Process control systems
February 1, 2010
When it comes to process control, it is difficult to decide where to start with all that is available in terms of control mechanisms, power monitors and all the gadgets and widgets that go toward making a system work and function at its best. The obvious answer is the same as with everything else in life — you start at the beginning.
With all goods that are received at a mill, you have more than one aim or objective when handling. First, is to ensure that what arrives at the mill is what you expected and is, indeed, what you bought. Secondly, you seek to discharge the contents of the bulk vehicle or the cargo on a flat-bed truck with the minimum amount of labor. Gone are the days when several men sat around waiting for deliveries only to find the shipment was coming in later than expected, requiring you to pay overtime to retain staff beyond the end of their shift to accept incoming goods. Nearly all deliveries today are prebooked for a given time of day, and invariably the driver unloads his own vehicle without the use of mill employees.
To deal with the contractual side of intake procedures, you would normally use the weighbridge as the receiving point where paperwork is checked and verified. Grain and other bulk commodities will be sampled and tested, invariably using NIR analysis, to get a quick result and ensure there is nothing untoward with the delivery.
You also will have the weighbridge data entries linked to the accounts package so that all receipts will upgrade contract balances, update inventory valuations and provide data to accounts at month’s end to enable monthly accounts to be produced. Any variations against contract, such as high moisture content, will automatically generate a discount in your purchase ledger and send claim forms to the seller. Intake weights automatically update the blending records and enable you to do forward production projections to ensure having enough raw materials available for the finished products schedule to be manufactured.
Bagged goods, supplements, minerals, etc., are received in similar fashion, only they may well be quarantined pending sampling and testing before use. Bar code readers are now becoming more common, and most mills have the ability to scan a coded label and update inventory programs and accounts packages at the same time, often creating batch records for bagged goods that require further tracking of their usage.
Liquids are treated the same way as bulk commodities but with samples being drawn by the tanker driver during discharge.
Many mills use programs which control the total site operations. Many new programs are now available, but almost all are based on the principles of packages like Mill Master which were prevalent in the late 1980s and were derivatives of bespoke programs that the formulation companies put together. They were all very basic as we look back now, but at the time these programs were at their zenith, we all went through major reconstruction of our workforces and reduced labor significantly, some more bravely, and as it turned out, wisely than others.
Once tested, and gross incoming weights are known and recorded, you need to transfer goods from vehicles to their designated storage area (silo, storage tank, quarantine area, etc.) as quickly and efficiently as possible, giving the driver a turnaround time of no more than 30 minutes for a 29-tonne load, ideally 20 minutes in normal circumstances. This can be done by utilizing a simple site traffic management system in which traffic lights are strategically positioned around the site which the driver follows.
When a vehicle passes over the weighbridge, the plant control system recognizes it has done so and a site route is automatically determined for transfer of vehicle content into the designated silo. The vehicle then waits its turn in the queue and follows the traffic lights to the designated intake pit. On arrival at the pit, the touch screen adjacent to the intake pit will ask the driver to confirm the origin of the vehicle contents and other pertinent data. Once this is done, the vehicle discharges, is weighed again and leaves the premises. The intake system will then shut down sequentially if there are no more vehicles to discharge or it will start up the route for the next vehicle to discharge. Records are updated and transactions completed.
Simple mechanics enable routing to be effected by using electro-pneumatic slides or fully electric slides that can be energized remotely. Safety of the plant is not compromised and rotational sensors are used on elevators and conveyors along with smoke detectors and heat sensors placed strategically around the plant to pick up on any hot spots which could present a danger. In such circumstances, the plant will shut down and make calls to sound the alert and summon the necessary personnel to address whatever circumstance has arisen, such as a bin being too full, a conveyor running slow, etc.
Almost all intake points are covered nowadays with either drive-through covers or, where vehicles back onto the tip, there is an open-fronted canopy which stops birds gaining access.
In the case of new mill installations, the planning authorities insist on intake points being covered and will not allow, in Europe at least, access to grain and other bulk feed ingredients by birds and other vermin. Sheets used to cover the vehicles in transit are only allowed to be removed either at the intake point or at the time of loading with feed. Obviously, this is to reduce possible contamination and, as far as access to the cargo by the driver is concerned, to address and eliminate the potential risks of individuals falling from the vehicle.
When ingredients are stored in silos, their use is dictated by the feed formulae being used at the time. Whether the mill is pre-grind or post-grind, the drawing forward of product through grinders to mixers is the next step in the chain. Product is moved as efficiently as possible with air flows being automatically monitored and adjusted. As product passes through the grinders, you can expect full control of the hammermill or whatever means you have chosen to use as a grinding media. Grist spectrum is of paramount importance and affects the power consumption and performance of subsequent processes, namely pelleting presses. For this reason it is better to post-grind, as a more representative sample can be examined than a simple, single raw material sample whose grist spectrum indication is often a bit of a compromise when taken in isolation. Monitoring hammermills and roller mills will tell you when bearings need lubricating, and most competent systems will apply lubricants correctly, on time and in the right place and right quantity. Screen wear and beater wear will tell you when attention is needed, and most systems will print out a timely work orders for maintenance to be done on a timely basis.
Full performance reports are presented at whatever time interval you deem appropriate, which can mean monthly to fit in with stock-taking periods but also weekly to suit maintenance schedules. The advent of “true-life” costings is generating some interest in the trade, and new software systems are being used to address the exact costs of performance of key areas such as hammermills, roller mills, pneumatic systems and similar large power consumption areas.
A cost of $3 per tonne of product ground is not unusual, and the application of this charge is a matter of some debate when formulating. Some millers believe this cost should be applied to a mixer head cost, because if it precludes the use of an ingredient and then favors the use of an unground ingredient, then this cost is not incurred. Other millers simply apply a grinding charge across the board to all ingredients equally on the basis that it is the nutritional value that he seeks irrespective of the cost.
I favor the first choice where a charge is only applied when it truly applies. A pound saved being a pound earned, as they say. However, irrespective of what your preferences are, control systems
will do exactly what you tell them, and you can rest assured that, unless you have provided it with false information, the results will be accurate.
In years gone by, millers used labels to record their processing and measured their use of electricity by getting up early on a Monday morning to read the plethora of meters that were positioned around the mill. Today’s control systems do exactly the same thing, only they do it constantly so that information is precise, up to the minute and accurate. You can make decisions quicker based on the data output and you can make just as many mistakes, only quicker. Therefore the skill of the miller to interpret the available data and treat it with respect and understanding is still crucial.
Jonathan Bradshaw is a consultant to the agribusiness and food processing industries, specializing in project management and bespoke training programs through his company, J.B. Bradshaw Ltd. He has extensive experience in flour and feed milling in Africa, the Americas, Europe and the Caribbean. He may be contacted at: email@example.com .