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PROCESS PRODUCT MANUFACTURING

AACE INTERNATIONAL • • • •

Quoting from one article [5]: Studies indicate that when a job is placed on overtime there is a sharp drop in productivity during the first week with a substantial recovery which holds for about two weeks and is followed by a fairly steady decline. At the end of seven to nine weeks the productivity on an overtime basis is no greater than the productivity would be on a 40-hour week. In this period there is an increase in work accomplished of about 12 percent. After seven to nine weeks of operation, productivity continues to decline and the work accomplished is less than would have been accomplished on a 40 hour per week schedule. After 18 to 20 weeks there is no gain in total work accomplished . . . .

direct maintenance labor, 35 to 40 percent; direct maintenance labor, supervision, 7 to 8 percent; maintenance materials, 35 to 40 percent; and contract maintenance, 18 to 20 percent.

Then as the project evolves toward a final staffing plan, the factors can be improved and are finally replaced with numbers generated from the staffing table. The back calculation allows one to estimate the number of people required early in the project. When operating at less than 100 percent of capacity, maintenance costs generally increase per unit of production. Such operations can be estimated as follows:

Percent of capacity 100 75 50 0

SUPERVISION AND MAINTENANCE COSTS As discussed above, supervision costs should be established, if at all possible, through a staffing table and tabulation of associated costs. Unfortunately, for most preliminary estimates, particularly those for proposed new processes, this is not possible. In such cases, costs of supervision can be roughly estimated by taking a fixed percentage of direct labor costs based upon company experience. In the absence of prior data on similar operations, a factor of 15 to 20 percent is generally satisfactory. The validity of the latter factor can readily be seen when considering the fact that one front-line supervisor can effectively manage no more than 8 to 10 workers, i.e., supervision workhours of 0.100 to 0.125 per direct labor workhour. With frontline supervision (i.e., foremen or forewomen) at labor rates approximately 50 to 60 percent above general labor rates, 15 to 20 percent supervision factor is evident. Maintenance labor costs, like supervision costs, should be delineated in the labor staffing table if at all possible. However, other than company records of existing similar plants, reliable data on maintenance costs are generally not available, and the staffing table approach is usually not feasible. For this reason, maintenance costs are often estimated as a fixed percentage of depreciable capital investment per year. For complex plants and severe corrosive conditions, this factor can be 10 to 12 percent or higher. For simple plants with relatively mild, noncorrosive conditions, 3 to 5 percent should be adequate. Maintenance costs are a semivariable category, which is generally distributed about 50 percent to labor and 50 percent of materials. For a preliminary estimate, the various factors making up plant maintenance can be back calculated from the total maintenance number using the following approximate percentages:

Maintenance cost as percentage of cost at 100 percent capacity 100 85 75 30

Maintenance generally increases with age of equipment, although most estimates use an average figure to simplify the estimate. This apparent error is offset in the overall estimate by use of average or “straight-line” depreciation, whereas accelerated depreciation is in fact generally used for tax purposes. Figure 10.3 illustrates the validity of using an average maintenance figure over the life of a project. Finally, for major projects, it may be necessary to include costs for additional maintenance supervisors. However, for small plant additions, additional supervision is generally not required.

OPERATING SUPPLIES AND OVERHEAD COSTS Generally, operating (or factory) supplies are a relatively minor cost of operations. Nevertheless, these must be included in the operating or manufacturing cost estimate. Such costs include miscellaneous items, such as lubricating oil, instrument charts, wiping cloths, etc. Lacking more detailed information, they may be estimated as a percentage of payroll. This percentage can range from a few percent to 20 percent or more, depending upon plant complexity and whether or not routine maintenance items and supplies are included or accounted for separately. For example, 6 percent of payroll is probably an adequate allowance for operating supplies in a coal preparation plant, while 20 percent is probably more reasonable for an oil refinery, a more complex operation requiring a cleaner environment. The best source of such costs, however, is always company records of similar past projects.

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