Conveyor Belt Scales in Mining: Best Practice Installation, Calibration and Maintenance

Transcription

1 Conveyor s in Mining: Best Practice Installation, Calibration and Maintenance Sheldon V. Shepherd International Sales Manager Tecweigh Tecnetics Industries, Inc. Abstract: Conveyor belt scales are designed to continuously calculate bulk material flow rate by measuring belt load and speed. They can be applied in many locations throughout a mine or mineral processing operation for rate control, production and inventory monitoring. Proper selection is important because with increased accuracy comes greater cost. Belt scales respond to vertical forces, both desired and undesired. Many issues are a result of problems with the conveyor or improper commissioning. Careful evaluation of the application, proper installation, calibration and routine maintenance will yield many hours of continuous and reliable service. Keywords: Dynamic Weighing, Material Handling, Conveyor s, Continuous Weighing 1. INTRODUCTION Fifty years ago in the bulk materials handling industry belt scales were entirely mechanical devices based on a complex weight balancing design. Today s systems employ load cells to assess material weight, belt speed sensors, and microprocessor-based integrators to continuously compute the rate of material transferred along a conveyor. Belt scales become an important asset to a plant by helping to maximize the use of raw materials, control inventories, and aid in the manufacture of a consistent product. Choosing and successfully installing a belt scale system may seem complicated, but it can be simplified by following a few guidelines. 2.1 Belt scale system 2. PRINCIPLE OF OPERATION A typical belt scale system is composed of a weigh bridge structure supported on load cells, electronic integrator, and belt speed sensor as shown in Figure 1. The rate of the material conveyed is computed using the equation Weight x Speed = Rate. Material weight on the belt is measured by load cells, which produce a voltage signal that is sent to the integrator. The integrator also receives input in the form of electronic pulses per revolution from a belt speed sensor connected to a tail or bend pulley. Using these two points of data, the integrator calculates the rate of material transferred along the belt in pounds or tons per hour. Belt Speed Sensor Weigh bridge with load cells Fig. 1. Principle of operation: weight x speed = rate 2.2 Conveyor components Material Load Integrator Experience shows that most training classes on belt scale systems devote about 75% of the time to discussing the conveyor. This is because the reliability of a belt scale is directly proportional to the fitness of the application, quality and maintenance of the conveyor. A basic conveyor configuration is shown in Figure 2 to provide a basic guide for the terminology used to describe conveying systems Rate Total Load Speed

2 Table 1. Belt scale cost analysis Tail \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ Screw Take Up Return Idlers Bin Fig. 2. Conveyor components 3. SELECTING A CONVEYOR BELT SCALE 3.1 Belt scale selection Designs range from complete high performance systems, to scales that may be purchased and assembled on site. Before selecting a supplier, it is important to consider the desired accuracy, dependability, cost, ease of installation, and availability of factory service personnel. Higher accuracy requirements typically increase the cost of a belt scale because of the need for precision components, tighter tolerances, and a stronger metal structure. Therefore, one might want to perform an economic analysis of accuracy versus the system cost. Modern systems require fewer idlers to achieve equal or better performance than older design, bulky scales. 3.2 Cost analysis Impact Idlers Skirting Bend Direction of Gravity Take Up Guides Carrying Idlers Snubbing Take Up Weight Head Analyzing the cost of a belt scale system in comparison to the cost of the material one may want to weigh is relatively simple. There are many potential costs resulting from errors in estimating or weighing the amount of material conveyed in a process. Some of these include inconsistencies in end product quality, inaccurate inventories, and excess use of raw material. For this exercise, let s assume that the cost associated with accuracy is solely the excess raw material fed into a process. The example in Table 1 presents an economic analysis of a typical conveyor moving approximately 280 tons/hour while operating 16 hours/day for 300 days out of the year. The cost of the material is about $10/ton resulting in a total cost of $44,800 worth of material conveyed daily. Without a belt scale, the designed rate of the conveyor would be substituted for an actual weighing device, although the accuracy achieved might be around 5% at best. According to the analysis, installing a ½% belt scale in this application would provide raw material cost savings annually of about $604,800 and pay for itself in just over 4 days. Excess Material Cost Daily Payback Accuracy Daily Annually System Cost Savings (days) 5% $ 2,240 $ 672,000 No Scale - - 1% ,400 $ 4,500 $ 1, /2% ,200 8,500 2, /4% ,600 11,500 2, An accuracy of ¼% is typically required in instances of high material cost, tight process constraints, or where the scale is being used to weigh materials for sale. For example, an increased material cost of $30/ton would reduce the payback period for a ¼% belt scale to less than 2 days. 3.3 Certified belt scales Some belt scales can be certified for custody transfer. The procedure involves calibrating and performing material tests on the scale that are traceable to national standards. While certified scales are needed for a few applications where a static scale is not practical, it is an expensive and timely undertaking. Often, the total cost to obtain certification can be 3 to 5 times the cost of the belt scale. Additionally, there are significant periodic costs to update the certification. 4. APPLICATION DATA 4.1 Data required for belt scale sizing After deciding on the desired accuracy, we need to gather application data on the conveyor to provide the belt scale supplier the information required to recommend the appropriate equipment. Most suppliers will have an application data form to complete and return. Contact the supplier if any questions arise as they have personnel and representatives which are available to provide assistance in collecting the information. The data form will at a minimum typically require: Material to be weighed Belt operating capacity (tons per hour) Belt speed Belt width Carrying idler spacing Idler/trough angle and diameter Conveyor incline angle Any additional information provided to the supplier will help to make certain the most appropriate belt scale system is selected for the application. Other useful information includes minimum, normal, and maximum rate, belt length, environment (outside, inside, corrosive, temperature, etc.), and hazardous area classification. 4.2 Evaluating the supplier In addition to accuracy and cost, one should evaluate system reliability, ease of installation, and the availability of service personnel. The cost and quality of a belt scale will vary

3 depending on the supplier. The analysis from Table 1 illustrates that minor differences in the system price are not as critical to the bottom line as consistent accuracy and reliability. The design and manufactured quality of the belt scale should be the most important evaluation criteria after deciding on accuracy. Next, evaluate the different suppliers in terms of what is needed to install a particular belt scale. Contact the factory and request an installation guide describing the equipment being considered for purchase. Furthermore, ask how many service personnel they have available, the cost, and the typical lead time required before an end-user can have a factory authorized technician at the plant site. It is also important to consider the features and functions of the recommended electronic integrator package. Options include rate and total output, alarm relays, and digital communications capability with the plant s control system. 5. BELT SCALE INSTALLATION 5.1 Importance of correct installation The scale measures the forces it senses on the conveyor. A survey of belt scale suppliers on the results of service visits revealed that about eight out of every ten service calls were to resolve a problem with the conveyor and its affect on the scale rather than a component failure or problem with the belt scale system. A properly installed ½% belt scale on a wellmaintained conveyor will typically produce an accuracy of ½% or better. Some of the areas of concern are: 5.2 Belt scale location The scale should be located a sufficient distance from the infeed section so the material has time to become properly profiled and settled on the belt. This distance will vary depending on the conveyor design, flow rate, and material; however, about 4 to 5 times the belt width is usually acceptable. In addition, it needs to be adequately isolated from any vibrations caused by material falling on the belt at the infeed. One must also consider conveyor belt tension because it increases closer to the head pulley as shown in Figure 3. Installing a scale in an area of high tension along the belt can significantly decrease the accuracy. It is possible to configure a scale to operate in an area of high tension; however, special care must be given to the installation, particularly idler alignment. Section 5.6 discusses idler alignment. Area of least belt tension. Preferred location for scale. Fig. 3. Conveyor belt tension Since many conveyors may curve up or down along some point, it is important to locate the scale an appropriate distance away from the tangent points of the curve. Distance guidelines from the tangent points are illustrated in Figures 4 & 5 for concave and convex curved conveyors. For concave curved conveyors, the recommended minimum distance for mounting the scale is 12 meters from the tangent points of the curve. With convex conveyors, the minimum distance is 6 meters on the approach side and 9 meters on the retreat side. Minimum 12 meters Fig. 4. Concave curved conveyors Area of greatest belt tension. Least preferred location for scale. Tangent Points of the Curve Direction of Minimum 12 meters The center of the curve lies above the conveyor Installation too close to the tangent point of the curve at the bottom portion of a concave conveyor is a common problem in the mining industry. It often occurs in the engineering stage when designs are completed without evaluating the physical requirements for dynamic weighing. Minimum 6 meters Tangent points of the curve Minimum 9 meters The center of the curve lies below the conveyor Fig. 5. Convex curved conveyors Violating the guidelines presented in Figures 4 & 5 causes major issues with repeatability and accuracy.

4 5.3 Conveyor support structure The conveyor should be rigidly supported so there will be no deflection caused by the weight of the material. The structure and components also need to be free of excess vibration. A3 A2 A1 Weigh Span R1 R2 R3 5.4 Belt tension A gravity tensioning device should move freely and place consistent tension on the conveyor belt. The amount of weight should conform to accepted conveyor design specifications. Ensure that significant water or debris does not collect on or inside the take up weight, thereby causing excessive tension that can create problems for the conveyor and belt scale. A problem definitely exists if the gravity take up does not move at all and therefore maintenance will be required. The screw tensioning device should be adjusted according to the conveyor manufacturer s specifications, usually just enough to keep the drive pulley from slipping. Material weight is transferred through the belt and on to the weigh bridge. Changes in tension throughout the day will have a proportionally negative effect on scale repeatability. It is critical to maintain constant and consistent belt tension. 5.5 Carrying idlers Many types of carrying idlers are used on conveyors. Several types of carrying idlers are suitable for use with belt scales: in-line troughed idlers, flat roll idlers, and picking idlers. Offset troughed idlers can be used but special attention needs to be directed toward making sure that all scale area idlers near the scale are properly aligned. Suitable idlers are shown in Figure 6. Idlers not appropriate include impact, adjustable transition, spiral catenary, roll catenary, belt training, and wire rope idlers. A3 A2 A1 Conveyor Stringer Fig. 7. Scale area idlers: A3 - A1 and R1 - R3 5.6 Idler alignment It is critical that a minimum of two idlers on each side of the scale be aligned with the belt scale, ideally to tolerances of 0.8 millimeters. In some applications, it may be advisable to go to three idlers on both sides of the belt scale. In high accuracy installations of ¼% accuracy or better it is recommended to strive for even better alignment if possible. Consult the manufacturer s guidelines for proper idler alignment depending on the belt scale purchased. Alignment Strings Conveyor Stringer APPROACH SIDE Troughed Idlers Weigh Span CL Alignment Strings Scale Idlers Scale Idler R1 R2 R3 Scale Idlers RETREAT SIDE Flat Roll Idlers Flat roll idler Picking idler Careful placement of the strings is critical to attaining a close tolerance alignment of the scale area idlers. In-line troughed idler * Care needs to be taken in using belt scales with offset troughed idlers. All scale area idlers must be properly aligned. Fig. 6. Idlers suitable for belt scales Offset troughed idler* The carrying idler used for the belt scale will often require some minor modification to the supports. Follow the manufacturer s recommendations for modifying the scale idler. For high accuracy applications, scale quality idlers that are manufactured to close tolerances can be supplied. It is recommended that all of the idlers in proximity to the scale area idlers be of the same make and model (A3-A1 and R1-R3, as illustrated in Figure 7). During installation, replace any worn and damaged idlers to ensure reliable measurement by the scale. Fig. 8. Idler alignment: proper placement of strings Figure 8 shows the proper placement of idler alignment strings for a typical belt scale installed in a flat or troughed conveyor. The USA National Institute of Standards handbook HB44 recommends inch piano wire or equivalent nylon line. Fluorescent deep sea fishing line can also be useful because it is easy to view while strung across the idlers. The strings should be tightly secured to a rigid part of the conveyor structure underneath the carrying idlers. After placing strings across the idlers, shims are usually added between the conveyor stringer and the mounting brackets of the scale area idlers to bring them into proper alignment. In addition, all of the scale area idlers should be spaced at equal distances. Lack of alignment is often the primary issue with belt scale problems. 5.7 Shipping bolts/stops Belt scales are often sent from the factory with bolts or stops installed to protect the load cells and structure during

5 shipping. To offer extra protection during installation, the shipping bolts can be removed after securely mounting the scale to the conveyor stringer. Shipping stops will usually be clearly marked and often with a distinct colour. Be careful not to remove any unknown bolts from the belt scale. It is also good practice to reattach the shipping bolts during major maintenance on the conveyor like belt replacement. 5.8 Speed sensor The speed sensor should be attached to the tail pulley or bend pulley so there is no slippage in the connection. It is important that the speed sensor be securely mounted and free of excessive vibration. Wheel-type return belt speed sensors are another option sometimes preferred because it is better protected inside the conveyor. Care should be taken to avoid installing the return belt speed sensor in an area that may cause the wheel to bounce. Follow the manufacturer s guidelines for installing the type of speed sensor purchased with the belt scale. 5.9 Cabling and wiring Recognized instrumentation wiring practice should be followed to protect the load cell and speed sensor signals from radio frequency interference and induction. Terminal blocks and grounded metal conduit should be used for all wiring. 6.2 Material test calibration Material test is the most accurate method because the scale is calibrated under actual operating conditions to known standards. With material tests, a known weight of material is transferred along the conveyor belt and then compared to the total from the scale integrator. The calibration parameters in the integrator are then adjusted to compensate for the difference. It is important to note that all of the known weight of material should pass across the belt scale. Sometimes the test material can become stuck in the bin or infeed section, or fall from the conveyor. The known weight of material can be obtained by weighing the test load in a bin supported by load cells or by a truck scale. The material can be weighed before or after the test. It is critical that the weighing device used for the test load be accurate and calibrated to a reliable standard. A discussion on calibration would be incomplete without addressing repeatability and accuracy. Figure 9 demonstrates five shots fired at three different targets to illustrate the relationship between repeatability and accuracy. When calibrating a belt scale, the first desired result is repeatability. After obtaining repeatable outcomes during calibration, the scale and integrator can usually be adjusted into accuracy. Non-accuracy Non-repeatability 5.10 Integrator The integrator should be mounted on a vertical surface that is free of vibration and protected from the weather. 6. CALIBRATION There are four commonly used methods to calibrate a belt scale: static weights, test chains, material test, and electronic. Refer to the operating manual for exact details describing the method appropriate for a particular scale, installation, and desired accuracy. 6.1 Static weights and test chain calibration Static weights are simple to use and are fine for the average application, or during a first-time calibration before material tests. Often the belt scale may come with an integrated calibration weight storage system allowing for easier and faster calibration. This also reduces the chance of misplacing a calibration weight. For very high accuracy applications, one might consider test chains that are unrolled by a mechanism onto the belt to simulate an actual load. There is disagreement in the industry as to whether test chains offer higher accuracy in calibration and provide any significant benefit over static weights. Test chains have their place but require consistent maintenance for reliability and can be dangerous if they break during calibration. Accuracy & Repeatability Fig. 9. Accuracy and repeatability 6.3 Post calibration best practice After the initial routine calibration it is good practice to note load cell value and speed sensor readings. This information should be recorded with the belt running empty, test load applied (if any), and while the conveyor is transporting a typical rate of material. This will speed the process of diagnosing any problems at a later date because the user can indentify what exactly has changed. 7. MAINTENANCE Proper and consistent maintenance on the conveyor and belt scale is essential for reliable measurement operation. The scale should be routinely calibrated according to the manufacturer s recommendations with consideration given the nature of the application and desired accuracy. Generally, the duration between calibration checks can be extended after the scale has proved reliable for a reasonable period of time.

6 It is also important to maintain calibration records to help discover hidden problems. Routine inspections should be made of the scale area idlers, take-up, belt, speed sensor, and scale to make certain they are in proper working order and that material build-up is not hindering operation. Belt scales can be susceptible to problems caused by material build up because debris can collect in structural portions of the scale and prevent the material weight from being sensed by the load cells. 8. CONCLUSION Many companies hire the services of an authorized factory technician to provide on-site assistance and training during installation, start-up, and commissioning. This is recommended for first time buyers or for those companies that want to ensure an optimum installation. A belt scale can be successfully installed and set up by appropriate plant personnel if they follow established procedures. These guidelines along with the instructions in a belt scale manual or installation guide should produce satisfactory results. Contact the factory or an authorized representative if there are concerns about whether a conveyor will work in an application or a scale cannot be installed according to these guidelines.