Two speeds with different roles
A decanter has two rotating parts that matter here: the bowl and the scroll inside it. They do not turn at the same speed. Flottweg describes the difference between them, the differential speed, as necessary because it is what allows the scroll to transport the solid particles that collect against the inner diameter of the bowl. If the two turned together, there would be no relative motion to push the solids along.
This makes differential speed a different quantity from the absolute speed of the bowl. Bowl speed relates to the centrifugal field in which the solids settle, differential speed relates to how the settled solids are carried towards the discharge. Flottweg also states that differential speed determines how long the solid material remains in the bowl. A reader who sees a number labelled "speed" on a trend should therefore ask which of these two quantities it describes before drawing any conclusion.
Why changing feed matters
The scroll has to move whatever quantity of solids arrives, so the load on it depends on the feed. Flottweg explains that changes in the feed change the amount of solid the scroll must transport and therefore the loads that arise, which shows up as a change in scroll torque. Scroll torque is thus a way of seeing how hard the scroll is working to move the solids.
The manufacturer also ties differential speed to the compression the solids undergo and to the total dry solids of the discharged solid. Because product properties in the feed can change, it describes the ability to set differential speed variably as important: adapting it is presented as the way to keep the dry solids at an optimum level and to react to a wide range of feed properties. The same page lists overload and blockage protection among the benefits of a controllable differential speed. The practical reading is that no single fixed value is offered as right for every feed. The sources explain why the value needs to be adjustable, not what it should be.
A manufacturer-specific control example
Flottweg's Simp Drive® is one way of handling this. The manufacturer says it regulates the differential speed between bowl and scroll depending on the prevailing scroll torque. It achieves this with a special planetary epicyclic gear unit, which lets bowl speed and differential speed be adjusted independently of one another during ongoing operation. The gear unit input shaft is driven by a variable-speed motor, and a second variable-speed motor drives the bowl.
Flottweg lists several stated benefits: consistently high dry substance, high throughput from high bowl speed combined with a precisely adjusted differential speed, display and regulation of scroll torque including overload protection, and the ability to turn the scroll even at standstill so the machine can be cleared out. This is a description of Flottweg's own drive arrangement. The Flottweg differential-speed page separately says that in modern decanter drives the differential speed adjusts automatically to the load condition in the machine. The pages reviewed here describe Simp Drive® and do not describe the transmission or control strategy of any other machine, so nothing should be inferred about those.
What this does not establish
The public pages do not provide torque limits, speed limits or a tuning sequence for any installed machine. They also do not give a method for diagnosing a particular problem from a torque or speed reading. The fact that bowl speed and differential speed can be adjusted independently does not mean that the machine's operating limits can be ignored, independence of adjustment says nothing about what values are permissible.
When a trend or a proposed change is being reviewed, the useful discipline is to name which quantity is involved: bowl speed, differential speed or scroll torque. Each answers a different question. Bowl speed concerns the field in which solids settle, differential speed concerns transport and residence time, and torque concerns the load on the scroll. Limits and settings come from the machine documentation and approved site procedures, not from a manufacturer's feature overview.