Maintenance: just-in-time or just-in-case?
October 01, 1995
by Teresa Acklin
Consultant David Sugden outlines the most effective maintenance techniques to keep a flour mill operating smoothly.
The trick with flour mill maintenance is to ensure that the plant stops only when scheduled and does not break down in between. Maintenance becomes more critical the longer a mill runs. But what is maintenance?
Maintenance is the means by which a mill is kept running reliably with minimum downtime or stopped time and at minimum cost. That seems simple, but it is not. For there is clearly a point at which a certain maintenance cost brings a benefit in better performance such as yield, capacity and quality. It is possible to measure the cost against the benefit.
There are two types of maintenance, “just-in-time” and “just-in-case.” Just in time conceptually means that required work is carried out at the last minute or even after an unnecessary breakdown; it also can signify a lack of planning. On the other hand, just-in-case typically means a carefully thought out, planned, costed and practical scheme, often known as preventive maintenance.
Peace of mind is important. Mills running long hours, such as those averaging more than 160 out of the 168 hours available per week, sometimes have enormous difficulty holding to their schedules unless a good quality maintenance scheme, including personnel, is in place.
With so much equipment in a typical mill, so much can go wrong that a good scheme is very well worth examining. Let's consider a relatively new flour mill with all the necessary facilities, including an elevator of 10,000 tonnes, a mill of 600 tonnes of wheat per 24 hours plus packing equipment.
The plant superintendent will have at his right hand an individual who serves as the chief engineer, maintenance supervisor or maintenance manager. In well run plants, the chief engineer probably will supervise five other people at a minimum. This team of six is split evenly between fitters and electrical specialists.
The keys to team success are the skills of the individual team members, who should approach the work with enthusiasm and common sense. The team will use its skills in very specialized and particular areas of the total plant.
Teamwork includes appreciating, planning and controlling work schedules. lmportantly, work schedules not only account for vacations and sickness but also encompass contracting or hiring out those tasks that are not specialized to flour milling. These tasks include painting; repairing or replacing lighting fittings, sprinklers, boilers, elevators and fork lift trucks; and sundry building and repair work.
Specialized work carried out by the team covers the monitoring of all machine performance, instrumentation and process control. The latter includes programmable logic controllers, computer software and hardware and electrical overloads.
Maintenance of a total plant can be broken down into two distinct categories. One deals with machinery, accessory equipment and projects that can only be worked on during a shutdown. Examples include plansifter sieves, dust collectors and replacement of worn parts in continuously operating and powered equipment.
The other maintenance category consists of machinery and equipment that can be handled at any time, including weighbridges, laboratory equipment, certain building repairs and the like. It can also include machines normally running continuously but which can be bypassed for the duration of maintenance. Examples include certain roll changes and production weighers; some wheat cleaning machinery also can be circumvented if bypass spouting is in place and product quality is not jeopardized.
How is it possible to measure the costs and benefits of maintenance? This is normally a straightforward mathematical calculation that takes into account the financial effect of a breakdown. If a mill can reduce its stoppage time by 1% over a year meaning the plant can operate efficiently for an extra 1% of the time and sell the extra flour, then significant savings can be made.
A mill grinding 600 tonnes of wheat per 24 hours and extracting 75% of flour produces 18.75 tonnes of flour an hour. If the mill can reduce its downtime by 1% a week by operating 161.6 hours instead of 160 hours, the mill will produce an additional 1,500 tonnes of flour in a 50-week operating year (1.6 hours per week x 50 weeks x 18.75 tonnes per hour).
Some overhead expenses, such as administration and manpower, will be paid regardless of running time. But additional overhead costs for power and transportation will accrue, as well as costs for the additional wheat. Depending on the country in which the mill is operating and trading, the economic benefits clearly will vary.
Another measure of cost/benefit is the matter of extraction and capacity. Suppose the mill's extraction rate (and therefore, capacity) drops by 1%, to 74% from 75%; the mill superintendent identifies the cause to be certain sifter sections that are underperforming, requiring a stop for maintenance.
Every hour the mill continues to operate with the faulty sifter sections represents a loss of at least 250 kilograms of flour, which over a 160-hour week means a loss of 40 tonnes. The cost implications are clear if the mill is allowed to run, and the mill superintendent would be totally justified in stopping the mill for about two hours to rectify the fault.
The issue of a quality drift is less easily quantifiable but can be analyzed, assuming it is a problem with the plant and not with the wheat. Assume the ash or mineral content has increased by 0.05%, and therefore the flour is out of specification by that amount.
The mill superintendent in this example has identified the cause to be insufficient water in tempering, but this problem will take a few hours to fix. If the mill continues operating, he will have no choice but to reduce extraction to keep his customers happy. This leads to a calculation similar to the one above to stem loss of money.
Turning to machinery and electrical equipment, the chief engineer and his team will always be looking ahead to the next necessary maintenance project. The team carries this out by gaining information from several sources.
These sources include the mill's 24-hour log, mill production personnel and the maintenance team's own observation. The schedule of work coming up for completion, which often is managed these days by computer, is another consideration.
Monitoring by the team will include a daily tour of the whole plant. Team members will be looking for unusual signs, such as temperature, vibration, noise, odor and appearance.
Some of these aspects, especially pressure, flow, temperature, speed, power consumption and vibration, can be monitored by instruments. These instruments can often be permanent or hand held. There is nothing like quantifying a question because judgments will always need to be made as to how long a situation can be allowed to run. The most frequently heard question in mill maintenance is “How much time have we got/can we have?” Other sources of information are the laboratory, as well as production records. The records will indicate a stable or deteriorating situation, although in no case will the cause of a difficulty be clear without investigation.
If hand-to-mouth maintenance is not to be recommended, what is? There are a number of off-the-shelf software maintenance packages available for personal computers that are ideal for preventive maintenance. The programs have wide ranging facilities that allow memory jogging, budget and cost control and planning.
Facilities include a menu and a schedule in which all assets and spare parts can be loaded into the program, together with their costs. The software also can store information on and help analyze inventory forecasts, plant history, a log of when and why breakdowns occurred, frequency of maintenance per asset, lists of personnel call-outs including reasons, indications of the need to design out a problem, reminders, trend plotting, defect reports and analyses, test results, estimated and actual man hours per job, schedules of upcoming work, unscheduled work completed, goods received and used, resource availability and calendars, training needs and records, worksheet generation and instruction and budget and cost control.
The only thing “wrong” with this preventive maintenance system is that it must be kept up to date and edited daily. If it is not, it is useless. On the other hand, it can be an excellent tool for allowing proper control over the whole maintenance of a plant. It actually results in cost reduction over time while increasing efficiency.
So, the answer to the question of how to stop the plant only when scheduled while avoiding most unscheduled breakdowns is preventive maintenance, organized by a small team of specialists under the chief engineer. The ultimate requirement for the maintenance crew is skill, enthusiasm and common sense.
It is my view that corners cannot be cut in maintenance for proper competitive efficiency; and a good, small maintenance team is priceless.