Belt Weighers And Energy Monitoring
To the old adage ‘the only certain things in life are death and taxes’ should be added ‘…and spiralling energy costs’. One of the most significant costs in the production of most bulk solids is the amount of energy consumed to produce them. This is particularly relevant with materials such as graded quarry products and asphalt, and other processes that use dryers and heaters. In quarrying, where primary, secondary and tertiary crushing take place, the costs can be enormous.
The cost per unit weight (in the case of quarries: per unit tonne) must be known, as this cost has to be passed on to the customer. A significant part of this costing is the amount of energy used to produce the product. To do this accurately on a day-to-day basis, measurements need to made on the plant itself. If these measurements are required on a real-time basis, one of the best ways to achieve this is to use belt weighers. Belt weighers produce real-time and long-term information about mass flow within a plant. Belt weighers can also be sited between various pieces of equipment to monitor individual items of interest.
The information produced by the weigher needs to be collected and correlated to the power consumption over the same measurement period. Measuring the electrical power used is a reasonably straightforward process using a power meter with analogue and pulse outputs giving instantaneous power and integrated kW/h pulses. The output from these units can provide useful information such as peak demand, phase balance, line voltages etc.
If a relatively simple unit is employed with pulse output only, the frequency of the pulses can be translated into real-time power consumption in kilowatt hours (kWh). Depending on the level of sophistication, such units are reasonably cheap and easy to install. CT’s are used to interface to the three-phase supply, giving good galvanic isolation between the power supply and the monitoring system.
What is the best way to correlate all the information produced by the plant transducers, and make the information easy to read and valuable for analysis of plant performance? It is important that data from the belt weigher can be read in the same time frame as the information from the power-monitoring system.
Lodestone use multi-channel data loggers to store, assess and display this information both in real time and as historical data. The data are time and date stamped. The frequency of data recording is programmable. Depending on the memory size within the logger, in excess of 12 months’ information can be stored and accessed. The information can be displayed in real time on a PC and stored to disc for later analysis. The data can be either transmitted via a serial link (RS232 or USB) or downloaded from a data stick, which can be plugged into the data logger and then removed when the information is required and plugged directly into a PC. This obviates the need for wiring and also the need for a dedicated PC connected to the system. The information collected can easily be transferred into a spreadsheet (eg Excel) for further analysis. A basic spreadsheet is supplied with the data-logging system.
The advantage of using data loggers is that other plant transducers can be introduced into the system at very little extra cost. Industry standard process signals, such as 4–20mA, pulses, frequency, thermocouples etc can all be monitored. The data logger has essentially become a modern-day equivalent of the chart recorder, with the added flexibility of being able to easily manipulate the data collected on a PC.
As with all measurement systems, the information being produced has to be accurate and repeatable (rubbish in equals rubbish out), but achieving this is not always easy. When designing a belt-weigher installation, the method of calibration should always be considered very carefully. The proper way to calibrate a belt-weighing system is by ‘live’ load checks, ie running a known amount of material over the weigher and checking the actual amount conveyed against the measured amount. This material can be either pre-weighed or post-weighed, ideally over the site weighbridge. Failing this, the use of a pre-calibrated front-end loader weigher can be used, although this may not be as accurate as a stamped weighbridge.
The use of calibration weights, chains or material belt cuts is not good enough if the result is important to the final price calculations. One of the major reasons for using material weigh checks is that the weigher is tested under actual operating conditions. This takes into account problems such as changes in belt tension, conveyor deflection under load and no-load conditions and belt tracking empty and full. All these variables, and others, can cause weighing errors.
Weights and chains do not provide these effects and tests using material belt cuts have to be done with the belt stopped. A belt weigher will show different weights on the load-cell in static and dynamic conditions mainly due to the differences in belt tension. Also, the ratio of material from a belt cut against the amount of material conveyed is considerably different and the resolution is too low for the results to be really useful. This method can only confirm that the weigher in roughly the correct area of calibration.
All methods other than a weighed load check will merely show that the weigher is ‘about right’. The user has to be confident that the information generated by the weigher is correct, which is another very important consideration when using weighed load checks, as this method actually proves that the weigher is performing correctly.
Calibration checks should be carried out as often as possible and until the user is confident that the weigher is holding its calibration over a useful period of time. This usually translates to a check once every three months or so, depending on the site. If the weigh area is kept free of spillage and regularly inspected, the calibration should be expected to hold for a minimum of three months. Site personnel usually get a feeling for the numbers produced and can tell if they are correct or not.
In critical applications it is important that a site survey is carried out to establish whether the conveyor in question is suitable for a belt weigher. The person responsible for the survey should have an in-depth knowledge of weighing system applications and should be able to advise the customer what, if any, modifications will need to be made to achieve the required accuracy and repeatability.
To give an accuracy statement of ±0.5% without giving a length of time that the equipment will hold this accuracy is meaningless. Most belt weighers will produce a good result at the time of testing; the real test is how long will the system hold this accuracy. This should be one of the first questions asked when purchasing a belt weigher. The only way to assess this is to inspect the site and calculate from the information accrued what weigh length is required to give short-, medium- or long-term accuracy.
The following list highlights some of the conveyor conditions that are crucial for good belt weighing:
- The conveyor should be rigid in the weigh area. Any structural deflection in this area can cause weighing errors.
- Ideally, the belt tension should be controlled by a gravity tension unit (GTU).
- The belt weigher should be sited at the non-driven end of the conveyor (normally the tail end). This part of the conveyor tends to have the least change in tension. It should be far enough away from the feed point so that the material has time to settle before it reaches the weighing area.
- The conveyor should be as full as possible (volumetric utilization).
- The troughing angle should be as small as is practical. The steeper the troughing angle, the stiffer the conveyor.
- The conveyor speed should be as low as possible. The longer the weigher has to measure the product on the conveyor, the better the measurement will be. Higher belt speeds demand longer weigh lengths if reasonable weighing accuracy is to be maintained. This is known as material resident time in the weigh area. There are a number of other factors that govern the weigh length (see ‘The Black Art of Belt Weighing’, QM, July 2005).
Another calibration method that has been employed successfully in situations where it has not been possible to run a weighed load check is the use of long-term calibration. This requires the user to keep records of both the belt weigher throughput and the weighbridge totals. Over a period of weeks and months the two can be correlated and a calibration figure calculated.
One of the major indicators of the performance of a belt weigher is the belt zero. When the belt is running empty, the rate reading should show a negative and positive reading as the weight of the belt changes along its length. The belt zero should be checked each day as this is often a major source of inaccuracy. A belt zero measures the weight of the belt over a predetermined number of revolutions and calculates the average zero, which becomes the ‘zero’ value. This process can be done automatically although this is not recommended. A belt weigher should never be regarded as a ‘fit and forget’ device as there are too many variables that can effect its performance.
The LodeMaster belt weigher has an auto zero tracking (AZT) option built into the system. When this function is selected the system monitors the amount of material flowing over the weigh area. When this amount drops below a predetermined weight, the AZT function begins to operate. This is known as the ‘dead band’ and is the point at which integration ceases; it is software programmable by the user. The system allows the belt to travel for a number of revolutions and if during this time the system stays within the dead band an automatic belt zero routine is instigated. This is similar to the manual zero routine selected via the keypad. If during the AZT routine the system remains within the dead band, at the end of the period a new zero is stored by the system. If the zero passes outside the dead band the AZT routine is aborted. This process will continue until material is flowing on the belt.
The disadvantage of using AZT is that if there is a trickle of material on the belt or a there is a build-up of material on the weigh bridge which then falls off, weighing errors can be introduced and the zero can rise beyond the prescribed limits. If this occurs, the AZT is cancelled and an alarm is shown. This situation usually requires a phone call to the vendor to establish if the weigher is still functioning correctly. It is always far better to perform a manual zero with someone observing what is taking place.
Once it has been established that the transducers are performing accurately and repeatably, the information generated in real time can be invaluable. Just a small change in feed rate to various items of plant can be monitored to assess if the change increases or decreases the plant efficiency. The possibilities are manifold; for example, a more efficient gearbox may be added to a system and the effect this produces can be read in real time. The combination of belt weighers and other plant transducers provides a very powerful management tool. To measure is to know.
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