Process control in the flour mill
October 01, 1994
by Teresa Acklin
Defining your objectives and requirements is the key to automating a mill successfully.
Process control is of great interest to flour millers and apparently is difficult at first glance because of the myriad and multiplicity of possibilities available.
Many specific elements must be taken into account by the flour miller. Great care must be taken not to go overboard with fancy features; many are seldom used, not necessarily reliable and often lead to runaway capital costs.
The recommended method is to start by describing or formalizing a requirement specification. This becomes the basis for process control engineers to write a functional specification, design a system and develop a price quote.
The objectives of process control should be stated first. What are the objectives of the miller? In a word, efficiency. This covers the whole spectrum and includes:
consistent, cost effective and accurate product quality;
efficient use of wheat that is the right quality at the right cost;
simplicity and ability to expand cheaply;
increased yield/extraction through longer runs, quicker changeover;
accurate inventory control;
lower power consumption;
return on investment;
Once the miller's objectives are clear, a requirement specification should be prepared. Table 1 shows an oversimplified example of a theoretical mill with a capacity of 420 tonnes of wheat per 24 hours.
Of course, the complete requirement specification would detail fully each of the separate elements shown in the example. The flour miller knows very well the necessary options he needs, and the process control engineer must have a very clear understanding of these needs.
For instance, in describing how wheat receival should function, the miller might want an intake system that could take a sample from the truck automatically and send it to the laboratory. Once the laboratory accepts the wheat, it could issue a printed circuit key card coded for that load. The truck driver would insert the key card into a device that would route the load to a defined bin that has equipment programmed with automatic predamping instructions.
Other examples of functions that could be process controlled include conveyors and elevators, which can be monitored and alarm controlled with malfunction rotation and tracking devices. Wheat cleaning functions can incorporate aspiration remote control valves for preprogramming different wheats. Conditioning times can be adjusted remotely according to variety or class, as can wheat blending.
The mill also may choose to include partial or total roll gap setting, grist by grist, for accurate and fast changeover through the programmable logic controller. Flap valves for dividers and purifier air valves also can be motorized and remotely controlled.
The P.L.C. is especially suited for blending flour with other flours and selected additives by weight. The avoidance of mistakes in this area is positively helped by P.L.C. use.
While millfeed and germ processing are straightforward, the monitor and control of this area completes the system.
After discussion and careful review of the requirement specification, the engineer will write the functional specification. After agreement on the functional specification, the engineer will design and quote a suitable system.
A process control system can be designed to allow the entire plant, end to end, to operate under full automation 24 hours a day, day after day, with occasional operator intervention. Table 2 lists typical features widely available at cost effective prices.
The notional mill in the example would have at least two work stations: a visual display unit with a keyboard in the control room and a parallel unit for the plant superintendent. A parallel unit also could be provided for the chief maintenance engineer.
The P.L.C. still is the most-used system for flour mills and is very reliable, but P.L.C.s are only as good as the field instruments feeding information to the controller. Instrumentation comes in all shapes and sizes and includes bin level indicators, proximity capacitance devices, pressure gauges on blowlines, electronic weigher signals and on-line quality analyzers.
Process control systems also feature real time display monitor and control that is, as it happens. This releases the miller from mundane tasks more suited to computers and allows the miller to spend time on checking and tuning the plant or dealing with problems.
Avoiding pitfalls is tricky. Not only is there a great need for definition by the miller, but this must be balanced by the requirement to keep it as simple as possible.
If matters are not thought through logically and carefully, money can disappear at a frightening rate because of overkill. The temptation to add on feature after feature, which may well be desirable but not strictly necessary, is great. If the miller starts from scratch with simple policy objectives in mind, it is easy to add extra or other features later.
Another potential trap is to purchase a system that requires frequent and expensive visits by a specialist from the supplier. This is especially true in remote areas. Reputable suppliers of process control systems provide good and effective training courses on site, as well as effective backup.
All in all, the miller needs to define most carefully his requirements to end up with an effective system. The features should be sufficient for him to gain his objectives of efficiency in the widest sense: return on investment with finished product quality and consistency clearly in mind.
By David Sugden, a consultant to the grain industry.Table 1: Sample requirement specification
Table 2: Process control system features
|Capacity||420 tonnes of wheat per 24 hours|
|Running hours||156 per week|
|Wheat grists||eight classes, ranging from 100% hard to 100% soft|
|Intake||100% by road in bulk over a weighbridge|
|Silo/elevator||two weeks storage or 5,200 tonnes, with preliminary cleaning, predamping and|
|blending facilities, plus turnover|
|Wheat cleaning||comprehensive, 20 tonnes per hour|
|Conditioning||24 hours or 420 tonnes, with fully flexible blending capabilities|
|Mill||straight grade and divide flours of patent and low-grade types with an extraction of up|
|to 80% on white flour; wheat germ extracted at up to 0.8%; straight millfeed at 28%|
|Flour blending||four days or 1,300 tonnes, capable of adding gluten (maximum 1%) by closed loop,|
|ascorbic acid, fungal alpha amylase, etc.; on-line flour quality monitor and control;|
|automatic sampling required for laboratory; flour blending with additives to make 40|
|grade maximum with up to 70% bulk and 30% in 25 kilogram bags; 20-tonnes per|
|hour flour intake and reprocessing system required|
|Millfeed||36 hours of storage of 180 tonnes, bulk outloading in loose form|
|Germ||10 tonnes storage in bulk, then bagging in 25 kg bags|
Inventory monitoring and control from start to finish using production weighers tied to
weighbridges for inbound and outbound goods, with shortage prediction;
Bin level monitoring and control;
Blending of wheat, as well as flour and additives;
Stop/start of all machinery in sections and sequence;
Queuing of jobs automatically, one after another, including plant changeover, valve and bin
Least-cost formulations with specialist software;
Maintenance scheduling, inventory, monitoring and costing;
Production reporting statistics and hard copy records;
Password access for different staff members for security;
Diagnostics and fault-finding and reporting;
Reconfiguring by local site staff after short training;
Product tracking of batches of wheat and flours for quality reference and in case of customer
Capability of transferring information to the management host computer for visibility,
administration and financial control;