Choosing materials

In choosing materials of construction, we have to compromise between various factors. Kirby [16] uses the acronym SHAMROCK to summarize and remember them. 

S = Safety what are the consequences of failure If they are serious, a more resistant material than usual may be justified. For example, on a plant where leaking v. ater would react violently with process materials, the water lines were made from a grade of steel resistant to stress corrosion cracking (from the chloride in the cooling water) as well as rust. 

H = History if a plant has used material successfully for many years, and the staff members know its strengths and limitations, how to weld it, etc., hesitate before making a change. For example, a fiberglass-reinforced plastic had given excellent service for many years when another composite from the same company, with the same name but a different number, was used instead, it failed overnight. 

A = Availability before a salesman sells you the latest wonder-working material, ask how easy it will be to get replacement supplies in a hurry. 

R = Reparability a plant bought some vessels wdth a new type of plastic lining instead of the one they had used for many years. The new material had better temperature resistance than the old, but when it did need repair, the patches would not stick. In time the problems were overcome, but reparability should have been considered before the change was made. 

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Electron donor molecules are oxidized in solution easily. Eor example, for TTE is 0.33V vs SCE in acetonitrile. Similarly, electron acceptors such as TCNQ are reduced easily. TCNQ exhibits a reduction wave at — 0.06V vs SCE in acetonitrile. The redox potentials can be adjusted by derivatizing the donor and acceptor molecules, and this tuning of HOMO and LUMO levels can be used to tailor charge-transfer and conductivity properties of the material. Knowledge of HOMO and LUMO levels can also be used to choose materials for efficient charge injection from metallic electrodes. 

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. 

From what we have said so far it should be obvious that the first requirement that we should look for in choosing materials that are resistant to creep is that they should have high melting (or softening) temperatures. If the material can then be used at less than 0.3 of its melting temperature creep will not be a problem. If it has to be used above this temperature, various alloying procedures can be used to increase creep resistance. To... 

Electrical isolation Heat radiation Cooling coils Recent incidents Vacuum relief valves Accidents at sea Fires Problem sources Emulsion breaking Chimney effects Interlock failure Choosing materials. 

It is the intention of the present author to discuss the aforementioned developments and not to give a complete review of all work concerning thiophenes which has appeared during the last 7-8 years. Because of the subjectivity inherent in choosing material for such a survey, it is possible that important contributions will unintentionally be overlooked for which the author would like to apologize. 

Cowan, C. T, Choosing Materials of Construction for Plate Heat Exchangers, Chem. ng ,June 9 (1975) p. 100. 

Choose materials with a high resistance to erosion and wear. Keep erosion corrosion in mind while designing the equipment. 

Establish corrosivity levels of process streams and choose materials and protection systems appropriately. 

A direct relationship is found between the hardness of a material and its ability to withstand mechanical wear, and as a rule the harder the material, the greater is its wear resistance. A related factor is that harder materials usually take a better polish and, as a result, the wear due to surface roughness mentioned above can be minimized. Choosing material combinations with a low coefficient of friction is also an important factor in reducing mechanical wear. 

Consider the environmental implications when choosing materials for hard landscaping, soil improving, and so forth. 

Experimental studies, such as those to be discussed in Section 10.2.5, have been key to demonstrating the validity of these flow models. Many of the more sophisticated models [3,6,8] have shown to follow the resin flow process accurately. These models are extremely useful for choosing material systems and for determining optimum pressure cycles and bagging procedures. 

Viscoelastic creep is important when considering long-term structural design. Given loading and temperature conditions, designers can choose materials that best suit component lifetimes. 

Table 9 presents information on the corrosion resistance of some common metals, nonmetals, and gasket materials. Table 10 presents similar information for various types of plastics. These tables can be used as an aid in choosing materials of construction, but no single table can take into account all the factors that can affect corrosion. Temperature level, concentration of the corrosive agent, presence of impurities, physical methods of operation, and slight alterations in the composition of the constructional material can affect the degree of corrosion resistance. The final selection of a material of construction,... 

One important aspect of the writing process is the student s ability to incorporate effective supportive material into a given piece of writing. The selection of quotations, examples, anecdotes, and other forms of detail show the writer s expertise in choosing material that enriches a given writing task. The choices a writer makes and the explanations attached to those choices, provide insight into the writer s ability to synthesize ideas. ... 

A random sample means that every item in a population has an equal chance of being chosen. Simply choosing materials by eye does not satisfy this criterion. Each of the dose units should be assigned a number, starting at 1 and ending with the last number (i.e. the number of items in the sample). The materials to be chosen should then be picked by using either a computerized random-number generator or random-number tables. Whichever method is used, it should be documented. 

Galvanic compatibility is an important consideration for choosing materials for hydrogen systems. It is based on the relative position of metals in what is called the electrochemical series, galvanic series or electromotive series. 

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