Construction materials selection

Construction material selection (minimum use of natural resources), design for disassembly, efficient and durable envelope, durability... 

The headframe construction material selection is an important consideration, and there are two primary choices concrete or steel. The concrete headframe was selected based on a concentrated construction schedule, a more robust structural system, a reduced headframe complex footprint and structural durabihty. These are some of the key reasons laid out in full in the Headframe Design Selection—Steel vs. Concrete paper (Butler and Schneyderberg, 1981). Once selecting concrete as a material, the optimal choice for a construction method was shpforming. 

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. 

Selection of Equipment The principal factors which influence mixing-equipment choice are (1) the process requirements, (2) the flow properties of the process fluids, (3) equipment costs, and (4) construction materials required. 

The primaiy purpose of materials selection is to provide the optimum equipment for a process application in terms of materials of construction, design, and corrosion-control measures. Optimum here means that which comprises the best combination of cost, life, safety, and reliability. 

The selection of materials to be used in design dictates a basic understanding of the behavior of materials and the principles that govern such behavior. If proper design of suitable materials of construction is incorporated, the eqiiipment should deteriorate at a uniform and anticipated gradual rate, which will allow scheduled maintenance or replacement at regular inteivals. If localized forms of corrosion are characteristic of the combination of materials and environment, the materials engineer should still be able to predict the probable life of equipment, or devise an appropriate inspection schedule to preclude unexpected failures. The concepts of predictive, or at least preventive, maintenance are minimum requirements to proper materials selection. This approach to maintenance is certainly intended to minimize the possibility of unscheduled production shutdowns because of corrosion failures, with their attendant possible financial losses, hazard to personnel and equipment, and resultant environmental pollution. 

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. 

Select appropriate constructional materials (flexible hoses)... 

Corrosion resistance it is critical to select construction materials such that neither the gas and solute nor the scrubbing liquid will corrode the packing materials. Ceramic and plastic materials, are commonly selected for this reason. 

The process designer must also consider the corrosive nature of the fluids involved when selecting construction materials for the various process and utility service lines. Some designers attach these materials designations to the line designation on the flowsheets, while others identify them on the Line Summary Table (Figure 1-24D). Some typical pipe materials designations are ... 

The pump specified identifies the design data, key portions of the construction materials and driver data as required informadon for the pump manufacturer. If the pump is to be inquiried to several manufacturers this is all that is necessary. The individual manufacturers will identity their particular pump selection and details of construction materials and driver data. From this informadon a pump can be selected with performance, materials of construction, and driver requirements specified. 

Typical Materials of Construction for Selected Centrifugal Applications... 

Selection of construction materials for rotors and casings is more limited than for centrifugal compressors. 

The screw configuration is normally either a ribbon-type helical screw or a series of paddles mounted on a common shaft. Materials of construction are selected based on the specific application and materials to be mixed. Typically, the screws are either steel or stainless steel, but other materials are available. 

The construction materials should be selected to withstand the operating conditions and the condition of the pollutants. Galvanized sheet steel and black mild steel are the most common for general work. Corrosion or heat applications will have ducting constructed in stainless steel or plastic. 

The principal economic implications of corrosion of a plant are the initial cost of construction, the cost of maintenance and replacement, and the loss of production through unplanned shutdowns. The initial cost of the plant is influenced by material selection, and a choice of material that is more corrosion resistant than is necessary for the safe operation of the plant over its design life is a very expensive error. This cost involves initial outlay of money, and plants have been built which could never be profitable because of the inappropriate materials selection. 

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. 

Steel is the most common constructional material, and is used wherever corrosion rates are acceptable and product contamination by iron pick-up is not important. For processes at low or high pH, where iron pick-up must be avoided or where corrosive species such as dissolved gases are present, stainless steels are often employed. Stainless steels suffer various forms of corrosion, as described in Section 53.5.2. As the corrosivity of the environment increases, the more alloyed grades of stainless steel can be selected. At temperatures in excess of 60°C, in the presence of chloride ions, stress corrosion cracking presents the most serious threat to austenitic stainless steels. Duplex stainless steels, ferritic stainless steels and nickel alloys are very resistant to this form of attack. For more corrosive environments, titanium and ultimately nickel-molybdenum alloys are used. 

If changes have been made to the process (e.g. if incoming water quality cannot be maintained or other uncertainties arise concerning the corrosion behavior of the construction materials) it is possible to incorporate coupons or probes of the material into the plant and monitor their corrosion behavior. This approach may be used to assist in the materials selection process for a replacement plant. Small coupons (typically, 25 x 50 mm) of any material may be suspended in the process stream and removed at intervals for weight loss determination and visual inspection for localized corrosion. Electrical resistance probes comprise short strands for the appropriate material electrically isolated from the item of plant. An electrical connection from each end of the probe is fed out of the plant to a control box. The box senses the electrical resistance of the probe. The probe s resistance rises as its cross-sectional area is lost through corrosion. 

The interaction of a metal or alloy (or a non-metallic material) with its environment is clearly of vital importance in the performance of materials of construction, and the fact that the present work is largely confined to a detailed consideration of such interactions could create the impression that this was the sole factor of importance in materials selection. This, of course, is not the case although it is probably true to say that this factor is the one that is the most neglected by the design engineer. 

In selection of construction materials, the tendency is therefore to play safe stainless steel is on this account generally specified and is always used in cases of doubt. The suitability of other materials should not, however, be overlooked . Prior to the general advent of stainless steel in 1924, tinned copper was much in vogue. Today titanium, although expensive, represents a possible alternative". ... 

Atomic power production The necessity for avoiding contamination of operative liquids, together with other requirements which must be met in selecting constructional materials in this highly specialised field has resulted in the choice of austenitic steels for applications in heat exchangers, pressure vessels, pipelines and fuel processing. 

The continuous development of new materials has resulted in changing attitudes towards materials selection for corrosion control, and the range of materials now available can be gauged from the Materials Selector Review", which becomes considerably thicker each time it is updated. Plastics are replacing metals for a variety of applications and a recent application is the use of g.r.p. in place of metals for the construction of hulls of hovercrafts the corrosive action of the high velocity spray of sea-water is such that very few metals are capable of withstanding it and the use of g.r.p. represents the... 

GP 1[ [R 1[ A change from aluminum to platinum as construction material results in reduced micro-reactor performance concerning oxidation of ammonia, decreasing N2O selectivity by 20% [28]. This is explained by the lower thermal conductivity of platinum, which causes larger temperature differences (hot spots) within the micro channels, i.e. at the catalyst site, e.g. due to insufficient heat removal from the channels or also by non-uniform temperature spread of the furnace heating. 

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