How GEA's active torque control for decanter centrifuges increases economic efficiency.
Creaking doors, squeaking brakes or stuttering pneumatic cylinders and discontinuously running carriages on machine tools – consequences of a phenomenon that is generally unwanted in everyday life as well as in industrial applications: the stick-slip effect.
This detrimental effect can occur when the static friction between the solid bodies moving against each other is greater than the dynamic friction. If the slipping speed of the solid body becomes too slow, it gets stuck and needs additional energy to start slipping again. This resembles irregular chattering: sticking-releasing-slipping-braking-sticking-releasing-slipping and so on.
Invisible from the outside with expensive consequences The stick-slip effect can also occur in decanter centrifuges, irrespective of the machine manufacturer and the design, and its build-up cannot be detected from the outside: The solids of the product to be processed are flung against the bowl wall by the centrifugal force and from there conveyed by the scroll to the discharge. In the case of products that are relatively difficult to convey in the centrifugal field, such as starch or whey in casein production, this can trigger the stick-slip effect.
The solids move inconsistently creating dynamic loads on the mechanical components. Possible effects: Strong torque shocks and oscillations, which create loads on the drive shaft. This leads to increased fatigue and wear to machine components such as the bowl, scroll, gearbox, shafts and coupling. Stick-slip increases the risk of unplanned, expensive downtimes with costly repairs.
Identifying the origin and eliminating it through efficient control Decanters work with an optimally adjusted working point at which the solids are discharged in a state as dry as possible. At times, the optimal point is at the boundary region where stick-slip is a concern. If stick-slip occurs, the effect can be eliminated by adjusting the working point.
Option one: By means of an increased differential speed, the solids can be conveyed out of the bowl faster (reduction of the solids load in the bowl), but less dry.
Or option two: The bowl speed is reduced to permit easier conveying of the solids, but with a corresponding reduction of the capacity of the system. Both options have one thing in common: compromised economic efficiency. The system operator only knows that the working point needs to be adjusted to avoid stick-slip if there is damage detected, but not by how much. If the correction is too small, mechanical damage is imminent. If it is too big, the solids exiting the decanter are wetter than necessary, and needlessly increases energy costs for the downstream thermal drying.
The task is therefore to reliably detect the occurrence of the stick-slip effect and to control it in such a manner that the best possible process result is achieved while taking the stick-slip effect into account. This is exactly what GEA has now achieved with the development of Active Torque Control (ATC).
GEA solution: Correction of the differential speed by means of active torque control Here the research & development experts at GEA engaged in some detective work: The detailed analysis of damage patterns showed that the stick-slip phenomenon had to be detected on the decanter’s drive train. During the investigation, the coupling between scroll motor and gearbox was visually checked using a high-speed camera and stroboscope.
The results revealed a visible oscillation of the coupling components generated by a dynamic torque when operating at a working point within the stick-slip boundary region. For GEA engineers this meant the development of a new sensor-based monitoring and control method to precisely detect the onset of torsional vibrations on the coupling and, depending on their intensity, avoiding them by automatically changing the working point.
Advantages of ATC for the user GEA’s innovative solution relies on an automated correction of the scroll speed which is rotating faster than the bowl to create the differential speed to convey solids out of the decanter. The user benefits from automated, precise changes during ongoing operation. Even with products susceptible to stick-slip conditions, the decanter can be operated as close as possible to the optimum working point with the best possible yield of dry substance. And it pays off: In starch production with a GEA CF 6000 decanter, for example, a dry substance value of the solids that is only one percent higher can achieve annual savings of approximately EUR 20,000 in the use of the dryer.
In the casein process, too, a 1-2 percentage point higher dry substance can be achieved, which can result in approximately EUR 15,000 lower drying costs. In this application, the system displays its strengths in allowing greater system capacity and in a primarily more uniform structure of the casein flakes and the associated more stable dryer operation. And, finally, the risk of dreaded unplanned shutdowns is drastically reduced.
ATC equipment for new and existing machines GEA offers both new machines of the CF decanter series with ATC equipment and the option of retrofitting existing CF decanters with the revolutionary ATC automated machine monitoring and control system. Experience with decanters without ATC shows the operating staff is almost “blindly” increasing the differential speed to be on the “safe” side. With ATC, the negative consequences in the form of additional unnecessary costs for downstream drying can be avoided and the operation and maintenance staff can see the results.
Conclusion With the automatically operating Active Torque Control for CF decanters, GEA has developed a market innovation that makes the production process for starch and casein not only more reliable but also as profitable as possible under stick-slip conditions. The active torque control in process operation, compared to decanters without ATC, provides the user optimum machine utilization and availability, maximum process reliability and flexibility, optimum product quality and yield by increasing the dry substance. This reduces energy costs in thermal drying. If only one production stop caused by the consequences of the stick-slip effect is prevented by the ATC system, the investment would have already been profitable.