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by David E. Pierce, Diamond Alkali Co.
The Kilowatt-Hour Yardstick Maintenance profiles are useful in comparing plant departments against the same denominator plant control presup poses a suitable measure of activity. For a plant making one product only, or for a plant where the other prod ucts bear a substantially constant relation to one material, the best yardstick may well be that item. Thus, in a plant making chlorine by the electrolysis of brine, tons of chlorine per day makes a very good measure of activity. Using this term as the abscissa, the staff engineer can plot as ordinates such items as operating labor, maintenance cost, supplies, supervision cost, steam and power consumption, caustic soda pro duction, cubic feet of hydrogen gas delivered, and so on down the long list of pertinent items. Not onlycan such a chart be used for compari sons of performance within the plant at various periods but it permits com parison with other similar plants in the company or in the industry. Problems arise, however, when a comparison is desired after other de partments have been set up in the same plant, departments that use only part of the original major pro duction or have no relation to it at ^\DEQUATE
all. There may be a contact sul furic acid operation or a department manufacturing chlorinated hydro carbons or synthetic resins. Each department may use its own major product as the common denomina tor of its performance in this quarter compared to last year, and should do so, of course, but management still wants to have an over-all view of all departments, a "plant profile," as it were. For that purpose, the writer still suggests, as the common denominator of plant activity, the one that he proposed 10 years ago— the quantity of energy consumed, kilowatt-hours. Just as people have characteristic profiles that make them easily recog nizable in silhouette, so plants seem to have maintenance profiles that are unique. This is the conclusion that might be drawn from a study of three plants whose maintenance pro files are presented in Figures 1, 2, and 3. The features chosen to rep resent each plant are the average maintenance labor indexes (manhours per 1000 kw.-hr.) for each manufacturing department in the
plant. These indexes represent the total man-hours of maintenance labor charged to a given departmen tal account during a selected period, divided by the total thousands of kilowatt-hours metered to that de partment during the same period. The index is plotted at the average annual power consumption of the department. Plant A, whose departmental in dexes are plotted in Figure 1, has a few departments with power con sumption beyond the scale of the chart, so these are omitted in this presentation but are included on the larger chart used in actual study of plant performance. In Plant Β (shown in Figure 2) the operations are smaller and the rate of change in maintenance index is greater for the departments using the larger
Figure 2. Maintenance labor index of departments of Plant Β
Figure 1 .
Maintenance l a b o r i n d e x o f departments o f Plant A
Figure 3. Maintenance labor index of departments of Plant C VOL. 48, NO. 11
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NOVEMBER 1956
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Figure 4. Maintenance profile of operating departments of chemical plants
amounts of power. The situation in Plant C (charted in Figure 3) is quite different. Here, the depart ments vary severalfold in their power usage but all have indexes approach ing 30 man-hours per 1000 kw.-hr. Differences, such as those that have been illustrated by these three charts, make it evident that no one curve can be applied to every plant. It would be convenient to have a curve that would predict accurately the optimum maintenance labor index for any proposed new depart ment as soon as its anticipated power consumption could be given. This is not possible now, but perhaps some day a family of curves may be de veloped for this purpose. One pa rameter would be some measure of the corrosive conditions involved in the operation. Another would be batch vs. continuous equipment. Probably another would be the value of the installation.
Figure 5. 46 A
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As a start toward the set of dream curves that have been so hopefully suggested, Figure 4 is presented for testing by the readers of the column. Within the range of power consump tion up to 7 million kw.-hr. per year, most operating departments in a great many plants have reported indexes lying in the shaded zone of this chart. Also small plants, using 1 to 2 million total kw.-hr. have almost always found the over all maintenance labor index of the whole plant lying in this area. When there is considerable use of mineral acids with the corrosive conditions that usually prevail, the index for an entire small plant or for a major department of a large plant may be expected to lie above the curve, whereas plants or depart ments whose maintenance is mainly due to mechanical wear will usually have indexes in the lower portion of the shaded zone. There are, of course, exceptions. Some plants, like Plant C, will find their indexes well above the curve and even above the shaded zone of Figure 4. Others may report lower indexes, especially departments that involve considerable power usage with little or no corrosion, such as those having mainly pumps and compressors. When other kinds of power than electricity are used— e.g., steam pumps or gas engines— the energy must be converted into equivalent kilowatt-hours. Some readers will inquire about the maintenance material that is used by the labor whose index is calculated and charted. The an swer is that maintenance material
Maintenance profile o f chemical plants
INDUSTRIAL AND ENGINEERING CHEMISTRY
profiles have been drawn in the same way for the same departments, plotted points being dollars of ma terial per 1000 kw.-hr. They need to be based on dollars of some standard value, of course, but seem to follow t\he same general pattern as the labor index profiles. The next question may be "Having made such a maintenance profile for the operation departments of a plant, what good is it?" One of the advantages found by those who have been using such a chart is that it compares the various departments against the same background. Some times, one index stands out like the proverbial sore finger and demands attention that it may not have been getting. Sometimes, the calculated value seems so far from the curve as to make one doubt the accuracy of the data. This calls for a check of metering or accounting. In any case, subsequent quarters or years when plotted will show clearly what change has occurred in the maintenance of each department. To make real improvement, of course, the new indexes must be farther below the curve at the new rate of power consumption than they were on the original chart— that is, there is a built-in factor to cover merely increased size of op eration without improved efficiency. As an example of the use of a plant maintenance profile, Figure 5 shows the indexes for two consecu tive years of ten departments in an actual plant. Of these, some show very definite improvement from one year to the next—e.g., Departments 4 and 8—whereas others have made little change in the indexes—e.g., Department 7—or have reduced only in the amount that would have been expected because of increased production—e.g., Department 9. Keeping up such charts from quarter to quarter or from year to year, the individual responsible for main tenance can see the effect of his effort or lack of effort in trying to bring down this important item of plant cost. Having this knowl edge, the supervisor can determine whether to retain or to change his methods and personnel to accom plish planned sayings. With a re liable basis of comparison, he can proceed with confidence.
