl not to reach its designed operating speed. The following is a list of possible causes with individual corrective actions to fix each cause.
An engaged brake is the most common cause of slow centrifuge bowl speed. If the operator uses the mechanical brake on the centrifuge frame while stopping the centrifuge, the operator often starts the centrifuge without disengaging the brake.
The brake prevents the drum and, therefore, the bowl from attaining full operating speed. Checking and disengaging the brake (if engaged) before every startup is the fix to this cause.
The friction clutch is the crucial assembly that transfers the torque from the motor to the centrifuge transmission. Any malfunction of the friction clutch can lead to various operational issues for the disc centrifuge. Read our detailed article on Friction Clutch Maintenance.
The friction pads are consumable parts in the friction clutch. Worn-out friction pads cannot transfer the motor torque to the centrifuge shaft, which causes the centrifuge not to reach its operating speed.
The simple fix for this cause of incorrect centrifuge operating speed is to replace the friction pads periodically per the manufacturer’s instructions.
The friction clutch drum can, on occasion, gets a very smooth, glossy surface. This mirror-like surface could be due to previous metal-to-metal contact with totally worn-out friction pads.
The friction pads slip on this smooth surface and cannot transmit the torque from the motor to the centrifuge shaft. This slippage prevents the centrifuge from attaining it's designed operating speed.
The solution to this problem is to ‘roughen up’ the inner surface of the clutch drum to facilitate adequate friction between the pads and the drum surface.
The manual or automatic removal of sludge from the bowl is essential for a disc centrifuge’s proper and safe operation. However, it is possible to accumulate too much sludge in the bowl due to operator error or system failure.
In such cases, the bowl packs up with sludge causing the bowl mass to increase considerably. The power of the centrifuge motor may not be sufficient to rotate this added weight. This added weight prevents the centrifuge from reaching operating speed.
The cleaning or purging of the bowl is the only way to fix this cause of slow bowl rotation. Some situations may require a thorough manual cleaning of the centrifuge bowl, including the disc stack.
The failure of any bearing in the centrifuge will prevent the transmission components from free rotation. This resistance caused by the failed bearing prevents the centrifuge from rotating at its design speed.
Bearing failure is often accompanied by an exceptionally high noise level of the centrifuge. The centrifuge may also vibrate at a higher amplitude.
A total and thorough centrifuge rebuild is essential to fix any bearing failure-related issues.
A disc centrifuge incorporates a gear housing in the bottom frame. The main transmission gears operate in this housing. Gear lubricating oil fills this housing to a predetermined level, i.e., halfway through the sight glass, to splash-lubricate the bearings and the gears.
Suppose the operator inadvertently fills too much oil in the gear housing. In that case, the thick oil resists the rotation of the gears, thereby slowing down the centrifuge speed.
The operator needs to check the gear housing oil level regularly (through the sight glass) for too much or too little oil to ensure that the centrifuge operates per design.
For more details, read our article on Disc Centrifuge Lubrication.
Sometimes during the service or replacement of the centrifuge motor, the motor shaft gets misaligned from the horizontal shaft. Incorrect motor assembly or dent on the motor flange could be possible causes.
This misalignment causes a transverse load on the horizontal shaft causing the bearings to resist free rotation leading to incorrect or low bowl speed.
A motor alignment issue usually accompanies a high motor current and is easily identified by monitoring the motor current draw. Therefore, it is essential to monitor the motor current after motor replacement or repair.
The solution to this cause of low centrifuge speed is to check the motor alignment and fix it as needed.
Proper disc centrifuge bowl assembly is critical to the safe and efficient operation of the centrifuge. If there is a misalignment in the bowl part, the bowl does not fit correctly. In this case, the rotating parts of the bowl make contact with the stationary centrifuge parts.
Visit our blog for details on disc stack centrifuge bowl parts.
The contact between the centrifuge rotating and stationary parts cause resistance to rotation, reducing the bowl speed.
This contact between the rotating parts and stationary parts will cause considerable damage to the centrifuge parts, and the operator should avoid it. It is also a safety hazard.
The simple fix to this cause of low bowl speed is to dismantle the centrifuge bowl and reassemble it correctly.
A few electrical issues can prevent the centrifuge from reaching it's designed operating speed. The following is a list of the most common electrical issues.
The majority of centrifuge motors are three-phase electric motors. If an electrical fault occurs and the motor loses one or two phases, it cannot generate the required torque or rotate at the desired speed. This improper operation causes the centrifuge to rotate at a low speed and not achieve the desired RPM.
This high motor current draw accompanies this electrical malfunction and is thus easily detected.
A thorough electrical wiring check and troubleshooting are essential to identify the cause of the phase drop and get the centrifuge to operate at design speed.
The centrifuge motor can fail due to many reasons. Bad motor windings or bearing failures are common occurrences. In any case, a malfunctioning motor will directly cause low torque and therefore prevent the centrifuge from rotating at the specified RPM.
Motor repair or replacement is the fix for this problem.
All disc centrifuges have a specific direction of rotation which is critical to the operation of the centrifuge. The direction of rotation is indicated by the arrow on the centrifuge frame and often on the motor.
A phase reversal of the electric supply to the centrifuge drive motor will cause the centrifuge to rotate in the opposite direction. This reverse rotation is highly undesirable and must be avoided at all times.
The friction clutch design only allows the torque transmission when it is rotating in the correct direction. A clutch rotating in the opposite direction cannot transmit the torque for the bowl to rotate at the specified speed.
In addition, a reversal of rotation of the centrifuge will unlock the threaded parts of the bowl assembly (left-hand thread), possibly leading to catastrophic damage to the equipment and operators.
To prevent reverse rotation, the operator must check the motor rotation each time the motor is electrically disconnected or after any electrical systems service.
by Sanjay Prabhu MSME
Engineering Manager, Dolphin Centrifuge