Bulk weighing systems have become the most popular weighing system for rail car and barge/ship loading and unloading because of their ability to provide 99.9% accuracy in weighing while providing fast and flexible load-out into a bulk carrier. Time restrictions placed on the economic incentives for loading and unloading have made it imperative that material handling capacities be increased without sacrificing weighing accuracy. At the same time, bulk weighers provide certifiable weighment printouts, historical records, and stable data outputs.
A typical bulk weighing system consists of a mechanical hopper system, load cells, gates and a control system. Each system is sized to suit the application whether it is inbound or outbound weighing.
Careful consideration should be given to what materials are going to be weighed, to the location of the weighing system, the type of carriers that will be loaded or unloaded, and the type of weighing data that will be required to be obtained.
Bulk weighing systems can be used to weigh most free-flowing commodities such as cereal grains, flour, meals, coal and other minerals and even liquids.
Design and construction of each mechanical hopper system is very important, as more drafts per hour will have an impact on the wear characteristics of the material(s).
Cereal grains are generally easy to handle and have good flow characteristics. Flow angles of around 45 degrees will allow cereal grains to feed and discharge through the mechanical weighing systems freely. When weighing more abrasive grains like soybeans, for example, it is recommended that all hopper bottoms be lined with an abrasive resistant material such as urethane, AR steel or ceramics. It is not recommended to run materials in a straight flow through operation without weighing, as this will cause excessive wear to the hopper and may damage material(s) being weighed.
Meals on the other hand are less abrasive but tend not to flow as well. Therefore, flow angles need to be steeper, approximately 60 degrees, to allow good material flow through the hopper system.
assembly & Location
The majority of bulk weighing systems are now shipped as factory assembled units and are installed outside facilities rather than inside. Since handling capacities have become larger, so have the mechanical weighing systems. Installing them inside existing facilities or inside new construction is not very economical as you get into Class II-G requirements as well as possible split construction of hoppers. Sending the units factory assembled and installing them outside of a facility saves installation, electrical and plumbing costs on the job site.
Mechanical weighing systems up to approximately 500 tonnes per hour can still be shipped inside 40-foot export containers for overseas shipments. Systems above that capacity have to be shipped as freight on deck. When deciding on the location for a weighing system with-in a facility, consideration should be given to minimizing the elevations required to get to and from the scale, as too many elevations can cause breakage.
Also consider alternative uses for the system, such as the ability to use it as a shipping or receiving scale or as a means to measure inventory. Creating multiple uses for a scale makes it more economically feasible. In addition, most bulk weighing applications will involve the installation of an automatic sampling system.
Most sampling systems will only take up 6 to 8 feet of vertical space below the scale, but one must be aware of how the sample will be transported to the collection point. Gravity is the preferred method of transportation rather than having to go to the expense of a mechanical transport system. Additionally, proper application and use of a control gate above the sampler is highly recommended. This controlls flow through the sampler and into the car (less surging); serves as an emergency stop; and, at the end of the last draft on each targeted weighment, it can be closed and the next weighment started, which helps speed up the process.
Set Up & control
Bulk weighing systems may be fed via gravity or mechanical means such as bucket elevators, drag conveyors or belt conveyors. Gravity fed scales tend to simplify matters as flows are more constant and "cut-off" weights are more consistent. On the other hand, when you have mechanical feeds, flow rates are more inconsistent and target weights are harder to meet. Additionally, the upper garner of the scale system may have to be oversized to allow for extra run time in the event of a pause in the weighing sequence or to allow for "clean-out" of the mechanical feeder.
The most common set-ups for bulk weighing systems are "beside-the-track," "over-the-track" and "multiple track." Proximity of the track to the facility as well as location and elevation of the feeding device (bucket elevator or conveyor) will most often dictate which design is used. There is no significant difference in cost for any of the methods. The less work one has to do over a live rail line, the less involvement is needed in specific requirements of a serving rail line. (See Figures A, B, and C.) The second part to a fast and flexible bulk weighing system is the weighing control system. Technology has blossomed in the last few years with the use of PLC to control the complete operations of most modern facilities. The need to eliminate the manual input of data, the ability to monitor and modify data during the weighing operation, and the automatic transfer of data to central computers has energized the capabilities of weighing controllers.
Electronic car readers have become very popular. Railroad car data is available and can be loaded into the weighing control system. As the rail car passes a car reader, the bar code is read, matched up with the car data, and the information is automatically loaded into the weighing controller — eliminating manual input and possible errors.
During the actual weighing operation, the operator has a graphic view of the activity, enabling him to modify data that was entered, and add to cars that have been weighed. Weight certificates can be generated automatically and a PLC can monitor various phases of weighing and respond to any alarms. Many facilities now have inventory monitoring systems that allow blends during loadout and automatically update inventory.
Once carriers are loaded, data may be transferred to the central computer system where bills of lading can be generated and cars released for billing via the internet. Inventories are adjusted and invoicing becomes automatic.
Higher capacities and faster load-out times require data to be processed faster and automatically. Bulk weighing systems are meeting this challenge while providing higher accuracy and reliability.