Material Selection Procedure

It attempts only to indicate the system of parallel evaluation of corrosivity of different petroleum environments, the effect of process parameters, estimation of corrosion rates, determination of corrosion allowances for a given life, and a few guidelines in selection of materials in conjunction with corrosion control measures and economic principals. 

However, the knowledge of materials is so vast and far-reaching that close cooperation of the designer with metallurgists, material engineers, corrosion engineers, and other materials specialists is stressed. 

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However, in general these fabrication and performance advantages are common to all plastics and so a decision has to be made in regard to which plastic would be best for a particular application. Rather than compare the basic raw material costs it is better to use a cost index on the basis of the cost to achieve a certain performance. Consider again the material selection procedures illustrated in Section 1.4.1 in relation to strength and stiffness. 

The materials selection procedure for new or replacement plant is crucial to the safe and economic operation of that plant. There is no one correct way to select the appropriate construction material, since for small plant handling toxic or inflammable products the integrity of the plant is high in priority, whereas large plant producing bulk material in a competitive market is more likely to be made of cheaper constructional materials, and the occasional leak or failure may have fewer safety implications. 

The materials selection procedure should take into account the commissioning requirements of all parts of the plant in order not to include any materials that cannot safely be degreased or chemically cleaned. 

During the materials selection procedure isothermal corrosion testing may indicate the suitability of a material for handling a corrosive process fluid. In many cases where heat transfer is involved the metal wall temperature experienced in service is higher than the bulk process fluid temperature. This, and the actual heat transfer through the material, must be taken into account since both factors can increase corrosion rates significantly. 

The corrosion conditions can be different at the fluid line from the bulk condition. Aqueous liquids have a concave meniscus, which creates a thin film of liquid on the vessel wall immediately above the liquid line. Some corrosion processes, particularly the diffusion of dissolved gases, are more rapid in these conditions. Additionally, the concentration of dissolved gases is highest near the liquid surface, especially when agitation is poor. Locally high corrosion rates can therefore occur at the liquid line, leading to thinning in a line around the vessel. This effect is reduced if the liquid level in the vessel varies with time. Any corrosion tests undertaken as part of the materials selection procedure should take this effect into account. 

The four major thermoplastics—polyethylene, polypropylene, PVC, and polystyrene—together represent over 85% by volume of world plastics consumption. Because of their lower prices, these commodity materials dominate the market, and in any materials selection procedure there are good economic reasons for considering them first before turning to the more expensive engineering plastics. 

This chapter explores the influences leading to the use of particular plastics in particular applications. The first section identifies the characteristics which are decisive in motor industry applications, the second summarizes the strengths and weaknesses of the different polymer groups, the third offers suggestions for a materials selection procedure, and the fourth looks at the requirements of the different application areas. This is intended to provide a logical background to the detailed application surveys in Chapters 4, 5, 6 and 7. 

The most significant steps in the new material selection procedure described by Cebon and Ashby [16] is the determination of performance indices of properties which maximize material properties. The design of a mechanical component is specified by the following three factors ... 

The material selection procedure for design described above is a method of rating materials for a particular design application that is based on a single design variable such as varying the depth h or diameter d in Table 6.5. This allows use of the design variable in a linear form. 

The American Society of Mechanical Engineers (ASME) United Engineering Center 345 East 47th Street New York, NY 10017 The ASME Boiler and Pressure Vessel Code, under the cognisance of the ASME PoHcy Board, Codes, and Standards, considers the interdependence of design procedures, material selection, fabrication procedures, inspection, and test methods that affect the safety of boilers, pressure vessels, and nuclear-plant components, whose failures could endanger the operators or the pubHc (see Nuclearreactors). It does not cover other aspects of these topics that affect operation, maintenance, or nonha2ardous deterioration. 

These standards specify design, construction, and testing details such as material selection, shop inspection and tests, drawings and other uses required, clearances, construction procedures, and so on. 

In certain cases, it is necessary to choose materials for equipment to be used in a process developed in the laboratory and not yet in operation on a plant scale. Under such circumstances, it is obviously impossible to make plant tests. A good procedure in such cases is to construct a pilot plant, using either the cheapest materials available or some other materials selected on the basis of past experience or of laboratory tests. While the pilot plant is being operated to check on the process itself, specimens can be exposed in the operating equipment as a guide to the choice of materials for the large-scale plant or as a means of confirming the suitability of the materials chosen for the pilot plant. 

In this section a selection procedure will be developed for injection moulding, since this process is used for the widest range of materials. The choice available for other processes such as, for example, compression moulding, filament winding and vacuum forming, is much more restricted. The approach described will be less mechanistic than the systems described in the two previous sections, requiring the prospective user to be aware of the properties of the various materials available. Because the approach is somewhat different, it would be instructive to run it parallel to the above processes and compare the results. 

Another serious problem in heat exchangers is corrosion. Severe corrosion can and does occur in tubing and very often with common fluids such as water. Proper material selection based on a full analysis of the operating fluids, velocities and temperatures is mandatory. Very often, heavier gauge tubing is specified to offset the effects of corrosion, but this is only a partial solution. This should be followed by proper start-up, operating and shut-down procedures. 

In the ideal case the target level procedure for industrial ventilation can be compared with materials selection. Somebody who wants steel AISl 316, for instance, just selects and buys it by specifying this standard. The person will obtain steel with the desired properties, because the steel is made according to the producer s quality requirements and the producer guarantees its quality. 

The fundamental objective in this section is to describe the factors and procedures to select the right material for a specific structural application. The right stuff for a material, as for a fighter pilot or an astronaut, is a complex combination of characteristics. To select the proper material requires being able to characterize and evaluate various composite materials (or metalsl) and to compare their attractive characteristics with the behavioral features required for a particular structure. Finally, a materials selection example of a space truss design problem will be addressed. 

The technology of manufacturing the same basic type or grade of plastics (as with steel and other materials) by different suppliers may not provide the same results. In fact a supplier furnishing their material under an initial batch number could differ when the next batch is delivered and in turn could effect the performance of your product. Taking into account manufacturing tolerances of the plastic, plus variables of equipment and procedure, it becomes apparent that checking several types of materials from the same or from different sources is an important part of material selection and in turn their use. 

Plastics are families of materials each with their own special advantages. The major consideration for a designer is to analyze what is required as regards to performances and develop a logical selection procedure from what is available. 

Selection of processing equipment and materials Production procedure investigation Development and organization of processing facilities Preliminary processing Preliminary drying... 

Each chapter has its unique style of presentation that essentially comprises of the following vital features, namely brief introduction, theory with necessary details and relevant reactions, instrumentation, assay methods—with typical appropriate examples invariably selected from the Official Compendia including brief theoretical treatment of individual pharmaceutical substance and dosage form, materials required, procedures, calculations wherever applicable, cognate assays and lastly citation of relevant literature under Recommended Readings . 

This will not be covered in depth, but some types of material may occasionally be presented for analysis, so a few selected procedures and references will be given. 

Pumps and compressors are designed per technical specifications and standards developed over years of operating and maintenance experience. Table lO-lO lists some of these standards for pumps and compressors and for related equipment such as lubrication systems and gearboxes which, if not properly specified, could lead to many operational and maintenance problems with the pumps and compressors. These standards specify design, construction, maintenance, and testing details such as terminology, material selection, shop inspection and tests, drawings, clearances, construction procedures, and so on. 

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