For almost all silo operators, weighing systems are very important and essential for recording of silo capacity. It is not easy to make the right choice relating to the measuring methods and suitable components, because there are many weighing possibilities. But which criteria must be considered for silo weighing? We illustrate it on basis of the weighing modulesVC 3500 and LCM 460.
The selection of the load cells and modules is largely determined by the total load (weight of silo capacity plus net weight) of the silo. The LCM modules are available for rated loads from 2 to 100 tons, the VC 3500 for rated loads up to 200 tons. This means that the weighing of a silo with a total weight of up to 800 tons can be realised with a four-point support. Other parameters that influence the selection of the modules are the environmental conditions, e.g. temperature, installation location (chemical influences, humidity, etc.) and ATEX requirements. For outdoor installations, wind loads and associated forces on the load cells (e.g. tension and compression) must also be considered. Depending on the environmental conditions, the modules are available in painted, hot-dip galvanised or stainless steel. The precise double-ended shear beam load cell is made of stainless steel by default.
Weighing modules ensure the optimum force transmission and are a part of the complete silo construction. The VC and LCM modules have a very robust base plate and top plate with lift-off yoke as well as lock screws and split pins. This special construction guarantees an optimal function - without the use of any additional steering and lashing mechanism, which can easily result in errors and maintenance. The top plate of the weighing module is movable in two axes. This ensures that lateral and side forces can be deflected and misalignments due to slight inaccuracies in the steel construction can be compensated. The 50 tons LCM module is EN 1090-certified (optional).
The right choice of weighing module significantly determines the effort of silo installation and repairing. The VC 3500 and LCM 460 modules (up to 50 tons class) feature an interesting advantage: they are equipped with lock screws that stabilize the top plate on the base plate of the module. This protect the load cell from shock loads that can occur during the silo installation. Therefore the installation of the modules with a pre-installed load cell is possible. The built-in modules can be installed directly between the foundation and the silo support. The use of dummies during the silo installation and the subsequent exchange of load cell are not necessary. In case of a load cell failure: a replacement of the load cell is possible without removing the module.
The weighing of silos and containers is the most accurate and reliable weighing system for determination of silo capacity, if designed and implementation is accordingly. The measurement error of < +/- 0.03 % is long time stable and reliable if the load distribution in the silo is consistent, the force bypass are low and the installation of the modules is correct. The LCM 460 load cells from 5 to 20 tons have an OIML R60 approval and are extremely precise. So the LCM's are often used for legal-for-trade applications. For example, the weighing of a 60-tons-silo can be achieved a resolution of 20 kg.
Please note: The necessary control unit should be powerful and with a high measurement accuracy according to the load cells.
Silo weighing operates independent of bulk material and is free from influences such as geometry, dust, pressure and temperature. Even changing of the bulk material has no effect on weighing system. Silo weighing is characterized by a long life cycle, if the following conditions are fulfil:
- Load cells and modules are protected from aggressive environmental influences (corrosion damage) and mechanical damage.
- Deviations occurring in steel construction and in the distribution of bulk material as well as wind loads in outdoor installations must be considered for dimensioning of size or measuring range of the modules.
- It is better to use a ground cable on weighing module for bypass of electrical voltages that may arise during wel ding or overvoltage.
A further advantage of VC and LCM weighing modules is the minimal maintenance effort: The load cell should be kept free of deposits in the module and checked intermittently. Smaller vessels should be adjusted every year, larger silo plants every three years.
Today, weighing systems should be an integral part of every newly constructed silo. The right choice of the weighing modules minmizes the effort for installation and maintenance and ensures a reliable measurement during the silo life cycle. Silo weighing features a permanent and accurate inventory and process control as well as optimization of bulk material outflow and silo filling. This reduces the amount of work and down times as well as storage and purchasing costs. Besides this it amortizes the purchase price of the weighing system within a few years.
For a manufacturer of injection molded components, MTS equipped three new skirted silos with weighing modules. The silos made of stainless steel aren't installed on supports or steel girders - they are fixed directly on their pulled down skirt. The annular support is ideal for the installation on easily produced concrete foundations. The silo cone as weighing module connection is located inside of the usually accessible frame.
Due to the total load of 47,000 kg per silo and the outdoor installation, the VC 3500 weighing modules have been used. The silo weighing was realized with a three-point support (per weighing module with a rated load of 20 t). The weighing achieves a measurement accuracy of +/- 20 kg.
The modules were installed directly between the concrete foundation and the silo frame and thus integrated as a part of the complete silo construction. Lock screws on the module protect the load cell from shock loads that may occur during the silo installation.
In addition to the high measurement accuracy, the VC 3500 weighing module also offers a highly reliable mechanical fixation of the silo in horizontal and vertical direction without the use of any additional steering and lashing mechanism.
The evaluation of the measurement results is carried out by the multi-channel control unit G4. The G4 can be easily operated by a function keyboard and an integrated LCD display. The connection to the process control system is based on Ethernet.