A world leader in the cement industry had a project to develop the production of a new type of cement at their production centre located in Madagascar.
In order to do this, they had to replace the old cement and fly ash dosing equipment whose dosing accuracy (±2%) was insufficient for the new project. The two dosing lines (cement and ash) were made up of:
- A buffer hopper
- A knife gate valve
- A helmet dosing valve
- An impact flow meter
- A fluidisation booster
Based on their experience and expertise in bulk materials dosing and after studying the needs of their client, Sautelma Rotolok proposed an original dosing solution with two symmetric weighing hoppers operating according to the principle of weight loss. The two hoppers, each equipped with a screw extractor, constitute a high-precision dosing system (< 0.5%).
Each dosing station (cement and ash) is continuously fed by a screw conveyor. The flow rate is about 60 t/h for cement and 40 t/h for ash.
Each filing screw feeds a pneumatically operated “Y” type diverter that directs the product to one of the two weighed hoppers according to the level measured by a level probe (high).
In fact, they are two loss-in-weight feeders mounted on a common supporting frame and discharging the dosed material into a common chute.
The twin hopper weighing system has two fundamental advantages in this case:
1) Absorb a continuous flow of material because while one hopper is emptying the other is filling.
2) Avoid the “filling phase” during which, in the case of a classic loss-in-weigh feeder, the dosing is volumetric (without weighing) and therefore less accurate. The feeding by two hoppers is therefore 100% gravimetric.
The operating sequences:
To simplify the explanation, we will call the hoppers, Hopper 1 and Hopper 2.
At start up, the hopper 1 is filled and the hopper 2 is empty.
When the high level in hopper 1 is reached, the diverter changes position and directs the product to Hopper 2.
When the order to start is given the weigh feeding starts.
The MINISMART controller, equipped with a specific weight-loss dosing program, which controls the dosing unit, adjusts the speed of the dosing screw so that the weight lost per unit time (flow rate) is constant and in accordance with the set point.
When the low level is reached (measured by weighing the hopper) the diverter changes position and redirects the product to Hopper 1.
At the same time Hopper 2, which in the meantime has been filled, starts dosing.
If the feed rate is higher than the dosing rate, the hopper that is dosing will not have emptied its contents yet and the hopper that is filling would have reached its high level. A “filling defect” will appear which will stop the feed screw in order to prevent the product from overflowing.
In cases where this dosing system is placed under a silo and receives the product by gravity, a pneumatically operated knife gate valve is installed at the silo outlet. This valve is used to close the feed when the high level in one hopper is reached before the low level in the other is triggered.
This Sautelma’s Twin hopper loss-in-weigh feeding system has many advantages:
- Reduced investment amount compared to other existing systems on the market
- High dosing accuracy and perfect stability and repeatability of the measurement
- Calibration is done once at start-up and is no longer necessary afterwards
- No influence on the dosing accuracy of any product clogging because static weighing associated with a loss-in-weigh system (net weight lost per unit of time)
- Maintenance reduced to a minimum and limited to sealing gaskets on the shafts
This system combined with lean phase pneumatic conveying (positive pressure) can also be used to feed pulverized coal to the burner at the furnace inlet.