With the wide application of port loading and unloading systems such as ship unloaders, stackers and reclaimers, and belt conveyors, electronic belt weighers have been paid more and more attention by users as an important means for solving bulk cargo metering. The selection of a suitable electronic belt weigher system enables real-time metering in large loading and unloading systems, as well as scientific management of the loading and unloading system.
First, background introduction
Baoji Majishan Port is located on Majishan Island, about 1.5 kilometers southwest of the island of Shuo Jiao, Qixian County, Zhejiang Province. It is located in the middle of the north and south coastlines of China and at the mouth of the Yangtze River. It is a golden water area for China's north-south transportation and it also enters and exits Shanghai Port and the Yangtze River. The inevitable route to the river basin, which borders on international routes, is the closest large-scale ore transfer base to the Yangtze River estuary and has unique geographical advantages.
The second phase project of Baoji Majishan Port commenced construction on December 28, 2005. The second phase of the project is based on the first phase of the project. It will build a new berth of 300,000 tons of unloading vessels, 10,000 tons and 50,000 tons. There are 1 berths for each ship, forming a 0.32 square kilometer ore storage yard along the sea, as well as supporting and supporting facilities. The designed annual throughput is 30 million tons. After the second phase of the project is put into production, Majishan Port will have an annual capacity of 50 million tons.
At the beginning of the second phase of the construction of Majishan Port, the world’s advanced ports were used as a model to determine the goal of building a world-class modern port area and to realize the transformation of scale expansion to independent innovation. Baosteel independently researched and developed the “bulk cargo stack and automatic control system†to enable centralized control of production control, stock yard management, and unmanned operation of the stack reclaimed material at Majishan Port, becoming the leading domestic and similar type in the world. The advanced ports are comparable to information, digital, and intelligent ports.
Previously, we have used belt scale products from a number of companies at home and abroad, including multi-rollers, full-suspension, double-lever and other various types of belt scales. There are generally unstable performance, large fluctuations in measurement values, and poor repeatability. Large differences in calibration, can not meet the requirements of high-precision measurement; and some electronic belt scale can achieve very high measurement accuracy in the early stages of installation, but after a period of time, its measurement accuracy began to drift, and finally had to be carried out regularly or From time to time, calibration and maintenance work, on-site maintenance personnel have concentrated their opinions.
How to choose and use the appropriate electronic belt scale products, not only to achieve high-precision internal management and measurement, but also to reduce the daily maintenance of the electronic belt scale, to achieve the goal of unmanned operation. This has a crucial influence on the complete implementation of the system.
Second, the application requirements of the port loading and unloading equipment on the belt scales
The modern port bulk cargo handling system can be divided into: waterway-waterway; waterway-landline; land-waterway, etc. according to the flow of goods. Majishan Port serves as an important transit base for large-scale ores, mainly for the entry of sea routes, and land (cars, railways, and belt conveyors) to be exported to Hong Kong. The main application of electronic belt weighers and the direction of logistics in and out of Hong Kong are:
A, ship unloader. The on-board conveyor can be equipped with an electronic belt scale for controlling the dosing amount.
B, belt conveyor system. The system is equipped with an electronic belt weigher for monitoring instantaneous flow and production statistics.
C, bucket wheel stacker and reclaimer. In the state of stacking, the boom conveyor belt is equipped with an electronic belt weigher to control the instantaneous picking amount of the bucket wheel.
D, yard belt conveyor. It is used to monitor and count the amount of materials that are transported from the yard to each production unit and is often used as an internal management measurement.
Ship unloader
A large number of grab unloaders were used at Majishan Port, and the 2250t/h grab unloader provided by the large-heavy-duty crane in the second phase project reached the international advanced level in the late 1990s. The ship unloader bridge grab unloader is widely used in the unloading machinery type of large bulk cargo specialized docks in major coastal ports and inland ports in China (see Figure 1).
Fig. 1: Grab ship unloader (red circle is the belt scale installation position)
In general, metering belt scales are usually installed below the hopper and parallel to the transporter (as indicated by the red circle in the figure). According to the specific requirements of the application, the electronic belt weigher with internal settlement accuracy (0.5%) or trade settlement accuracy (0.25%) can be selected in the unloader configuration. At the same time, due to the particularity of the installation location, high requirements for the long-term stability and low maintenance of belt weighers are also generally put forward. Belt conveyors
Belt conveyors are friction-driven machines that transport materials in a continuous manner. It can be used to form a material conveying process on a certain conveying line from the initial feeding point to the final discharging point. It can not only transport the bulk materials, but also transport the articles. In addition to carrying out pure material transport, it can also cooperate with the requirements of the technological processes in the production processes of various industrial enterprises to form a rhythmic flow production line (Figure 2). Belt conveyors, in conjunction with their stackers and reclaimers, have become the only effective method for large-scale stacking of bulk materials (such as coal, ore, etc.).
Figure 2: Belt Conveyor
Belt conveyor belt conveyor selection is relatively simple, according to different precision requirements, you can choose 0.5% internal settlement and 0.25% trade settlement level.Bucket Wheel Reclaimer
A large number of highly automated bucket wheel stackers and reclaimers are used in the Majishan Port Yard, especially for the second-phase project, which is based on unmanned design and is used for the first time in China. High demands are placed on the equipment used. Bucket wheel stacker and reclaimer is a high efficient equipment for continuous loading and unloading of large quantities of modern industrial bulk materials. It is widely used in bulk materials such as ports, wharfs, steel plants, coking plants, coal storage plants, and power plants (ore, coal, coke, and sand The stacking and retrieving operation of the stockpiling yard (Figure 3).
Usually, the belt scale is installed on the belt conveyor of the stacker's boom. Because the stacker's boom will move up and down according to the stack height, the load cell of the belt scale is vertical. The force in the direction is always changing. Usually the angle changes from 0 to 20o, sometimes reaching 20 to 30o. Therefore, in order to correct the influence caused by the angle change of the stacker crane arm, the belt scale used in the stacker-reclaimer usually needs to be equipped with a tilt compensator to correct this error and ensure the high precision of the belt scale.
Stacker-reclaimer is a kind of equipment that can both stack material and take material. Therefore, a two-way measurement requirement is also put forward for the speed sensor, and an axial velocity sensor that can measure both in forward and reverse directions is generally used instead of the linear velocity sensor.
Because the belt weigher measurement of the stacker-reclaimer is relatively complicated, the installation environment is difficult to achieve an ideal state. Even if a trade-grade accuracy belt weigher system is used, it is difficult to achieve an accuracy of 0.25%, so it can be realized on the stacker-reclaimer. 0.5% of the company's internal measurement accuracy has also been quite good, and the main technical requirements are usually focused on the use of long-term stability and maintenance of the belt scale.
Therefore, in addition to considering the accuracy, the selection and application of the belt weigher on the port handling equipment need to fully consider the long-term stability and maintenance workload of the belt weigher, which saves the production cost and improves the production efficiency. Very important.
Third, belt scale working principle and choice
The electronic belt weigher is mainly composed of main components such as a load cell (including a scale stand), a speed sensor, an inclination compensator (used only for applications with variable angles), and a weighing integrator.
After many investigations and trade-offs, we chose a large number of Siemens electronic belt weighers based on direct load-bearing principles at the ship unloaders, bucket wheel stackers, and belt conveyors at Majishan Port.
The design of Siemens belt scale platform part is very special. It is different from the commonly used lever type scale frame. It adopts a direct load-bearing scale frame structure called "three nos", namely: no lever, no pivot No balance weight (Figure 4), ie no weighing carrier. The benefits of this design are obvious:
Ø Easy to maintain
There are no lever fulcrums on the scale frame, no moving parts, and no problems such as fulcrum wear. Lever-type scales are large in scale and have a large surface area in the horizontal plane. Large areas of ash need to be cleaned frequently, otherwise they will cause zero point changes, and the direct load-bearing scale frame will have a small size, the horizontal surface area will be smaller, and the amount of accumulated dust will be small. Ash deposits cause small changes in the zero point and can be cleaned without frequent cleaning.
Ø The scale structure is simple
The straight-supported weighbridge structure ensures that the weight of the material is directly added to the load cell through the weighing roller group. The taring and material weight are weighed together. Therefore, the weighbridge structure is very simple and the weight is very light.
Ø The scale frame has modular features and good versatility
The structure of the scale is not only simple, but also the commonality of components for different width belts. Except that the width of the static beam and the dynamic beam changes correspondingly, the rest is almost the same. At the same time, the modular design structure greatly reduces the workload and costs of repairing and replacing spare parts.
Ø High accuracy of weighing frame
Due to the high accuracy of the three-beam parallelogram load cell, the repeatability is 0.01%, and the nonlinearity and hysteresis are both 0.02%. Such a high degree of accuracy is not only sufficient but also has room for a nominal accuracy of 0.5% of the weighing feeder. Because of the direct load-bearing structure, the loss of measurement accuracy due to these components and the disturbance to the weighing system caused by the mechanical vibration during the material transportation process are correspondingly reduced.
MMI multi-roller belt weigher is a high-precision belt weigher, which can realize the purpose of trade settlement. It adopts a unique modular design structure. The weighing cradle is composed of two MSI single roller weighers and is equipped with 4 load cells. The accuracy index is 0.25% in the range of 20% to 100%, which brings great convenience for installation, maintenance, and replacement of spare parts.
The BW500 is a powerful calculator designed for high-precision belt scales. It has the following main features:
The patented dual load cell (up to four sensors) balance function eliminates the effects of non-uniform lateral loads without the need for sensor-matched mechanical balance;
Online calibration, batch processing, humidity compensation, tilt compensation and other functions;
Dual PID adjustment function can set PID response time and adjustment range according to different process requirements;
Effectively solve the sensor's characteristic matching problem, compared to the external potentiometer, can significantly improve the long-term stability of the system.
With the high-precision MD256 axis velocity sensor, bidirectional measurement in the forward and reverse directions can be achieved (see Figure 5).
Fourth, installation and application of the effect
At the initial stage of installing belt weighers, we also encountered many problems, mainly as poor accuracy, which could not meet the requirements. After technical communication with the manufacturer, on-site inspection, re-installation, and debugging, the main cause of the problem was discovered, and after calibration and testing, it was proved that the process requirements could be met.
The main installation problems encountered by Siemens Electronic Belt Scale at Majishan Port:
1, belt deviation problem (Figure 6). During the on-site commissioning of Baoji Majishan Pile Reclaimer, the horizontal deviation of the two sides of the bracket of the belt weigher idler is very large, according to Siemens MMI. The installation of belt weighers requires equipment calibration. It is found that the maximum deviation needs to be increased by 64 mm in order to ensure that the belt weigher is in the correct position, but this also results in belt deviation, with the maximum oscillating range of about 150 mm.
2. The installation site is too close to the bucket turbine and the site is too vibrating. It not only seriously affects the measurement accuracy of the belt scale, but also easily leads to the damage of the original components of the belt scale.
At the manufacturer's suggestion, we have taken the following measures to improve:
a. Change the installation position: Keep away from the bucket wheel, reduce the vibration, and find the position where the deformation is relatively small. After the weighing roller is moved backward five meters, the measured height drop is reduced to 14 mm, and then the leveling adjustment is performed. This is the best solution.
b. For the six groups of rollers, the roller offset before and after the corresponding roller height: reduce the belt deviation, reduce the height difference between adjacent rollers, alleviate the bulk material caused by the phenomenon of bulk material transport.
Siemens belt-type direct load-type belt weighers, due to the compact and compact structure, the entire rectification work quickly completed in accordance with the requirements of manufacturers. After the chain code calibration and physical calibration tests (Figures 7 and 8), the final calibration data can fully meet the original design requirements.
Since this batch of belt scales has been used for more than one and a half years since installation, none of the belt scales have been assigned to the maintenance of belt weighers, and the accuracy can basically meet the requirements of production. Siemens belt-type weighers based on the straight-support structure have the characteristics of low maintenance, which has played a key role in the management of the stock yard and the unmanned operation of the stack material in Majishan Port.