Materials selection life-cycle costing

Section 8 outlines the following criteria for selection of measurement devices measurement span, performance, reliability, materials of construction, prior use, potential for releasing process materials to the environment, electrical classification, physical access, invasive or noninvasive, and life-cycle cost. 

Selection of fluoropolymers is an integral part of the overall material selection process. This implies that all the available materials such metals, ceramics, and plastics are considered candidates for an application. The end user then considers these materials against established criteria such as required life, mean time between inspection (MTBI), ease of fabrication, frequency of inspection, extent of maintenance and, of course, capital cost. More often than not it is the initial capital cost, rather than the life cycle cost of equipment, that affects the decision made during the material selection step. However, the most important piece of data is the corrosion resistance of a material in the medium under consideration over the life of the equipment. This information is available in a different format for plastics than for metals. A comparison is appropriate. 

The available data from the services indicate that corrosion in weapons systems is the primary cost driver in life-cycle costs (46). Quantifying corrosion is difficult as neither the mechanisms nor the methodologies exist to quantify accurately. Analysis of field data reveals instances where questionable materials selection early in the acquisition process has led to enormous unanticipated increases in life-cycle costs because of corrosion (J Argento, US Army TACOM-ARDEC, Picatinny Arsenal, NJ, Personal Communication, 1999). In view of force reduction and a reduction in budgets, consideration must be given to the selection of advanced materials, processes, and designs that will require less manpower for corrosion inspection and maintenance. 

The selection of materials for refinery construction depends on the type of refinery, the type of crude oil to be refined, and the expected service life for each vessel. As with all materials selection, the life-cycle cost is of importance in addition to the purchase price. Table 4.45 lists some of the common alloys and their material costs relative to carbon steel. The costs listed are relative to carbon steel, which is assigned a value of 1.0. 

Finally, another more forward-looking trend relates to the more sophisticated material design and selection strategies used by the industry. These anal)di-cal strategies take into account the contributions of all the components in a compound and how they affect total raw ingredient costs, physical properties, processing rates and yields, and environmental footprint (life cycle costs) [2-34, 2-35, 2-36, 2-37, 2-38]. 

Sometimes, cost may be the only criterion in material selection. This is particularly true when it is known that the candidate materials are fairly evenly poised on all other criteria. This calls for a method that analyzes the costs associated with each candidate material. One such quantitative method is the Life-Cycle Cost Analysis (LCCA). LCCA helps ns get the overall picture of the costs associated with each material over its entire lifetime. It attempts to identify all CAPEX and OPEX heads involved in all stages enconntered over the lifetime of a particnlar asset or facility and therefore operates over a longer-term horizon. LCCA uses the discounted cash flow technique to reduce the future costs to present-day valnes. This makes LCCA an even fairer means to rationally compare the candidate materials. 

With all the above data at hand, we can now compute the life-cycle cost of each candidate material. The candidate material with the lowest overall life-cycle cost is selected. The life-cycle cost of a candidate material for a particular asset/facility is given by the following expression ... 

The most commonly used criteria for selection of materials for use in waterfront structures are least first-cost and prior experience. This is unfortunate as least first-cost construction usually does not result in least life-cycle costs when maintenance, repair, and facility life are considered. Prior experience also may not be reliable, particularly if the experience is from a diff erent geographical location or from a structure with a different set of service conditions. In addition, differences in materials, changes in the local environment, and differences in corrosion protection and maintenance may Umit the applicability of past experience to new construction. 

The procurement costs of the materials for initial construction are often the easiest cost data to obtain, and too often are used inappropriately as the sole criteria for materials selection. Materials costs based on a price per pound or other quantity usually are very misleading when used in lieu of total system costs to evaluate alternatives. Usually, in order to ensure that actual life-cycle costs are minimized, other cost factors such tis maintenance and repair must be considered. 

Figure 11.9 Generic process for selecting materials to improve corrosion resistance—a roadmap to life-cycle cost reduction.

There are no simple rules of thumb in defining the cost of reinforced plastic components. Their successful use has resulted from proper design, utilizing the benefits these materials offer, process selection, tooling cost advantages that fit the production needs, and consideration of life cycle economics. Each existing application illustrates the cost-performance advantage of reinforced plastic over the traditional material that is displaced. 

Selecting an elastomer for an application requires consideration (like for plastics and foams) of many factors, including the mechanical and physical service requirements, the product s life cycle, the material s processability, and its cost (see Figs. 6-24 and 6-25 and Tables 6-12 and 6-13). A wide range of properties is available, based on the many different compounds that can be produced. 

All materials have their limitations and the solution to high-temperature problems is often a compromise between careful materials selection when the cause of a problem is known, process control in order to impose a safe limit for temperature or gas composition, for example, and better design specifications to recognize mechanical constraints at elevated temperature or resulting from thermal cycling. The ultimate choice will be a compromise based on what is available and how much it costs. In some cases it is rational to accept a short life expectancy with a high reliability factor where the component is replaced on a planned time schedule [1]. 

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