Continuous monitoring of the mass flow or weight of solids is a vital area of process control in solids measurement, particularly in quarries and aggregates handling. The challenges for instrumentation are many, eg, dust, abrasion and impacts, varying bulk densities, changing moisture, build up, particle sizes and vibration.
'In line' weighing systems come in a variety of technologies depending on the product and conveyor type. Formats include 'loss in weight' systems, impact and dynamic belt weighing systems. They all involve various levels of process contact and/or constricted plant space for the weighing of material at a point in time, and all involve some mechanical and moving parts. Not ideal when handling abrasive products.
Another option is the indirect measurement of product flow through screw and drag conveyors via motor load consumption, acoustic or suspended conveyors and chutes on load cells, which have proved generally unreliable, high in maintenance and inaccurate.
These factors can make many existing systems' cost of ownership high through lost production, time and money spent on repair, unscheduled maintenance and specialist recalibration.
A way around the problem
There is another highly reliable alternative to consider, by using a completely externally mounted, low energy, high sensitivity nucleonic system 'around' the conveyor, with no intermediate machinery or additional space needed. Although the installed cost may be higher, the benefits of this type of system are clear.
These include no moving parts, so no mechanical wear or drift - installation without major modifications - little space required - measurement possible even during free fall of product - low source activity due to the use of highly sensitive scintillation detectors - extreme long-term stability due to digital electronics and decay compensation - determination of the dry weight possible with existing moisture signal, and more.
Concern over the use of radiation is a normal reaction. However, it is worth considering a little more carefully in light of the potential benefits outlined above and the demand for more predictive maintenance and 'fit and forget' equipment.
The source output levels are lower than ever, due to improvements in electronics and sensors and the radiation beam is also tightly collimated and directed only at the detector. Safety is carefully controlled and the sources held in very sturdy containers. Source sizes are always kept to the minimum.
How it works
The nucleonic system is mounted completely outside the process. A small gamma radiation source, just strong enough for the anticipated path length and materials directs a collimated beam at the detector mounted on the opposite side of the conveyor.
The source beam is attenuated by the product and the signal reaching the detector is inversely proportional to the weight of product passing through the beam, ie, with more product present the lower the energy at the detector. The detector measures the amount of radiation reaching it and can combine this with an input of conveyor or screw speed to provide conveyor loading and total weight.
This simple yet highly effective technique can deliver an accuracy of up to 2% on the most arduous applications with a very high degree of reliability. Other nucleonic systems use the same basic signal attenuation technique to measure density, level or interface of products in both solids and liquid applications in many industries.
How it works in practice
It can be applied on almost any size of conveyor and almost any product - from coarse aggregates to fine powders. The system can be installed easily without interrupting the process as it is non-contact and clamps around the conveyor. It is also suitable for hot processes.
Simple external caging to prevent access to the beam when the source shutter is open is normally required. Comprehensive support and advice is provided by the supplier. A final inspection to produce a critical examination report takes place before commissioning and start up of the system.
Installation, commissioning and operation
Source holders are safe to handle with low levels of external radiation when the shutter is closed. The design of the holder is such that no access to the source pill is possible by site personnel. A lockable shutter enables site technicians to install a system safely and isolate the source during any routine work on the conveyor.
Because of their reliability, ongoing maintenance is planned and routine, not reactive, as systems have integral compensation for source decay and there are no moving parts. Initial calibration of the system by the supplier is straightforward, once calibrated accuracy can be maintained by zeroing the system with the conveyor empty to take account of any build-up or wear within the conveyor.
The system supplier will provide all the information, support and help needed to install this very reliable answer for difficult measurements and process control.
For more information contact Alan Wynn, Vega Instruments, 011 958 1901, alan.wynn@za.vega.com
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