Load-out & Dosing Control Systems

Posted in: , on 29. May. 2007 - 12:47

There are a great number of techniques used by industry to load bulk material from containers into lorries trains and dumpers and other vessels. Some use volumetric methods but for good accuracy, weighing is usually involved. The type of system selected depends on a number of factors such as required load accuracy, resident time of lorries in the dispensing area, driver or operator controlled load-out and other factors. Most of the writer’s experience is within the mining and quarrying industry.

Volumetric Load-out

Probably the simplest and the cheapest technique is using slide or clamshell gates beneath a storage hopper and a timed gate controller. This type of system depends on the flowability of the material in the load-out vessel being consistent such as a graded single size material. Moisture and seasonal variations play a part so constant calibration checks may be required. This however is generally straightforward as the dumpers can usually be checked over a weighbridge after loading. The frequency required for calibration checks will soon become apparent. The loaded material from a volumetric system would normally only be used within its own environment such as a quarry, for uses such as dumpers taking material from a silo to say a coating plant and other similar applications.

In order to calibrate a volumetric system, an amount of material is instigated by the system and put into a dumper from the load-out point and a timer in the system is started. The timer is then stopped when the material has stopped feeding. The material is then taken away and weighed and the weighed value is entered as the weight calibration value in the controller. From this information, the system can calculate any discharge amount from the dispensing system. It is generally repeatable to within 5% of target.

The set point can be adjusted by the driver or operator as required. A display gives target load and time to load completion. Calibration information should be password protected as this information is entered into the system via the operator’s console.

Errors in this type of system usually occur when either the material type in the dispensing vessel is changed, or the flow characteristic change due to low volumes of material in the hopper or change in moisture content.

Static Weigher Load-out

A static weighing system offers the best accuracy. A system supplied by the writer involved the collection of the off-cuts from a number of steel presses being collected onto a conveyor system and then transported to a weigh hopper. A lorry is placed beneath the weigh hopper. When the load in the hopper reaches a predefined set point, the hopper automatically discharges into the lorry. In order to distribute the load correctly, a number of discharges are initiated so that the lorry driver can move and even out his loading. When the lorry has its correct load an alarm is sounded to alert that another lorry is required. A display indicates the progress of the load cycle. This loading process can take up to 24hrs. There are no tip-off or top-up facilities on site so the weighment has to be consistently accurate.

Dual Hopper weighing system

Other systems such as dual hopper static weighing systems are available but tend to be relatively expensive. This technique involves one hopper loading with the other hopper weighing and discharging. Generally the hoppers are fed via a bifurcated chute with a divert flap which depending on the material can be a source of high maintenance and wear. Properly designed, this system can give a continuous discharge of material into the receiving vessel and can be very accurate as the calibration is fairly simple using static weights. These can be attached to each individual hopper and put onto the hopper either electrically or with a manual lever system. Although technically this is a continuos weighing system, it uses static weighing techniques and is some times used for batching into larger vessels such as train load-out systems and blast furnace charging systems.

Simple static batching systems

Some static weighing systems simply weigh in the required materials and discharge the complete vessel when the set point is reached. One advantage of this method of weighing is that multiple ingredients can be added to the same vessel using multiple set points in the weighing controller or controlling SCADA system. The amount of each ingredient can be stored and printed out or sent to a data logging system for future analysis. The system also monitors the overall progress of the batch. This can either be done manually or by a continuous automatic batch system. Generally this type of batching would be interfaced into a plant control SCADA system or other plc type control application.

Errors in this type of system are generally caused by either seasonal temperature variations with the sun on one side of the silo causing mechanical movement ,windage and temperature changes causing the structure to move. Hoppers sitting within a building usually give the best results.

Loss-in-Weight controllers

The Loss-In-Weight controller works along the lines of a static weighing vessel which is mounted on loadcells with the added function of being able to integrate the discharge of material with time. The loss-in-weight controller not only indicates the amount of material being discharged, but also its rate in weight per unit time such as tonnes per hour or kilograms per second. This information can be valuable when integrating the controller with a proportional dosing system such as sand to cement ratioing using a PID controller. The cement stream is generally fed via a screw feeder in order to prevent flushing.

As the weight output is absolute the weighment is accurate generally to better than 0.2%. The main source of error is during the fill cycle. As weighment can’t be made when the vessel is filling, an assumption has to be made, and the last value before the fill cycle began is held and used by the system. It is therefor necessary to fill the system as fast as possible and ensure that the material has settled before weighing can start again. The best technique is to stop the feed whilst the vessel is filling but of course this is not always possible.

Load-out systems using belt weighers

The most common type of load-out control system is to have a load-out vessel feeding onto a conveyor and thence into a target vessel, lorry, train dumper etc. A belt weigher measures the flow of material from the dispensing vessel. The information from the belt weigher is then passed to the load-out controller. The load-out controller starts and stops the feed as required by reading the required tonnes and acting accordingly. In most systems in-flight material compensation will be required due to the distance of the belt weigher from the discharge point. This value is a parameter set at calibration time. The in-flight material is determined by setting the in-flight value to zero and the set-point controller to a required amount and then weighing the load. The difference between the required amount and the actual amount delivered is the in-flight material value. This gets more complex when dealing with multiple feed points. Generally each feed point will need to be calibrated against the belt weigher and its in-flight value determined. Then depending on which feeders are selected, the controller needs to be able to determine which in-flight values to use. This is generally done by digital inputs into the controller.

There may also be a requirement for accurate material sandwiching in such application as black top manufacture on continuous drum mix plants. Selecting the relevant feeders is done on a time basis, the furthest feeder starting first and then each feeder starting after the required amount of time. The reverse action is required when stopping the feed.

In order to get a more accurate loading a trickle feed can be introduced. This enables the bulk of the load to be delivered quickly and when a predefined point in the loading cycle is reached, the feed can be reduced until the set point is reached.

Sophisticated load-out using weighbridge feedback and lorry tagging

In the past 20 years or so with the advent of powerful PC’s and Super Quarries where quarry traffic can be at a very high volume, automated systems have been introduced in order to keep the throughput of traffic to a maximum. The need to eliminate time wasting such as topping up and tipping off is paramount to these types of operations. Also the system offers management information such as lorry resident time in the system and a host of other powerful management tools. The types of systems that the writer has been involved in are fully automatic load-out control systems for the quarry environment.

When the lorry driver enters the quarry and is tared off and he is given a tag that is similar to a credit card. This as you might expect has to be extremely rugged. A magnetic swipe card wouldn’t last very long in a quarry, so inductive identity cards and similar non-contact type cards can be used. When the card has been issued the driver is entered into the system and is now ‘tagged’ . This ‘tag’ contains all the drivers’ details, the required load and type of material required and the time and date this was entered into the system. He then goes to the load-out point. When his turn for loading comes he presents his card to a reader and he is directed to the correct loading point by a traffic light system. A series of sensors tell the control system that he is in place. The driver then presents the card again to another card reader in the load-out bay to confirm that load-out can commence and that the lorry has indeed arrived at the correct load-out point. The driver can start and stop the load as required via a local control panel. When the load is complete, the driver can now return to the weighbridge in order to get his ticket. The weighbridge operator then uses the same card to bring up the lorries details and create the invoice. Cards can be dedicated to frequent users saving time on data entry.

During the loading process, the system keeps a record of each weighment from the belt weigher on the load-out conveyor. When the weighbridge weighs a particular truck, its sends this information with its tag back to the load-out controller which in turn checks and if required adjusts the load out system to comply with the weighbridge value. It is assumed that the weighbridge, being point of sale, is the most accurate of all the weighers, hence the load-out control system is continually checked against the site weighbridge. If a single size has been dispensed, then individual feeders can be adjusted accordingly.

For best accuracy each feed point should have its own feeder weighing system. This would generally consist of a hopper with an extractor weigh feeder beneath it. Using this technique each feeder can be individually calibrated and controlled with a far greater accuracy than volumetric feeders, each feeder having it own control loop. The actual load out belt weigher on the feed to the lorry should be a multi-idler system for the highest possible accuracy and repeatability.

Data Interfacing

The transfer of information from site to Company Headquarters, Site Manager and others can be of great importance to the invoicing and general production information required. Some companies use dedicated modem lines in order to transfer this data, others still rely on the well-tested method of sheets filled in on site and posted. With the advent of the Internet, data transfer is now a very efficient process to anywhere in the world. There are now relatively simple systems for extracting all of the relevant information either on request or on a timed basis and sending it to its destination.

Data Logging

Data logging to a memory stick has also proved to be popular. The memory stick can be removed and the data downloaded into a PC and analysed when convenient. This can be displayed in graphical format or as a spreadsheet at the click of a button. Data security using this method is also very high and can be preferable to sending the information over the web. Once the data has been recovered it can easily be emailed. Depending on the sample rate and the storage capacity of the memory stick, up to 12 months worth of information can be stored. If the data exceeds this, the oldest data is then overwritten.

Jon Scarrott

Lodestone Electronics Ltd


Write the first Reply