Bolt-On technology is ideal for providing material weight in existing large bulk storage vessels. It provides a cost-effective alternative to obtain the material weight in the vessel compared to the retrofit cost of installing load cells. Load cells offer high system accuracy compared to Bolt-On systems. Bolt-On sensors are mounted to the existing vessel support structure while load cells require an empty vessel for installation. Level technology is commonly looked at as the alternative to load cells for bulk inventory when the tanks are already installed. Bolt-On sensors convert the level of the material to volume and then to weight, based on the material’s density.
The success of a Bolt-On system depends on factors such as vessel capacity, structure, minimal or loose attachments and an understanding by the user of how the system performs. It all starts with the vessel usage. Bolt-On is designed primarily for large bulk vessels that are used to store raw material or for load out.
These vessels range in capacities of 100,000 pounds and up. Smaller vessels are better suited for the retrofit of load cells more so than larger capacity vessels. If high accuracy is not desired, there are techniques that can be applied to minimize cost.
Another factor involving a successful installation is a sound symmetrical support structure. Loading will be consistent as the vessel rocks and shifts if the support members are identical in size and configuration. If the foundation sags, the vessel attachments can affect the readings more, showing up as measurement errors.
There will always be attachments on the vessel. In general, the more freedom the vessel has, the better the system accuracy. However, materials do have to be moved in and out of the vessel, which requires attachments. Catwalks and support ladders can share load changes between vessels. They also shift the weight of the vessel to its foundation, causing the material weight to bypass the Bolt-On sensors. Typically, these connections can be loosened to provide additional flex under changing loading conditions.
It appears that the hardest part about a Bolt- On application is communicating the performance of the system to users. The accuracy is estimated based on the full- scale psi loading of the structure. Accuracy includes the non- linearity and hysteresis normally associated with a load cell weighing system. It also includes the effects of temperature on steel, which makes the sensor believe there has been a change in vessel weight. Temperature affects are usually the greatest error on the system and are the most visible to the operator.
Accuracy is always stated as an actual +/- weight value of the full-scale load of the vessel. For example, a 100,000 pounds vessel with +/- 3% is stated to 100,000 pounds +/- 3000 pounds. If the user moves 500 pounds of material, the accuracy is still applied to the full-scale value, or +/- 3000 pounds, not 3% of 500 pounds. In certain applications such as a clam-shell truck load out which occurs in minutes, the material transfer can move faster than the thermal induced errors. Therefore, the load out performance is typically much better than the inventory accuracy. If material is augured out or pneumatically conveyed over hours, thermal changes will play a part in affecting the system accuracy.