Weighing
very import to consider the following when designing the belt and the accuracy required.
Material handling
Materials vary in shape, size, flow ability and bulk density; each material can look very similar, but behave totally different. It is therefore critical to establish how the material will behave whilst being handled, both during discharge on to the belt and whilst being transported to prevent flushing or material movement during transfer.
Discharge zone
Where the material falls onto the belt and the distance of travel to become static on the belt. If the belt is inclined it is important to establish at what point does the material become stable on the belt and does not continue to roll backward.
Influence zone (Z2)
The Influence zone is the area at which the belt is prepared to achieve the most effective weight measurement to take place. As this is a critical part of the exercise, the influence of poor preparation and insufficient attention to detail of the belts roller configuration will lead to the weigh span area under-achieving leading and to inaccurate results and poor weight measurement.
Weighing span (WS)
This is the weight of material passing over the weigh scale for a given time period and at a constant speed and the point at which the weighing calculation is made. The accuracy of a belt weigher is dependent on the rate between Weight on belt (Wb) and Belt weight (Bw).
Belt Ratio BR = Wb kg/m / Bw kg/m BR = 10 Ideal value: the belt weight is 10% of Ws BR = 2 Minimum values: the belt weight is 50% of Ws To increase the belt ratio (BR), we can decrease the belt speed (S), decrease belt weight, (Bw) increase the flow rate (FR)
Stringer spacing
It is at this point that the belt configuration and stringer design should be considered. Stringer interval spacing should be inspected to see if the roller prior and after the scale position is in contact or providing additional lift to the belt removing the influence across the weigh scale. The stringers should be designed and set in position to promote maximum effect on the lead into and lead out of the weigh span zone. Too close and the belt may be removed of some of the weight across the weigh scale span area. Too far apart and the belt may be inclined to droop and cause small pockets, allowing for movement material influencing weight accuracy. Good stringer interval spacing will provide the best practice to achieving accurate weight measurement.
The accuracy of a belt weigher is dependent on: the signal of ‘n’ load cells of ‘C’ capacity. Weighing Signal WS % = Q kg / n x C kg WS = 20 % for Trade Use OIML Class 1, +/- 0.5 % WS = 8 % for +/- 2 %
To increase this signal we can decrease the belt speed (S), increase the Weighing Length (WL) and decrease the Load cell capacity using an Aluminium Tare
Influencing the depth of material on the belt is dependent
20 Solids & Bulk Handling • March 2010
upon the signal (‘n’) and capacity (‘c’) from the load cell in relation to the weight on the belt. The better the signal the more accurate the measurement. Selecting the right capacity for the correct measurement to ensure the cell is operating within its recommended limits and therefore producing accurate results. There is a range of load cell types available and selecting which type of load cell to use may seem a daunting task. However once size type and mode of operation of weighing system is determined then choosing the correct cell becomes straightforward.
Load cells operate in two modes: the weighing vessel (or similar) either sits on one or more load cells comparison mode or hangs from one or more load cells tension mode. Although tension applications are common and easy to set up and gravity ensures optimum load introduction, weighing vessel design and safety considerations limit the practical load cell capacity, typically 5 tonne and below.
Once mode of operation has been established, system capacity usually determines which type of cell should be used. The majority of modern weighing systems rely on strain gauge load cell for conversion of weight or load change into usable electrical output. Modern electronics has dramatically outpaced the changes in load cell development, but it should not be forgotten that the overall performance of any system is still dependent on the primary transducers providing accurate weight data.
Belt tensioning device
Good belt stringer positioning design can be enhanced and complemented by correct belt tensioning. A good auto tensioning device such as suspended weight, spring loaded belt tensioner device and drum tensioner device will ensure a constant weight is present throughout the weigh scale span area and will minimise the effect of belt bounce and result of loading and unloading of the belt across the weigh scale. Poor tension and roller efficiency can contribute to belt slippage causing the belt to jump or shudder, leading to excess forces across the weighing scale and inaccurate measurement. Handling wet and sticky material can lead to material spillage and cause drums and rollers to become clogged with material. It is good practice to ensure correct cleaner and scrapper devices
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