Page 344 - Improving Machinery Reliability

310 Improving Machinery Reliability
Summary

Life cycle costs include cradle-to-grave costs. When failure costs are included, the
quantity of manpower required can be engineered to avoid the use of antique rules of
thumb about how maintenance budgets are established.
LCC techniques provide methods to consider trade-off ideas with visualization
techniques as described above, which are helpful for engineers. Likewise, LCC
analysis provides NPV techniques of importance for financial organizations, and
LCC details give both groups common ground for communication. With LCC
details, the financial organizations can complete DCF calculations.
Some chemical plants have cost values and failure data for ANSI pumps that are
different from that shown above. As examples, coupling costs are around US$lOO
and the associated logistics costs are perhaps US$75 for couplings with a MTBF of
-3 years, seal life is -1.5 years, shaft life is -4.5 years, impeller life is -3.5 years,
pump housing life is -6 years, and the cost of bearings is -US$140. Of course, using
these “not so commendable” values for a chemical plant will result in higher mainte-
nance costs and greater maintenance expenditures.
Each of the examples described above can be made more accurate by using more
complicated models. For one example, in the Monte Carlo model, the time for
repairs can be changed from a fixed interval to a statistical interval by simply using a
log-normal distribution. This will provide a more realistic idea of the time expended
and costs incurred. Spare part quantities can also be calculated.
Good alternatives for LCC require creative ideas. It is the role of the engineer to
suggest and recommend cost effective alternatives. Much lower LCC’s are obtained
when creative efforts are employed in the design area. Making changes downstream
in the operating plants has smaller chances for improvements because they come too
late in the improvement cycle. Design engineers are the most important link in devis-
ing cost-effective plants, and naturally, the burden of LCC falls on their shoulders,
but design engineers can’t perform an effective analysis unless they have reasonable
failure data from operations. Therefore, there is a need for plant and industry data-
bases of failure characteristics. Remember, to obtain good failure data, both failure
and success data must be identified. If only the failure information is considered, the
failure database will be too pessimistic; no one will believe it and few people will
use overly pessimistic data.

References

1. Goble, W. M. and Paul, B. O., “Life Cycle Cost Estimating,” Chemical Process-
ing, June 1995.
2. Paul, Brayton O., “Life Cycle Costing,” Chemical Engineering, December 1994.
3. Roscoe, Edwin S., “Project Economy,” Richard D. Irwin, Inc., Homewood, IL,
1960, pp. 18-20.
4. Bloch, H. P. and Geitner, F. K., Machinery Failure Analysis and Troubleshoot-
ing, Gulf Publishing, Houston, Texas, Second Edition, 1994, pp. 684-686.