Analysis and Countermeasures of explosion and leak

2022-08-12
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Analysis and Countermeasures of the explosion and leakage of the inlet and outlet elbows of the boiler's high temperature superheater

Abstract: through the statistical analysis of the explosion and leakage of the inlet and outlet header of the high temperature superheater and high temperature reheater of Hebei Xingtai Power Generation Co., Ltd. No. 8 boiler, effective countermeasures are put forward, the interference of external stress is eliminated, and the safe and stable operation of the unit is improved

key words: high temperature superheater; elbow; Explosion and leakage; Statistical analysis

abstract:based on statistical analysis of blast leaks occurred in inlet and outlet elbows of hightemperature superheater of unit 8 boiler in Hebei Xingtai Power Co., some effective treating measures of provision which optimizes the interference from outside stress and improves safe and stable operation of the unit

keywords:highteturn on the control computer (or controller) temperature superheater; elbow; blast leakage; Statistical analysis

boiler 8 of Hebei Xingtai Power Generation Co., Ltd. is a dg670/13.78 ultra-high pressure intermediate reheat single drum natural circulation solid-state slag removal pulverized coal boiler produced by Dongfang Boiler Plant, with negative pressure combustion in a single furnace chamber, once through burners arranged at four corners, and a 200 MW steam turbine generator set for one boiler

1 equipment overview

1.1 No.8 boiler technical specification

boiler design efficiency: 89.6%

superheated steam flow: 670 t/h

reheated steam flow: 579 t/h

superheated steam pressure: 13.7 mpa

reheated steam inlet pressure: 2.6 mpa

reheated steam outlet pressure: 2.4 mpa

superheated steam temperature: 540 ℃

reheated steam inlet temperature: 323 ℃

reheat steam outlet temperature: 540 ℃

Feed water temperature: 240 ℃

exhaust gas temperature: 144 ℃

1.2 superheated steam flow (Figure 1)

1.3 high temperature superheater structure

high temperature superheater consists of 49 panels, each panel consists of 6 pipe rings, a total of 294 φ 42 mm × 5 mm U-shaped pipe; The material is 12Cr2MoWVTiB (Steel Research 102) seamless steel pipe for high-pressure boiler, and the elbow material of the connecting part between the roof and the inlet header is 12Cr1MoV. The heating area of high temperature superheater is 795 m2, and the design average steam mass flow rate vw=789 kg/(M2 · s). The horizontal pitch of the pipe coils is 240 mm, the pipe group adopts the 6-coil single winding type, and the fixation between the pipes adopts the "pipe clamp" fixation form, and anti-wear measures are set. During operation, the high-temperature superheater can increase the main steam temperature by 56 ℃ on average, and the structure is shown in Figure 2

2 explosion and leakage of connecting pipes at the inlet and outlet of high temperature superheater

8 boiler has operated for 62500 hours from the minimum 179.30n113 in 19920 to 200006. From the part above the ceiling of the high-temperature superheater to, there have been five tube bursts, and the five tube bursts are all located at the elbow, and the phenomenon of explosion and leakage is the same

according to statistics, the first elbow leaks after passing through the ceiling three times outside the fifth row on the right of the outlet of the high temperature superheater; Leakage occurs at the last elbow of the 28th row on the right of the outlet of the high temperature superheater;, Leakage occurs at the penultimate elbow of the third circle outside the 22nd row on the right of the inlet of the high temperature superheater; At the same time, it was found that the leakage of the first elbow after passing through the ceiling was found at the third circle outside the 22nd outlet on the left of the high temperature superheater. The five times of explosion and leakage points are marked in Figure 2 with symbols a ~ E

in 2000, the same phenomenon of tube explosion occurred repeatedly in this part of high temperature superheater. Technicians analyzed the causes of tube explosion and formulated corresponding countermeasures

3 cause analysis of burst and leakage

3.1 tube burst phenomenon

see Figure 3 for the shape of burst mouth

the length of the burst is 30 ~ 60 mm and the width is 1 ~ 3 mm. There are 2 ~ 4 parallel cracks around the outside. The edge is rough and blunt, with a thickness of 4 mm, almost no thinning, and the color of the blast is black. There is a thin oxide layer on the inner wall and several parallel cracks on both sides of the outer arc crater

3.2 cause determination

when the pipe burst occurred at this position for the first time, it was initially determined that the pipe had original defects. After the second tube explosion at this position, the cause analysis of tube explosion shall be started. First, the possible causes of tube explosion are listed:

A. overtemperature

b. the pipe has original defects

c. the pipe has processing defects

d. stress corrosion

e. steam corrosion

f. corrosion fatigue

since the pipe material above the ceiling of the high-temperature superheater is 12Cr1MoV, the allowable temperature limit is 580 ℃, the pipe material in the high-temperature superheater furnace is Gangyan 102, the allowable temperature limit is 620 ℃, and the steam temperature at the superheater outlet controlled by the boiler is 540 ℃. The pipes above the ceiling are not directly heated by the flue gas, and the ceiling superheater with nearly saturated temperature is arranged at the lower part. If the pipe wall of this part reaches the temperature resistance limit of 580 ℃, the steam temperature should exceed 580 ℃. At this time, the Steel Research Institute 102 in the furnace may have overheated and burst, so it is judged that the possibility of overheating is unlikely. However, in order to confirm the inference results, the pipe burst sample was sent to the metallographic room for inspection. The inspection results showed that the metallographic structure of the pipe was spheroidized by 2 ~ 3 levels, and no carbide aggregation was found, indicating that there was no reason for overtemperature tube burst

since the pipe burst occurs at the elbow without exception, it is not clear that the pipes used by the manufacturer have original defects. After spot checking and measuring the external shape of some elbows, each shape index does not exceed the provisions of the relevant power specifications, which can only be determined under the same experimental conditions. Therefore, reasons B and C cannot be the cause of tube explosion of No. 8 boiler

through the analysis of the current situation of tube explosion from the aspects of D, e and F, sufficient reasons have been found. Therefore, it is considered that the pipe burst is the result of the combined action of stress corrosion, steam corrosion and corrosion fatigue

3.3 cause analysis

dg670/13 The dead point of the expansion of type a boiler is at the elevation of the boiler ceiling in the middle of the high temperature superheater. The inner part of the high-temperature superheater expands downward, and the outer part of the furnace and the header expand upward. The weight of pipes above the inlet and outlet headers and the ceiling is borne by 12 variable spring hangers. The steam collecting duct is led out from the left and right 1/4 parts of the outlet header and supported by variable spring hangers

bending test method of special reinforcement for concrete leakage of high temperature superheater during temporary repair and shutdown inspection, it was found that the dead boom of the ceiling was partially loose and did not bear the force at all, and the indicated value of the elastic hanger of high temperature superheater exceeded the design lifting force. Under the cold and hot state of the outlet header of high temperature superheater, the theoretical value of expansion displacement is 19 mm, while the actual value is 10 mm. The theoretical value of expansion displacement of the inlet header of high temperature superheater in cold and hot state is 16 mm, while the actual value is 8 mm. Therefore, it can be determined that there is a strong tensile stress in the cold state and a strong compressive stress in the hot state of the outlet elbow above the ceiling of the high temperature superheater

according to the data provided by Dongfang Boiler Works, the stress analysis and calculation of the high temperature superheater pipeline is carried out in accordance with the mec21 pipeline flexibility analysis program provided by CE company of the United States. The stress analysis results were evaluated according to the American national standard ansi/asme31.183 "power pipeline" and sdgj61990 "technical regulations for stress calculation of steam and water pipelines in thermal power plants" issued by the planning and design administration of the Ministry of water resources and power of China. Therefore, the pipe support and crane designed by the manufacturer should be credible

according to further analysis, the pipeline stress comes from:

a. it is caused by the deformation of the ceiling caused by the positive pressure of the furnace during operation or other reasons

b. during boiler installation, the lifting force of high temperature superheater was not tightened according to the design requirements

c. the elastic coefficient of spring hangers has changed after years of use

because the bending radius of some pipes above the ceiling of high temperature superheater is only 85 mm, and each pipe has four bends here, the distance between the two adjacent elbows is very close, and the residual stress at the elbow is large. The pipes of the original manufacturer are φ 42 mm × For 5 mm standard deviation pipes, the outer wall of the elbow is thinned, and the residual wall thickness is generally between 4.1 ~ 4.4 mm, so there are also reasons for low strength in terms of structure. The structural strength is weak, which creates conditions for stress corrosion

the steam flow vibration during operation, or the surge phenomenon during the water filling and drainage process will cause alternating stress damage to the elbow above the ceiling of the high temperature superheater. After the temporary repair, the surge of the pipe occurred during the water filling process, and up to 20 strong vibrations caused a certain degree of damage to the pipe

since the steam temperature here is as high as 540 ℃, the existence of stress also provides a good development environment for steam corrosion

in a word, the explosion and leakage of the elbow above the roof of the high temperature superheater furnace is the comprehensive damage result of steam corrosion, stress corrosion and corrosion fatigue caused by steam as a corrosive medium under the action of long-term compressive stress and alternating stress

4 countermeasures

4.1 replace the tubes

according to the current situation of the tubes, it is decided to replace the outer 3 ~ 6 circles of the tubes above the header roof at the inlet and outlet of the existing high-temperature superheater. In order to reduce the strength weakness factors existing in the structure, the following changes have been made in the processing and material selection of pipes:

a. all pipes used are positive deviation pipes. It is required that the wall thickness of all pipes should be greater than 5.1 mm, and the thinning of the outer arc wall thickness of the bending elbow should not exceed 0.5 mm

b. change the shape of the elbow, increase the spacing between adjacent elbows, and reduce the bending angle of the pipe, as shown in Figure 4

4.2 ultrasonic flaw detection

ultrasonic flaw detection was carried out on all elbows of the inlet and outlet headers of the high temperature superheater, all pipe seats and the 1 and 2 outer rings of pipes outside the inlet and outlet of the high temperature superheater. Inner wall defects were inspected. A total of 3 pipes were found to have inner wall cracks, and they were replaced

4.3 when processing new bends, bend and check them strictly according to the requirements of the regulations to ensure ovality and smooth inner wall. All argon arc welding is adopted for welds, 100% of which are subject to nondestructive testing and 20% of which are subject to radiographic inspection

4.4 re measure the elastic coefficient

the elastic coefficient of the spring hanger of the high temperature superheater, and compare it with the nameplate value. If the deviation exceeds 2%, replace the spring. Readjust the tightening force of the dead boom on the furnace top and the elastic boom of the high temperature superheater according to the design requirements, so that the bearing force of each boom meets the design requirements

4.5 recalibrate the expansion indicator

recalibrate the expansion indicator of the inlet and outlet header of the high temperature superheater, and install two expansion indicators in the middle of the header to monitor the expansion of the inlet and outlet tubes of the high temperature superheater

5 conclusion

through improvement, after the start-up of boiler 8, at full load, the upward expansion of the inlet and outlet headers of the high-temperature superheater are 18 mm and 16 mm respectively, basically reaching the design expansion, indicating that the high-temperature superheater elbow has reached the free expansion state, eliminating the interference of external stress. Since the operation of the improved No. 8 boiler, no tube explosion accident has occurred in the high temperature superheater tube, which proves that the transformation is successful, and it significantly improves the safe, stable and economic operation level of the boiler unit

reference

[1] DL, pressure vessel supervision regulation [S] (end)

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