Fabrication Tolerances

In order to arrive at the optimum material for a given use with some degree of efficiency and reliability, a systematic planned approach (obviously) has to be used that identifies and lists the product requirements with all the steps required to produce and deliver the products a few examples of plans have been reviewed in this book. The requirements include factors such as aesthetics, tolerances, fabricating... 

Mok, P.Y., Kwong, C.K. and Wong, W.K., 2007. Optimisation of fault-tolerant fabric-cutting schedules using genetic algorithms and fuzzy set theory, European Journal of Operational Research, 177(3), 1876-1893. 

Fabric fusion tape bonds permanently to fabric with a steam iron. Perfect for fabrics that are thick or difficult to sew, Fabric Fusion works on most heat-tolerant fabrics, including cotton and blends, polyester, fleece, suede, canvas, denim, and nylon. It can be machine washed or dry cleaned. 

Tank Shell. Another example of where thickness is set by minimums for fabricabihty but not for strength is in small-diameter tanks. For example, a water storage tank built using a steel of an allowable stress of 20,000 psi (138 mPa), 9 ft (3 m) in diameter by 21-ft (7-m) high, requires a shell thickness to resist hoop stress of only 0.023-in. (0.58-mm) thick. However, if built to API Standard 650, the shell would be fabricated at least 0.1875-in. (4.76-mm) thick. The code requires this thickness so that when fabrication, welding, and tolerances are considered, a tank of acceptable quaUty and appearance meeting the requirements of most services in most locations is provided. 

Most carrier-active compounds ate based on aromatic chemicals with characteristic odor. An exception is the phthalate esters, which are often preferred when ambient odor is objectionable or residual odor on the fabric caimot be tolerated. The toxicity of carrier-active compounds and of their ultimate compositions varies with the chemical or chemicals involved. The environment surrounding the dyeing equipment where carriers are used should always be weU-ventilated, and operators should wear protective clothing (eg, mbber gloves, aprons, and safety glasses or face shields, and possibly an appropriate respirator). Specific handling information can be obtained from the suppHer or manufacturer. 

Fabrication tolerances are covered in this subsection. The tolerances permitted for shells for external pressure are much closer than those for internal pressure because the stability of the structure is dependent on the symmetry. Other paragraphs cover repair of defects during fabrication, material identification, heat treatment, and impact testing. 

Rules for the welded fabrication of pressure vessels cover welding processes, manufacturer s record keeping on welding procedures, welder qualification, cleaning, fit-up alignment tolerances, and repair of weld defects. Procedures for postweld heat treatment are detailed. Checking the procedures and welders and radiographic and ultrasonic examination of welded joints are covered. 

Among the topics of the TEMA Standards are nomenclature, fabrication tolerances, inspeclion, guarantees, tubes, shells, baffles and support plates, floating heads, gaskets, tube sheets, channels, nozzles, end flanges and bolting, materi specifications, and fouhng resistances. 

Cleanable Granular-Bed Filters The principal objective in the development of cleanable granular-bed filters is to produce a device that can operate at temperatures above the range that can be tolerated with fabric filters. In some of the devices, the granules are circulated continuously through the unit, then are cleaned of the collected dust and returned to the filter bed. In others, the granular bed remains in place but is periodically taken out of service and cleaned by some means, such as backflushing with air. 

Fabric filters are limited by physical size and bag-life considerations. Some sacrifices in efficiency might be tolerated if higher air-cloth ratios could be achieved without reducing bag life (improved pulse-jet systems). Improvements in fabric filtration may also be possible by enhancing electrostatic effects that may contribute to rapid formation of a filter cake after cleaning. 

It must be pointed out that a Code stamp does not necessarily mean that the vessel is fabricated in accordance with critical nozzle dimensions or internal devices as required by the process. The Code inspector is only interested in those aspects that relate to the pressure-handling integrity of the vessel. The owner must do his own inspection to assure that nozzle locations are within tolerance, vessel internals are installed as designed, coatings are applied properly, etc. 

Closest standard plate thickness to fabricate vessel is 0.50 in. with - 0.01 in. and + 0.02 in. tolerances at mill. 

The bottom of the downcomer must be sealed below the operating liquid level on the tray. Due to tolerance in fabrication and tray level, it is customary to set the downcomer seal referenced to the weir height on the outlet side of the tray. Recommended seals, based on no inlet weir adjacent to the downcomer, and referenced as mentioned are given in Table 8-19. 

The actual number of tubes to be installed in the unit. Manufacturing tolerances may require elimination of some tubes that preliminary design layouts and tables indicated might be installed in the unit. Figures 10-25A-Kand Table 10-9 have considered known fabrication tolerances. Sometimes extra tie rods for baffles must be added, or in some cases, eliminated. The outer tube circle limit for each exchanger is determined by the type of shell to be used. That is, (1) if commercial pipe, greater out-of-round tolerances might be required or (2) if formed on shop rolls, the out-of-round tolerance will be known, but not necessarily the same for each diameter shell. 

Failure of the metal can be the most important effect of a corrosive water, but other eff ts may arise from small concentrations of metallic ion produced by corrosion. A natural water passed through a lead pipe may contain a toxic concentration of that metal with copper there is a greater tolerance from the toxicity point of view but staining of fabrics and sanitary fittings may be objectionable. With iron, similarly, discoloration of the water may be unpleasant and may cause damage to materials being processed. 

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