Failures of Boilersand Related EquipmentRevised by David N. French, David N. French, Inc., Metallurgists

FAILURES IN BOILERS and other equipment in stationary and marine power plants that use steam as the working fluid are discussed in this article. The discussion is mainly concerned with failures in Rankine-cycle systems that use fossil fuels or a nuclear reactor as the primary heat source, although many of the principles that apply to Rankine-cycle systems also apply to systems using other steam cycles or to systems using working fluids other than steam.It is important to learn as much as possible from each failure. In any metallurgical evaluation, the general aim is to understand the root cause of the failure in terms of both the material and the boiler operation. To that end, estimates can sometimes be made of metal temperatures and implied boiler conditions at the time of failure. This information may then be useful in the prevention of future failures.Procedures forFailure AnalysisProcedures for analysis of failures in steam power plants do not differ significantly from procedures for failure analysis in general or for analysis of specific types of failure. These procedures are presented in other articles in this Volume, particularly those dealing with basic mechanisms of failure; failures of specific product forms, such as castings, forgings, weldments, and pressure vessels; and elevated- temperature failures. Consequently, this article will discuss the main types of failure that occur in steam-power-plant equipment, with major emphasis on the distinctive features of each type that enable the failure analyst to determine cause and to suggest corrective action.Service Records. Most power-plant operators maintain relatively complete records. Records of operating conditions and preventive maintenance for a component that has failed, and for the system as a whole, are relatively good sources of background information. These records can provide valuable information, such as operating temperature and pressure, normal power output, fluctuations in steam demand, composition of fuel, amount of excess combustion air, type and amount of water-conditioningchemicals added, type and amount of contaminants in condensate and make-up feedwater, frequency and methods of cleaning fire-side and water-side surfaces of steam generators, materials specified as to alloy requirements and dimensions, frequency and location of any previous failures, length of service, and any unusual operating history.Precautions in On-Site Examination. Power plants are vital to most industries, and power-plant downtime has an adverse effect on the entire operation. Thus, the individual conducting the on-site examination should have experience in making preliminary determinations of cause and recommending corrective action on the spot.Certain deductions, such as the exact cause or mechanism of failure, frequently cannot be made without laboratory examination. However, some determinations—for example, which of several damaged components failed first—can be made on the basis of careful on-site examination. The relationship of the location of the failure to the locations of other system components—for example, the location of a tube rupture in relation to those of burners or soot blowers—is an important phase of on-site examination.

It is usually helpful for the individual making the on-site examination to have an assortment of plastic bags or paper envelopes in which samples can be placed. Identification of each sample, with its location and orientation marked on photographs or sketches, is recommended because the failed component usually must be repaired or replaced with as little delay as possible so that normal system operation can be resumed. This need for prompt return to service may also preclude a second on-site examination; therefore, the first examination should be thorough and complete.Precautions in Sampling. Because of the massive size and the fixed, sometimes remote location of power plants, detailed examination usually cannot be carried out at the scene and must be performed on selected samples taken from the failed equipment. Therefore, the methods used to obtain samples for laboratory examination are of utmost importance. including wrong material; improper operation. including improper maintenance and inadequate water treatment; routine normal operations, such as too frequent soot blowing; and miscellaneous causes. Of these general types of causes, improper operation, which includes most incidents of overheating, corrosion, and fouling, and fabrication defects, which include most incidents of poor workmanship, improper material, and defective material, together account for more than 75% of all failures of steam-power-plant equipment.Most steam-generator failures occur in pressurized components, that is, the tubing, piping, and pressure vessels that constitute the steam- generating portion of the system. With very few exceptions, failure of pressurized components is confined to the relatively small-diameter tubing making up the heat-transfer surfaces within the boiler enclosure.Overheating is the main cause of failure in steam generators. For example, a survey compiled by one laboratory over a period of 12 years, encompassing 413 investigations, listed overheating as the cause in 201 failures, or 48.7% of those investigated. Fatigue and corrosion fatigue were listed as the next most common causes of failure, accounting for a total of 89 failures, or 2 1.5%. Corrosion, stress corrosion, and hydrogen embrittlement caused a total of 68 failures, or 16.5%. Defective or improper material was cited as the cause of most of the remaining failures (13.3%). Although defective material is often blamed for a failure, this survey indicates that, statistically, it is one of the least likely causes of failure in power-plant equipment.Defective material does not always cause a component to fail soon after being put into service. Figure 1 shows cracking at the root of a longitudinal mill defect in a stainless steel superheater tube. This tube ruptured after 18 years of service because the normal operating pressure caused stress-rupture cracking to initiate at the mill defect.Boiler design is inherently conservative; thus, even massive defects may be present in some areas without causing fracture to occur until after a considerable period of operation. Some imperfections may be present without ever causing failure. Figure 2 shows a poorly made weld with incomplete fusion amounting to more than half the tube wall thickness, yet this weld gave more than 27 years of service without failure. Nevertheless, fabrication and repair procedures should be aimed at producing defect-free systems.

Failures InvolvingSudden Tube Rupture

In the basic design of a boiler, the heat input from the combustion of fuel is balanced by the formation of steam in the furnace and the heating of steam in a superheater or reheater. The heat-flow path through a clean boiler tube has three components. First, fire-side heatFig. 1 Micrograph showing stress- rupture cracking at the root of a longitudinal mill defect in a stainless steel superheater tubeThe tube ruptured after 18 years of service. Approximately 25)<

Fig. 2 Weld defect (lack of fusion)This defect did not cause a failure even after 27 years in a reheciter.

transfer from the flame or hot flue gases is by both radiation and convection. Radiation predominates in the furnace, where the gas temperatures may be close to 1650 °C (3000 °F). By the time the flue gas has left the furnace, it has been cooled to 925 to 1095 CC (1700 to 2000 CF), and convection is the predominant mode of heat transfer. Second. conduction through the steel boiler tubes transfers heat to the internal fluid. Although conduction is important, boiler tubes are not chosen for thermal conductivity but for strength, especially creep or stress-rupture strength. Therefore, the temperature gradient through the steel is not controlled by design but accepted as a consequence of material selection. Third, at the fluid interface with the inside-diameter surface is a second convective heat-transfer mode. The steam- side heat-transfer coefficient is a function of fluid velocity, viscosity, density, and tube bore diameter.Boilers in service for some time have a fourth component to the heat-flow path: internal scale or deposits. Steam reacts with steel to form iron oxide:4H20 + 3Fe — 3Fe3O4 + ‘2 (Eq 1)Furnace walls may also have other deposits from impurities in the boiler feedwater. Because these deposits and scale have a lower thermal conductivity than the steel tube, the net effect is an increase in tube metal temperatures. In a superheater or a reheater, such temperature increases can lead to premature creep failures, dissimilar-metal weld failures, and accelerated ash corrosion or oxidation. In furnace walls, deposits may also lead to hydrogen damage (additional information is provided in the article ‘Hydrogen-Damage Failures” in this Volume).An upset in any stage along the heat-flow path can upset the balance and cause a sudden tube rupture. Sudden rupture of a tube in a steam generator is a serious failure, because the steam generator must be shut down immediately to minimize or avoid erosion of adjacent tubes and furnace sidewalls by escaping steam, overheating of other tube banks due to loss of boiler circulation, and damage to other components in the system resulting from loss of working fluid. The downtime resulting from boiler failure and subsequent repair may require other operations to be curtailed or shut down, with an attendant economic loss.Tube ruptures (excluding cracks caused by stress corrosion or fatigue, which usually result in leakage rather than sudden fracture) may be classified as ruptures caused by overheating and ruptures caused by embrittlement. Each type has characteristic features.

Ruptures Causedby Overheating

When water is boiled in a tube having uniform heat flux (rate of heat transfer) along iL length under conditions that produce a state o( dynamic equilibrium, various points along tt tube will be in contact with suhcooled watei, boiling water, low-quality steam, high-quaht steam, and superheated steam. A temper.x

Fig. 4 Plot of scale thickness and oxide penetration versus LMPx = 595°C (1100 °F)12500 h, Y 620°C (1150 2500 Ii, Z = 650 °C (1200 °F)/2500 h. Source:

cause pressure was substantially lower than is’rmal, At above 815 °C (1500 °F), the tube as renormalized. No chemical cleaning was rformed, and the boiler operated for 16 years. \her this service history, the microstructure mildly spheroidized carbides, with pearlite lonies still well defined. Inside-diameter-ale thickness indicated an average metal ternerature of nearly 570 °C (1060 °F). For the rs.sterial in question, the two values were not in .reen1ent; the microstructure, after 16 years of ,r’ice at 570 °C (1060 °F), should have been bully spheroidized. It was not. Therefore, these ;alculations of tube metal temperature are estiimates—useful to be sure, but estimates only.Figure 5 indicates the effects that different e.it fluxes have on tube-wall temperature. In Ir.e region where subcooled water contacts the ise (at left, Fig. 5), the resistance of the fluid n is relatively low; therefore, a small temi e-.ure difference sustains heat transfer at all .it-fiux levels. However, the resistance of asr film in steam of low quality is relativelytherefore, at the onset of film boiling, atemperature difference between the tube. i and the bulk fluid is required to sustain ah.C heat flux across the film. The effect of thesr.ct of film boiling on tube-wall temperaturewars as sharp breaks in the curves for modhigh, and very high heat fluxes in Fig. 5. It h increasing heat flux, the onset of unstable r- boiling, also known as departure from mkeate boiling (DNB), occurs at lower steam tjJities, and tube-wall temperatures reach ncr peak values before stable film boiling, s ch requires a lower temperature difference thtain a given heat flux, is established.

Fig. 5 Variation of fluid temperature and tube-wall temperature as water is heated through the boiling point with low, moderate, high, and very high heat fluxes (rates of heat transfer)See text for discussion. Source: Ref 5

At very high heat fluxes, DNB can occur at low steam quality, and the temperature difference between tube wall and bulk fluid at a point slightly downstream from DNB is very high. Under these conditions, tube failure can theoretically occur by inciting of the tube wall, although in reality the tube will rupture because metal loses its strength, and therefore its ability to contain pressure, before it melts. Departure from nucleate boiling is an important consideration in the design of fossil-fuel boilers and nuclear reactors because heat flux can quickly exceed the failure point (burnout point) at local regions in a tube if the tube does not receive an adequate supply of incoming feedwater.In superheaters and reheaters, which normally operate at temperatures 30 to 85 °C (50 to 150 °F) higher than the temperature of the steam inside the tubes, heat transfer is primarily controlled by the conductance of fluid films at the inner and outer surfaces. Although higher heat fluxes cause higher tube-wall temperatures, deposits have a greater effect on tube-wall temperatures and therefore on overheating.A tube rupture caused by overheating can occur within a few minutes, or can take several years to develop. A rupture caused by overheating generally involves fracture along a longitudinal path, typically with some detectable plastic deformation before fracture. Longitudinal fracture may or may not be accompanied by secondary, circumferential fracture. The main fracture usually has a fishmouth appearance(Fig. 6a) and is either a thick-lip or a thin-lip rupture.Thick-lip ruptures in steam-generator tubes occur mainly by stress rupture as a result of prolonged overheating at a temperature slightly above the maximum safe working temperature for the tube material.

The failures are characterized by thick-edged fracture lips, little ductility or tube swelling. excessive internal scale, and other evidence of oxidation or corrosion. The fracture is normal to the tube surface and parallel with the tube axis (Fig. 6a). The microstructure (Fig. 6b and c) exhibits evidence of creep damage, creep voids or cavitation, grain-boundary separation. and outside-diameter and/or inside-diameter intergranular cracking or oxide penetration of the grain boundaries. The carbide phase of ferritic steels is fully spheroidized.Temperatures slightly above the design condition are caused by six factors. The first is increases in heat flux. In a superheater or a reheater, partial blockage of the convection pass by fly ash will increase the flue-gas flow to certain regions. Higher velocity will increase the steam-side heat-transfer coefficient, h0. and will increase metal temperature. Flame irmpingement on a furnace wall tube will dosame thing.The second factor is internal scale huilc-z The thermal conductivity of steam or waterscale or deposit can be about 5% that of An insulating layer on the inside-diameie ir face acts as a barrier to heat transfer. T’i e’