A twin hopper loss-in-weigh feeding system

In order to do this, the company had to replace the old cement and flyash dosing equipment, which had a dosing accuracy of plus or minus 2% - insufficient for the new project. The two dosing lines (cement and ash) were made up of the following elements:

• A buffer hopper.
• A knife gate valve.
• A helmet dosing valve.
• An impact flow metre.
• A fluidisation booster.

A dosing solution

Based on the company’s experience in bulk materials dosing, and after studying the needs of the client, it proposed an original dosing solution with two symmetrical 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 of less than 0.5%.

Each dosing station (cement and ash) is continuously fed by a screw conveyor. The flow rate is about 60 tph for cement and 40 tph 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, discharging the dosed material into a common chute.

The twin hopper weighing system has the following two fundamental advantages in this case:

1. Absorbing a continuous flow of material because while one hopper is emptying the other is filling.
2. Avoiding 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.

To simplify the explanation, the hoppers will be referred to as hopper one and hopper two. The operating sequences are the following:

At start up, hopper one is filled and hopper two is empty.

When the high level in hopper one is reached, the diverter changes position and directs the product to hopper two.

When the order to start is given, the weigh feeding starts.

The MINISMART controller, equipped with a specific weight-loss dosing programme to control 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 one.

At the same time, hopper two, which has been filled in the meantime, 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 twin hopper loss-in-weigh feeding system has the following advantages:

• A reduced investment amount compared to other existing systems on the market.
• High dosing accuracy and the perfect stability and repeatability of the measurement.
• Calibration is done once at start-up and is no longer necessary afterwards.
• There is no influence on the dosing accuracy of any product clogging due to the static weighing associated with a loss-in-weigh system (net weight lost/unit of time).
• Maintenance is reduced to a minimum and limited to sealing gaskets on the shafts.

Conclusion

This system, combined with lean phase pneumatic conveying (positive pressure), can also be used to feed pulverised coal to the burner at the furnace inlet.

Read the article online at: https://www.worldcement.com/special-reports/27122018/a-twin-hopper-loss-in-weigh-feeding-system/