Silo-Weigh Solves Weighing Problem At Castle Hill

AV Technology (AVT) has installed a high-capacity Silo-Weigh system on two large limestone silos, at Castle Hill Quarry in Cannington, Somerset. The two outdoor silos, with a combined capacity of 400 tonnes, are mounted on a common structure supported by six steel legs. It is difficult to achieve accurate contents measurement on such silos. Modern level-control systems may be able to achieve reasonable average-level measurement results, but changes in bulk density and surface topography during filling and emptying prevent them from providing sufficiently accurate weight-related contents.

Retrofitting traditional load-cells is disruptive and prohibitively expensive, with structural cross-talk further complicating matters. The Silo-Weigh has an excellent track record on large steel and concrete silos in applications where established measurement systems do not give reliable results. AVT's approach takes into account application problems such as low signal levels, temperature effects and cross-talk. The Silo-Weigh system uses specially developed 'low-strain' sensors, which are insensitive to moisture, long cable runs and low signal levels.

Castle Hill set a number of difficult challenges for the AVT system: it wanted to measure the individual and combined contents of each silo and also wanted to weigh out known amounts into waiting vehicles while the silo-filling process continued. The latter enabled optimum vehicle-loading prior to final check-weighing. The raw compressive strain levels in the legs are measured using six spot-weldable strain sensors, mounted so that any bending components of strain are cancelled out.

Data from these, together with information from the adjacent temperature sensors, is fed back to a powerful Campbell Scientific CR1000 data logger. Engineers applied protective coatings to the sensors. UPVC thermal shielding wrapped around the legs minimises the temperature effects on the strain gauges. There are four individual weight displays in the central control and readout unit. The first three show the combined weight contents of the two silos and the calculated individual-content weights.

The fourth display normally indicates the average fill rate in tonnes per hour, but is also used during load-outs to show the amount taken out of the silos. This is calibrated from 0-50 tonnes and is zeroed just prior to vehicle loading by pressing a button adjacent to the display. This action also increases the system-measurement update rate to improve accuracy. To compensate for the material entering the silos as product is simultaneously discharged, the calculated weight taken out is automatically adjusted using an historic 10-minute fill-rate measured just prior to vehicle loading.

The Silo-Weigh system, which is totally non-intrusive, can be fitted without having to stop the process or empty the silos. Calibration at the Castle Hill Quarry site was achieved using theoretical values and known physical-load additions. During the commissioning phase, data from the system was transmitted via modem to AVT's offices in Cheshire.

This remote access to data allowed AVT engineers to fine-tune the system over a period of several weeks, without the need for unnecessary site visits. Stephen Ford, operations manager at Castle Hill, said: 'We can now reliably measure combined silo capacity better than 1.5 per cent. 'Being able to measure and control how much material is loaded into vehicles is extremely useful and results are better than expected. 'Prior to the Silo-Weigh installation, over-loaded vehicles had to return from the weighbridge and discharge part of their load before departure from site.'.