Tubes failure

Lamping, G.A. and AiTowood, R.M. "Manual for Investigation and Correction of Boiler Tube Failures." EPRI Research Project CS-3945, April 1985. 

Stress Corrosion Crocking. Stress corrosion cracking occurs from the combined action of corrosion and stress. The corrosion may be initiated by improper chemical cleaning, high dissolved oxygen levels, pH excursions in the boiler water, the presence of free hydroxide, and high levels of chlorides. Stresses are either residual in the metal or caused by thermal excursions. Rapid startup or shutdown can cause or further aggravate stresses. Tube failures occur near stressed areas such as welds, supports, or cold worked areas. 

The split apparent in Fig. 11.9 was located along the top of the tube facing the steam inlet nozzle. This is one of several tubes in this area having similar longitudinal splits. Leakage of river water from these tubes resulted in feedwater contamination, which turned out to be a major factor in tube failures in the boiler. 

Several sections of 90 10 cupronickel condenser tubing were received. The sections were from tubes that had been plugged because of earlier failures. No tube section contained the original tube failure, however. 

Another eommon eombustion problem eoneerns the erossover tubes. Cross-over tubes are used in ean-annular eombustors to assure eombustion in all ehambers and to equalize pressure. Many times the flow of hot gases through the erossover tubes is inereased due to bloeked fuel nozzles, whieh ean lead to tube failures as shown in Figure 21-17. 

Camp, E. Q., Phillips, Cecil, and Gross, Lewis, Stop Tube Failure in Superheater by Adding Corrosion Inhibitors, National Petroleum News, 38 (10) R-192-99, March 6, 1946. 

Remote contingencies such as heat exchanger tube failure or in case of closure of a CSO valve. 

Overpressure and tube failure may also result from valve closure on the inlet side of a fomace, or from feed pump failure, etc, if the coil remains pressurized by downstream equipment. In these cases, however, overpressure occurs at or below the normal operating pressure (due to overheating at no-flow conditions), and a PR valve cannot provide the necessary protection. 

Tube failure in a water-cooled or steam-heated exchanger used in hydrocarbon service can result in the contamination of the effluent cooling water or the condensate by the process stream, especially if the latter is at a higher pressure. Such effluents must be disposed of in such a maimer that the hydrocarbon contaminations can be safely contained. The following are some safe design practices ... 

It is generally desirable to minimize the diameter of a tubular reactor, because the leak rate in case of a tube failure is proportional to its cross-sectional area. For exothermic reactions, heat transfer will also be more efficient with a smaller tubular reactor. However, these advantages must be balanced against the higher pressure drop due to flow through smaller reactor tubes. 

At other times the burning of waste products in furnaces to save fuel and reduce pollution has caused corrosion and tube failure. 

Heavy ends can further degrade into carbon deposits on the insides of furnace tubes and lead to tube failure. Sometimes the tube blocks completely and prevents a serious spillage, but at other times spillages have produced costly and spectacular fu-es, as in the incident descried in Section 10.7.2 (though that one was not due to accumulation of heavy ends). To prevent tube failures, keep the concentration of heavy ends below 5% and follow the recommendations on furnace operation in Section 10.7.2 [27]. 

Finally, the HRA analyst would calculate the expected frequency of condenser ruptures as a result of improper isolation. The frequency of condenser tube failures is 0.33 per year (1 every 3 years), and the calculated probability of improper isolation is 0.05. Multiplying these two numbers shows the expected frequency of improper isolation of a failed condenser is 0.017 per year, or about once every 60 years. The manager can use this number to help compare the costs and benefits of improvements proposed as a result of the HRA or other studies. 

Figure 4-438. SSC failure, drillpipe (a) tubing failure (b) coupling (c). (From Refs. [184,218].)...

Some nucleate boiling still takes place in high heat-flux designs. It is critical that this process continues in order to remove heat, thus cooling the heated metal and consequently reducing the risk of tube failure through overheating. 

High heat-transfer rates and sufficient cooling via internal, two-phase flow and circulation are consequently necessary to ensure that modem boilers operate below their particular critical heat-flux (CHF) conditions. This minimizes poor thermal efficiency performance, risks of thermal metal fatigue, and probable tube failures from excessive temperatures. 

A specific waterside problem that affects many economizers is normally one of oxygen corrosion. This affects the internal, carbon steel tube header, first-pass tubes, and primary tube bend areas because these areas first receive cold FW. This form of corrosion commonly results in red oxides, economizer pitting and tuberculation, and potentially premature tube failure. 

In fossil fuel-fired WT boiler plants, boiler tube failure is perhaps the leading cause of forced outages, so identifying the chain of cause and effect is particularly important. 

For example, if tube failure occurs due to caustic gouging corrosion, the root causes are most likely related to the effects resulting from the availability offree sodium hydroxide in the BW, coupled with the development of localized caustic concentration. Control generally requires a twofold approach to remove the causes of this particular problem ... 

A further example is the effect of tube failure resulting from longterm overheating. Here it is likely that the principle contributing causes are a combination of deposit formation and stresses resulting from mechanical operation of the boiler in excess of design limits. To minimize or eliminate the risks of deposits and the subsequent effects they produce within the boiler, control again requires a twofold approach ... 

Where feed lines have short pipe runs, where hot wells or FW tanks are of small volume, or when FW is too cold, there often is insufficient time for full DO scavenging to take place, even when using catalyzed scavengers. The inevitable result of this lack of contact time is the formation of oxygen-induced corrosion products, which by various secondary mechanisms may settle out to form permanent deposits within the boiler system. These deposits may develop in several forms (e.g., where DO removal is particularly poor, they often appear as reddish tubercles of hematite covering sites where pitting corrosion is active). Active pitting corrosion combined with the presence of waterside deposits ultimately may lead to tube failure in a boiler or other item of system equipment and result in a system shutdown. 

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