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Available online at aracer.mobi .. May ), aracer.mobi;. This article represents findings of a PDMA task force studying measures of product development success and failure. This investigation sought to identify all. PDF | Corporate failure is of very important interest to economic, financial and corporate managers. Corporate failure could be seen in terms of the inability of a .

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Note the spalled and cracked oxide resembling tree bark caused by expansion of tube during bulging and thermally induced stresses. Brittle black magnetite patches still adhere to the rupture mouth. Elsewhere the oxide has spalled. Such throughwall oxidation is most common in low-pressure boilers, where internal pressures are not sufficient to cause premature failure.

Note the small secondary fissures and the relatively thick rupture edges.

The Queer Art of Failure | Books Gateway | Duke University Press

Often a small longitudinal fissure will be present at the apex of a heavily oxidized bulge Fig. In other cases, the rupture will be larger and have a fish-mouth shape Fig. The rupture will usually have blunt and slightly ragged edges. Similar, but smaller, longitudinally oriented ruptures and fissures may exist nearby. Chain graphitization A somewhat uncommon form of damage, chain graphitization, can also occur after long-term overheating.

The damage begins when iron carbide Figure 2. Note the fish-mouth ruptures. Compare with Fig.

The graphite nodules, if distributed uniformly in the steel, rarely cause failure. However, nodules sometimes chain together, forming planes of cavities filled with graphite. The nodules usually form at microstructure defects, in places where there are chemical impurities, and along stress lines. Stresses generated by internal pressures cause tearing of the metal along chains of nodules, much as a postage stamp is torn from a sheet along the perforated edges.

Chain graphitization is usually found at welds Fig. Less commonly, damage occurs away from welds and forms helical cracks spiraling around tube surfaces Fig. Such failures are sometimes confused with creep damage, but careful microscopic observation will reveal the presence of graphite nodules at or near fracture edges.

Critical Factors Long-term overheating is a chronic, rather than transient, problem. Heavy internal deposition on both hot and cold sides of water- Figure 2.

Note the blunt, rough rupture edge. Such failures occur after long-term mild overheating. These fissures formed along lines of maximum shear stress. Experience has shown that when the ratio of deposits on hot to cold faces of water-cooled tubes exceeds 3, fire-side heat input is substantially higher on the hot face.

When this ratio approaches 10, heat input on the hot face relative to the cold face can be quite excessive. In many cases, the ratio will be below 3 if the heat input is not excessive on the hot face; when chemical water treatment is deficient; or when water chemistry is overwhelmed by contaminants. In most steam-cooled tubes, heat-flux differences between the hot and cold sides are not pronounced.

Resulting deposit patterns are characteristic of contamination rather than excessive heat input. Other sources of overheating include overfiring, incorrect flame pattern, restricted coolant flow, inadequate attemperation, and improper alloy composition. Identification A thick, brittle magnetite layer near the failure indicates long-term overheating. At greatly elevated temperatures short-term overheating reduc- tion in metal strength is such that failure occurs before significant amounts of oxides can develop.

Bulging and plastic deformation are almost always present if the tube is pressurized. Tubes ruptured as a result of long-term overheating usually show bulging and plastic deformation at the failure site. The rupture is almost always longitudinal, with a fish-mouth shape. Rupture edges may be knifelike or thick depending on the time, temperature, and stress levels involved.

Multiple bulges may occur. Water-side deposits will usually be present and will often be hard and stratified. Deposits will usually be "baked" onto the wall and will become hard and brittle.

Deposits tend to show multiple layers of different colors and textures, the innermost layers being hardest and most tenacious.

Visual inspection is adequate for oxidation, spalling, and bulging.

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Thermocouple measurement during service frequently supplies good information. The best way to ascertain that long-term overheating has occurred is by metallographic inspection of a failed tube. Elimination Eliminating long-term overheating requires removal of a chronic system defect. Headers, U-bends, long horizontal runs, and the hottest areas should be inspected for evidence of obstruction, scales, deposits, and other foreign material.

Excess deposits should be removed by chemical or mechanical cleaning and prevented from recurring. Firing procedures, Btu value of fuels, and in-service furnace temperatures near overheated areas should be checked. Attemperation procedures should also be reviewed. If needed, changes in metallurgy, tube shielding, and the judicious use of refractories should be considered. The source of significant deposits must be identified and eliminated.

Each potential cause must be addressed methodically.

The Queer Art of Failure

Cautions It is incorrect to assume that long-term overheating automatically produces significant tube damage. While small microstructural changes may occur in the tube wall, these changes often do little to reduce service life or significantly weaken the tube. However, if overheating continues for a long time, failures will eventually result. Long-term and short-term overheating failures may appear similar.

Frequently, evidence of both long-term and short-term overheating will be present at the same failure. Failures due to long-term overheating are sometimes associated with chemical attack and other significant metal wastage, while chemical attack in short-term overheating is rare. The presence of corrosion does not exclude consideration of long-term overheating.

Because a brief episode of short-term overheating may follow long-term overheating, the sequence of thermal events can be difficult to diagnose without microscopic examinations. The thermally formed oxide fractured and spalled, substantially reducing wall thickness Fig. Thinning was more severe along the side of the section abutting a firebrick wall.

Most metal was lost from external surfaces and was caused by thermal oxidation at temperatures between and O0F and 0 C. Gas channeling and higher temperatures were present along the tube side abutting the firebrick.

These factors accelerated oxidation and spalling processes. In the past, nearby tubes had ruptured as a result of thinning associated with thermal oxidation. No deposits were present on internal surfaces, and attemperation was not used.

Fire-side slagging was not present in this intermittently run boiler. Failures were a chronic problem associated with design and operation. The proximity of failures to the firebrick wall strongly linked the overheating to boiler design.

The laminated nature of the scale indicates multiple episodes of spalling and oxide reformation. Surfaces are covered with an irregular tan slag layer, and the external surface near the through-fissure is checkered. The rupture and fissuring were caused by very long-term exposure of the metal to temperatures between and O0F and C. Evidence suggests that exposure to these temperatures may have been occurring for several years.

Overheating was caused by excessive heat input relative to coolant flow rate. Note the through-fissure at the bulge apex. Nearby tubes showed similar, although less severe, attack. The boiler had been cleaned 3 years prior to failure. Thick layers of hard, tenacious iron oxide cover each bulge, except where spalling has dislodged the oxide. Internal surfaces on the hot side are covered with spongy, porous deposits, which cover a hard, black magnetite layer. The back wall at the same elevation saw many failures.

The boiler had frequent load swings and was operated intermittently. The tube was overheated in a temperature range between and O0F and 62O 0 C at the bulges for a long time. Formation of deposits was due to an imbalance between coolant flow and fire-side heat input. Deposits were caused by exceeding the solubility of inversely soluble species, by evaporative concentration, and by mechanical entrainment of particulate matter.

Because of the frequent load swings and intermittent operation, closer operational monitoring was suggested. Each bulge is ruptured at its apex. The legs are slightly expanded. Fragmented thermally formed oxide surrounds the rupture. Bulges are shown in Figs. Internal surfaces are free of significant deposits, while external surfaces are covered with a tenacious, fragmented oxide layer. Eighteen tubes were similarly affected.

This boiler operates on waste heat from a slab furnace. The wall strength was decreased at elevated temperatures, and tubes were thinned and weakened by thermal oxidation. As a result of these two forms of weakening, the legs bulged and then ruptured. Operating records showed an increase of almost 10O0F C in steam temperature approximately 2 months prior to failure.

This increase was correlated with changes in firing associated with alteration of the slab mill's operation. The rupture occurred immediately downstream of the weld. The tube was bent into an "L" by the burst. The rupture terminates at one end in a pair of thick-walled transverse tears Fig.

A thick, tenacious magnetite layer covers external surfaces, except those near the Figure 2. Note the circumferential weld just below the failure. Elemental copper and spotty deposits are present on internal surfaces.

Failure was caused by prolonged overheating at temperatures above O0F 57O 0 C. The direct rupture was a stress creep rupture. Coolant flow irregularities immediately downstream of a partially intrusive circumferential weld, along with internal deposition, which reduced heat transfer, were contributing factors. Additionally, a switch from oil to coal firing likely changed fire-side heat input. The superheater had a history of boiler-water carryover, and load swings were common. Previous failure had occurred in this region at least 2 years before the current rupture.

Chapter Short-Term Overheating Locations Failures caused by short-term overheating are confined to steam- and water-cooled tubes including downcomers, waterwalls, roofs, screens, superheaters, and reheaters.

Because of their high operating temperatures, superheaters and reheaters are common failure sites. Failures due to short-term overheating almost never occur in economizers, where temperatures are limited. When low water level is the cause, failures will often occur near the top of waterwalls near steam drums. A single ruptured tube in the midst of other apparently unaffected tubes suggests pluggage or other flow-related problems.

Poor attemperation usually will not cause short-term overheating, although long-term overheating may occur. Failures of superheaters and reheaters can also occur during start-up, when steam flow is limited.

General Description Short-term overheating occurs when the tube temperature rises above design limits for a brief period. Depending on temperature, failure may occur in a very short time. Failure is usually caused by a boiler operation upset. Critical Factors An occurrence of short-term overheating is caused by an unusual set of circumstances, such as an upset, occurring during a brief period.

Therefore, pinpointing unusual events immediately preceding failure may be extremely important in identifying the cause of failure.

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