Weighing system handles 500 tonne silo

Conventional content measurement methods struggle to provide acceptable data for silos as large as the one at Pilkington's Greengate flat glass manufacturing site.

AV Technology's Silo-Weigh system has successfully completed a six-month trial on a huge 30m high, 500 tonne soda ash silo at Pilkington's Greengate flat glass manufacturing site at St Helens, Lancashire.

Rather than buy the weighing system, Pilkington has opted to rent it from AVT, who will provide full ongoing support, charging them a modest monthly fee for an accurate 4-20mA signal proportional to weight content.

The Greengate site has two float lines for producing flat glass which are operational 24 hours a day, 365 days of the year, without a break.

Together the lines produce around 8000 tonnes of glass a week, enough to cover more than 100 full size football pitches.

Soda ash is a critical ingredient in glass making, acting as a flux to lower the fusion point of the special silica sand used.

With a daily soda ash usage of over 200 tonnes, monitoring the contents of the two silos to ensure a consistent supply of material is of paramount importance.

Contents data from the silo is used as part of a web based VIM (Vendor Inventory Management) system installed by Brunner Mond, who supplies the soda ash from their plant in Northwich.

Soda ash has a relatively low bulk density and the simultaneous emptying and filling that takes place causes significant dust generation.

As a result, conventional content measurement methods struggle to provide acceptable data for such large silos.

Process Improvement Engineer Dave Grundy explains: "The original ultrasonic level control systems had proved to be inaccurate and at best erratic".

"Given the design and location of the silos, it was impossible to fit conventional load cells and it appeared that standard retrofit sensors would not give us reliable data".

"Understandably, therefore, we were somewhat sceptical of the AVT approach".

"How could four sensors, not much longer than a match box, attached to the outside of the silo, give accurate weight data?".

" However, having inspected the application, AVT were confident they could provide us with reliable weight data and we invited them to conduct the trial".

"Six months down the line we are very impressed with the results".

"These give us a good level of confidence and our supplier can optimise their delivery schedules to maintain the optimum levels of soda ash for our process".

While most current retrofit sensors are based on electrical resistance strain gauges, which struggle to provide sufficient stability and accuracy, the Silo-Weigh system uses specially developed "low-strain" sensors.

These devices are inherently waterproof and are insensitive to moisture, long cable runs and low signal levels.

The Silo-weigh system, which is totally nonintrusive, can be fitted without having to stop the process or empty the silo and has proved to be effective on both steel and concrete silos.

The main error sources in retrofit systems are temperature, differential support deflection and structure interaction (cross talk).

Temperature changes in different parts of the structure will affect the sensors to different extents and the magnitude of these errors is usually significantly greater than the required system accuracy.

AVT's Senior Manager Neil Parkinson said: "The accurate measurement of these extremely small and often irregular strains is not as straightforward as it might seem and many early attempts at strain gauge based weighing systems have given the technique a bad reputation".

"However we believe our practical and logical approach is effective in minimising inherent system anomalies and it is in the handling and post-processing of data where we have been so successful".

"After some eight years of ongoing trials and development work, we have now developed compensation techniques which are effective in reducing these errors to acceptable levels".

"Rather than try to adopt standard weighing instrumentation, we use a robust multichannel control unit to handle the data".

"This provides multifunctional capabilities for introducing nonlinear calibration factors, combined with individual calibration factors for each silo support if necessary".

"In parallel, temperature data is used to provide real-time thermal compensation".

"The system can also carry out intelligent data validation and signal conditioning together with data storage for weight trending".

"A single control unit can be expanded to accommodate any number of silos, thereby providing a very cost-effective solution for multiple silo applications".

In the Pilkington application, the sensors are spot welded to the silo at four points near to the base of the silo, below the bottom cone.

Each one has its own temperature sensor, which accurately measures the temperature of the steel where the sensor is fitted.

The silos are indoors and therefore see relatively stable temperatures.

For outdoor applications additional temperature sensors are normally installed onto the structure to provide additional compensation.

When the system was installed, an estimate was made of the actual contents at the time of calibration.

Three months into the trial Pilkington allowed the silo to empty and this gave AVT the opportunity to check the system calibration.

The results showed a positive offset of approximately 55 tonnes when the silo was actually empty and the system calibration was adjusted shortly afterwards to compensate for this.

Neil Parkinson said: "We believe we are the first UK company to offer fully installed permanent silo weighing systems on a contract hire basis, a concept normally restricted to smaller portable items of instrumentation and office equipment".

"This offers distinct advantages to our customers, whose main priority is simply obtaining an accurate silo weight signal and who may be concerned about actually owning some 'weird and wonderful' system that they may not be able to service themselves".

"The built-in modem gives us remote access to the system and we can monitor the performance and carry out any system adjustments from our Stockport offices".

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