Judge the cause of tube explosion according to the characteristics of explosion mouth

The explosion and leakage of boiler tubes is a problem often encountered in boiler operation, which affects the safe and stable operation of the unit. The causes are complex and involve many interdisciplinary subjects, so it is often difficult to make a correct judgment on the causes. The “rapid” emergency repair method can not fundamentally solve the accident, so that the tube explosion of the same reason may occur again, resulting in unnecessary losses. According to the common characteristics of tube explosion in the field, this paper summarizes the methods to judge the cause of tube explosion according to the characteristics of tube explosion.

The explosion and leakage of boiler tubes (water wall tube, superheater tube, economizer tube and reheater tube, the same below) is a problem often encountered in boiler operation, which affects the safe and stable operation of the unit. Although China has made some achievements in preventing and controlling the explosion and leakage of four tubes, the explosion and leakage rate of four tubes is still high. Compared with industrial developed countries, there is still a big gap. The unscheduled outage hours of 200MW and 300MW Thermal power units caused by boiler tube accidents account for 37.8% and 38.1% respectively, while that of the United States is only 7%. Therefore, the task of preventing four tube explosion and leakage is very arduous.

The only way to prevent forced shutdown caused by tube explosion is to establish formal preventive measures suitable for boiler tube explosion in our plant. Therefore, the technicians of the power plant can fully grasp the mechanism of four tube waterfall leakage, have a clear understanding of the causes of boiler tube explosion in the plant, and continuously accumulate the experience of preventive measures suitable for boiler tube explosion in the plant by establishing tube explosion records, which is an important means to reduce and eliminate four tube explosion leakage.

Types and definitions of four tube explosion and leakage

Four tube explosion and leakage refers to the rupture and leakage of water wall, superheater, reheater and economizer in boiler heat exchange surface due to overheating, corrosion, wear and other reasons, resulting in boiler tube failure and even boiler accident shutdown. According to different causes, they can be classified according to table 1.
Table.1 failure types of boiler tubes

20210814001109 29785 - Judge the cause of tube explosion according to the characteristics of explosion mouth

According to the statistics of power plants in China, the first cause of four tube explosion and leakage of 200MW unit is wear, accounting for 30%, mainly fly ash wear, followed by mechanical wear. The second reason is the weld quality problem, accounting for 26%. The third reason is overheating, accounting for 16.5%. The first reason for the explosion and leakage of 300MW and above units is the weld quality, accounting for 24.4%, followed by wear and overheating, accounting for 18.5% and 17.2% respectively. The explosion and leakage caused by corrosion accounts for about 3% in large units.
The method of judging burst characteristics is an important means to determine the cause of tube burst on site. The characteristics of crater mainly refer to:

  • (1) Location of blast hole: which heating surface is located? The specific position is the fire side or the back fire side.
  • (2) Crater shape: 1) whether the fracture surface is perpendicular to the axial direction; 2) Whether there is obvious thinning at the edge of the burst, whether it is a sharp edge or a blunt edge; 3) Whether there is obvious scale on the inner wall of the blast hole, the oxidation of the outer wall, and whether there are macro cracks on the inner and outer walls near the blast hole; 4) Whether there are obvious corrosion pits on the inner and outer walls near the blast hole; 5) The crack trend on the inner and outer walls near the blast opening.
  • (3) Metallography near the crater: including phase composition, quantity, shape, size and distribution, as well as various metallographic cracks (property, size, shape, trend and its relationship with microstructure), size and distribution of micro holes, spheroidization degree and graphitization degree of pearlite, decarburization, overburning, overheating, etc.

Superheated tube burst

Overheating can be divided into short-term overheating and long-term overheating.

Long term overheating

For long-term overheating tube explosion, the explosion opening is usually small, the fracture section is rough and uneven, the tube wall is not thinned much, the edge of the fracture is blunt and not sharp, and there are many axial cracks parallel to the fracture near the fracture.
Due to long-term operation at high temperature, there is often thick black oxide skin near the explosion mouth. In terms of creep principle, the break should be plastic fracture, but creep tube burst is often accompanied by stress corrosion, which makes the burst mouth show the characteristics of brittle fracture.
When the tube is overheated, the tube diameter will expand at an accelerated creep rate. Usually, obvious creep intergranular cracks can be seen in the metallographic diagram of the burst, accompanied by serious spheroidization.
Due to the long-term operation at high temperature, the oxidation inside the crack also occurs while the crack develops. As a result, an oxide layer is formed on the inner wall of the crack, especially at the coarse creep crack.

Short term overheating

Short term overheating is due to the serious decline of mechanical properties of the tube under severe overtemperature, and the plastic deformation and even explosion of the tube under the action of pressure. According to the degree of overheating, short-term overheating tube explosion can be divided into: (1) instantaneous overheating tube explosion, with the temperature above AC3. (2) Short term direct superheat tube explosion. (3) Small drum burst tube.
The rupture of the instantaneous overheating burst tube is in a horn shape, the tube is seriously thinned and expanded, the edge is sharp, it is a ductile fracture, and the appearance is blue black oxide structure. The inner wall of the break is very smooth because the steam water mixture in the tube rushes out rapidly, and the tube expands seriously. The outer wall of the tube is generally blue black; There are not many axial cracks parallel to the break near the break, and the structure at the break is feathery bainite.
The explosion opening of the short-time direct overheating tube is large, the shape is irregular diamond, the microstructure carbide is spheroidized, the break edge is sharp, there is a certain expansion near the break, and the tube is also expanded to varying degrees at a distance from the break. The fracture structure is ferrite plus massive pearlite, and pearlite has been spheroidized to a certain extent.
The small bulge tube blasting is a local overheating blasting, the expansion of the non blasted part is not obvious, there is an obvious small bulge at the break, and the break is also sharp and smooth. The fracture structure is ferrite plus massive pearlite, pearlite has been spheroidized to a certain extent, and there are cementite balls on the grain boundary.

Wear tube burst

The characteristics of the crater of the worn burst are that there is obvious thinning of the tube wall near the crater, there is no obvious change in the metallography of the crater, which belongs to ductile fracture, and the blasting edge is thin.
Four tube blasting caused by wear of heating surface can be divided into the following categories due to different wear mechanisms:
(1) fly ash wear
Fly ash wear is one of the most important causes of low temperature heating surface wear, leakage and tube explosion. The test shows that for the carbon steel surface, the part with impact angle of 30 ° ~ 50 ° is the most seriously worn, which will cause a wear edge on the tube wall surface. At the same time, in the boiler, the tube explosion of low-temperature heating surface caused by fly ash wear is mainly due to the existence of flue gas corridor.
(2) mechanical wear
The reason for mechanical wear is that the tube clamp on the heating surface tube row will often open welding due to overheating deformation or weak welding, resulting in tube vibration and wear with the tube clamp, or the water-cooled wall will collide or rub with other adjacent components, resulting in the wear and thinning of the tube wall. When the tube wall is thinned to a certain extent, the tube will explode under the action of internal pressure. Therefore, obvious mechanical friction marks can be found on the surface of the tube.
(3) soot blowing wear
The operation of soot blower will cause tube wall wear. The appearance of soot blowing wear is similar to that of fly ash wear, and the metallography of tube wall is also similar. Usually, it is only mechanical wear, plastic damage occurs, and the wear part of tube wall is obviously thinner. Generally, the places where wear and burst occur are located at the soot blowing tube row of the soot blower.
(4) coal particle wear
Coal particle wear is generally caused by the burnout and deformation of the tertiary air nozzle (or main nozzle) and the scouring of the pulverized air flow to the surrounding water wall. For coal particle wear, the break is characterized by cracking at the thinnest wall thickness on one side of the center line of the fire face, and then opening with the other side of the fire face as the axis. The edge of the blasting opening is in the shape of a knife edge, one end is torn, and the blasting tube and the tubes on both sides are free of swelling and bulging. One side of the center line of the fire surface is seriously worn and thinned, and the tube near the explosion mouth may be slagging. The metallographic structure of the edge of the burst and the fire side of the adjacent tube changes little, and the ferrite grain at the edge of the burst has no obvious elongation, indicating that the plastic deformation of the water wall is small during blasting.
(5) slag falling and wear
There are few wear cases caused by coke dropping, which will produce dot perforation leakage on the inclined plane of the cold ash hopper.

Corrosion tube burst

The tube burst caused by the chemical or electrochemical action between the external medium and the heating surface tube is called corrosion tube burst. Although the proportion of corrosion tube burst in the total number of tube bursts is low, due to its sudden and unpredictable nature, once corrosion occurs, the damage range is large, often resulting in large-area heating surface damage.
According to the location of corrosion, tube burst can be divided into smoke side corrosion and water side corrosion.

Flue gas side corrosion

Flue gas side corrosion can be divided into high-temperature corrosion and low-temperature corrosion due to different parts and conditions. When burning high moisture and high sulfur fuel, the high-temperature heating surface tube is corroded, which is called high-temperature corrosion. Low temperature corrosion refers to the corrosion of the tail low-temperature heating surface.

High temperature corrosion

The tube burst caused by high temperature corrosion on the flue gas side has the following corrosion mechanisms:

  • (1) the action of corrosive gases such as SO2, SO3 and H2S;
  • (2) High temperature corrosion mechanism of sulfate type;
  • (3) High temperature corrosion mechanism of sulfide type.

A large corrosion area can be clearly identified near the break, and the corrosion area is uneven; Near the break, the tube wall is thinner, and the explosion opening is in tension crack shape, with a long crack.
The difference between corrosion tube blasting and wear tube blasting is that the tube wall near the wear tube blasting is very smooth and has a worn edge, while the corrosion tube blasting is uneven and has no obvious edge; The metallographic structure of the break has no obvious change, and the grain at the break is elongated, which is a ductile fracture; There are sulfur-containing deposits outside the tube wall of the corrosion explosion tube on the flue gas side, and the side close to the substrate is generally black deposits, which are closely combined with the tube wall.

Low temperature corrosion

Low temperature corrosion tube burst mainly occurs on the economizer with low feed water temperature. There are also uneven corrosion areas near the crack of low-temperature corrosion burst tube. The crack is in the shape of tensile crack, there is no obvious change in metallographic structure, and the grain at the crack is elongated, which is a ductile fracture.

Water side corrosion

Boiler tubes will also burst due to water side corrosion. Water side corrosion mainly includes in-service corrosion caused by local concentration of water in the boiler, oxygen corrosion caused by oxygen in feed water and caustic embrittlement caused by stress. Caustic embrittlement mainly occurs in expansion or riveting boilers, which is rare in large power station boilers.

Acid corrosion and alkaline corrosion

The corrosion process during operation caused by local concentration of water in the boiler can be described as follows: the boiler water is locally concentrated under the sediment in the tube, the gap of evaporation heating surface and the part where steam plug is generated in the furnace tube, resulting in concentrated acid or concentrated alkali, which destroys the Fe3O4 protective film on the surface of the furnace tube, resulting in acid-base corrosion on the metal surface of the furnace tube, which can be called acid corrosion and alkaline corrosion respectively.
Alkaline corrosion often occurs under porous sediments. The adhesion between corrosion products and metal surface near the blast hole is poor. The corrosion products contain furnace water components such as phosphate and silicate. After removing the corrosion, there are uneven corrosion pits. Under the condition of concentrated alkali, there are few hydrogen ions, the generated hydrogen is easy to diffuse, and will not penetrate into the steel to cause decarburization. The metallographic structure and mechanical properties of the metal under the pit have not changed, and the metal still maintains its ductility. The tube explosion is caused by the thinning and overheating of the tube wall due to corrosion damage.
Acid corrosion often occurs under dense sediments. Due to the high concentration of hydrogen ions under concentrated acid conditions, the generated hydrogen is not easy to diffuse out, and part of it infiltrates into the steel and reacts with cementite Fe3C in the steel. Therefore, acid corrosion and hydrogen embrittlement are always accompanied. The corrosion products near the crater are firmly combined with the metal surface, and there are corrosion pits on the metal surface. The inner wall surface of most of the craters is decarburized, and there are many microcracks on the tube wall. These cracks are connected into a network, mostly intergranular fracture. The expansion of the blast mouth is not obvious, and the fracture section is flat, rough and blunt, showing the characteristics of brittle fracture.

Oxygen corrosion of boiler

The oxygen corrosion of boiler is electrochemical corrosion. Oxygen corrosion mainly occurs on the inner wall of the heating surface at the inlet section of the economizer, and in serious cases, it can reach the middle of the economizer to the boiler water wall. Its main feature is ulcer corrosion. Many small drums are formed on the corroded metal surface, with great differences in diameter. The surface color of the drums ranges from yellowish brown to brick red, the secondary layer is black powder, and there are corrosion pits on the metal surface. The change of metallographic structure of explosive mouth is not obvious, which is ductile fracture.

Fatigue failure

The boiler tube is subjected to alternating thermal stress and mechanical stress due to startup and shutdown or load change. At the same time, due to the weight of the tube and the working medium in the tube, the tube also bears the effect of gravity. When the tube vibrates for various reasons, the stress in the tube also changes periodically, resulting in thermal fatigue and vibration fatigue of the furnace tube.

Vibration fatigue

Vibration fatigue is often caused by support failure or unreasonable arrangement. There is no obvious thinning at the fracture, which is transverse fracture.

Thermal fatigue

Thermal fatigue can be caused by periodic cooling of water side metal caused by intermittent steam stagnation or rapid cooling. Generally, there is no obvious thinning at the fracture, which is a transverse fracture. At the lower part of the fracture, multiple cracks of different sizes parallel to the fracture plane can be found. Through the metallographic observation of the crack tip, the crack is intergranular crack, there are secondary intergranular cracks, and the crack tip is discontinuous, Its expansion direction is perpendicular to the direction of the tube axis. During soot blowing, rapid cooling or slag layer intermittently infiltrating the heating surface tube will also cause periodic changes in temperature, resulting in fatigue damage of the furnace tube, and multiple transverse cracks with “elephant skin” densely distributed on the outer surface of the furnace tube.

Corrosion fatigue 

There are various corrosion phenomena in the boiler, and the fatigue damage caused by the action of component corrosion medium is corrosion fatigue. The outer or inner surface of the crater of corrosion fatigue usually has a corrosion layer or oxide layer attached. Generally, there is no bulging and bulging, no wall thickness thinning, no plastic deformation and brittle fracture. The fracture surface of the crack is flush and perpendicular to the tube wall thickness, but not smooth. At the beginning of corrosion fatigue failure, it is often carried out in the way of multiple crack sources, so it often has unique multi tooth characteristics on the section.

Welding quality and dissimilar steel welding

The weld quality problem has always been high in the explosion and leakage of four tubes in China. Taking manual arc welding as an example, the easy defects are: undercut, overflow, weld bead, concave (waist collapse), incomplete penetration, slag inclusion, air hole, crack (including hot crack, cold crack and reheat crack). These welding quality problems cause stress concentration at the welding position and decline of mechanical properties of the joint, The welded junction becomes a weak part and causes tube explosion. Welding quality tube burst is easy to identify, because its break always occurs at the welding position, the break is generally broken along the defective place, and the crack is relatively straight. The welding part of dissimilar steel is also a part that is easy to cause tube explosion, and circumferential fracture will occur at the welded joint due to differential thermal expansion.
The explosion and leakage of boiler tubes occur frequently. Identifying the causes of these damages will help to reveal the hidden dangers of the boiler. If they are not handled, they will lead to more serious problems. Most of the damage can be attributed to one of several root causes. Comprehensive metallographic fault analysis can usually reveal the root cause; However, metallographic analysis of all damaged tubes is not required. The appearance of the damaged tube can provide intertwined information on the cause of the damage. This training is helpful to narrow the scope of possible causes of tube blasting. Sometimes, combined with some knowledge of boiler operation, it is enough to determine the cause of damage.

Source: China Boiler Tubes Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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