Weighing
Maximise throughput
A modern advanced control system can get the maximum throughput from
a plant without compromising quality, says Paul Girdham, DSL Systems
It is almost always worthwhile optimising weighing and batching
plants because the benefits re-occur day in, day out, year after year. Saving 30 seconds twice an hour works out at an extra 2 hours production per week, saving manpower and energy. All weighing and batching plants have a bottleneck in one area; sometimes this is weighing, sometimes mixing, grinding or other process. In some cases, long conveyors or elevators take time to transport material and then even longer to clean out. In all areas, there are things which can be done to speed up production; sometimes for every batch, sometimes on product change or maybe only at the start of a shift.
Deciding what batch to make next
For complex plants it is important to schedule the “best” batch or run of batches automatically. Many plants leave this to the operator and as a result, there are times when the operator is busy and the plant sits idle for a short while. The best technique here is for the control system to look at the orders, each
ABOVE: Drax control room
downstream process, the amount of product to process, any batches in transit and then decide the “best” batch to start. For animal feed plants, the key objective is to keep the pellet presses running and this may be calculated from knowing three key values: the throughput of each pellet press in tonnes per hour, the number of tonnes ready to process and batches already in progress. If a plant has at least xx minutes of processing ready, the control system can then look for a more urgent demand. Of course, there are many further rules which the control system has to follow.
For batching plants which are serving multiple production lines, it is important for the control system not to let the batching plant block up otherwise batches cannot get to the other processing lines. Sometimes the control system has to be programmed to take a calculated risk knowing that a particular scenario is very unlikely, thus blocking the plant. Often the operator has a good feel of these situations and can be relied on to override certain checks.
It helps too to pre-warn the operator of pending problems for future production. For example, if there will be a shortage of a particular raw material in 30 minutes, it helps the operator to prepare to get more raw material ready. If there is a cross contamination risk between batches then the operator has time to re-schedule production if necessary. If some piece of equipment will need its settings changed then the operator can arrange to do it in plenty of time.
Weighing
Some plants are restricted further downstream, for example at a mixer. You could assume that there would be no point in optimising anything upstream but optimising weighing upstream will be beneficial at times, for example at the start of a shift
16 Solids & Bulk Handling • March 2010
www.solidsandbulk.co.uk
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