Invalidation analysis of low-temperature superheater for a 1000MW ultra-supercritical boiler

doi:10.16085/j.issn.1000-6613.2018-1705

Abstract

Abstract:

Reasons of oxide failure for low-temperature superheater in a 1000MW ultra-supercritical boiler were studied in terms of macroscopic morphology of broken tube, X-ray examination of oxide scale accumulation, chemical composition and hardness testing of the sample tube, the microstructure and composition analysis of oxide, and the failure state of oxide. The results showed that elbows under vertical section were blocked by oxide, which caused short-time overheat detonation. The chemical composition and hardness of the sample tube accorded with requirements of 12Cr1MoV steel under the standard. Meanwhile, the equivalent metal temperature of the low-temperature superheater was lower than the design value and the anti-oxidation temperature of 12Cr1MoV. Since the tube thickness and steam velocity were high of large capacity boiler, its thermal stress and turbulence intensity were high. These factors led to the critical thickness of oxide failure much lower than that of small capacity boiler. The wall temperature of the middle part in low-temperature superheater was lower than that of both sides along the width of the furnace in a certain area. High wall temperature also increased the rate of oxide formation and the probability of tube detonation. The results also showed that the oxide was layered on the inner wall of the sample tube, including Fe₂O₃ layer, Fe₃O₄ layer and Cr-rich oxide layer from the outside to the inside. The grain size decreased and the surface structure became dense gradually from the outer layer to the inner.

Key words: boiler, low-temperature superheater, surface, oxidation, broken tube, tube temperature, safety

摘要：

通过爆口宏观形貌分析、氧化膜堆积量射线检查、样管基体的化学成分及硬度检测、样管内壁氧化膜微观形貌及成分分析、氧化膜脱落失效机理分析，对某发电公司1000MW超超临界锅炉低温过热器爆管原因进行了深入研究。结果表明：低温过热器垂直段下弯头氧化膜堵塞引起短时过热爆管；样管完全满足国家标准或行业标准对12Cr1MoV钢的化学成分及硬度要求，低温过热器当量金属温度也远低于低温过热器壁温设计值和12Cr1MoV的抗氧化温度；大容量锅炉低温过热器管壁厚、热应力大和大容量锅炉蒸汽流速大、扰动强度高等因素共同导致氧化膜临界脱落厚度远小于小容量锅炉同管材氧化膜正常脱落厚度，增大了低温过热器氧化膜堵塞爆管的概率；在一定区域内低温过热器壁温沿炉膛宽度方向分布为中间壁温低、两侧壁温高，两次低温过热器爆管位置均在壁温最高区域，壁温高也增加了氧化膜生成速率和爆管的概率。研究结果还表明：样管内壁氧化膜有明显分层现象，由外到内分别为Fe₂O₃层、Fe₃O₄层和富Cr氧化物层等3层，晶粒尺寸由外层至内层逐渐减小，表面结构由外层至内层逐渐致密。

关键词: 锅炉, 低温过热器, 表面, 氧化, 爆管, 壁温, 安全

CLC Number:

TK224

Shuai WANG, Xin WU, Kun LU, Qingguo ZHANG. Invalidation analysis of low-temperature superheater for a 1000MW ultra-supercritical boiler[J]. Chemical Industry and Engineering Progress, 2019, 38(06): 2633-2640.

王帅, 吴新, 路昆, 张庆国. 1000MW超超临界锅炉低温过热器爆管原因分析[J]. 化工进展, 2019, 38(06): 2633-2640.

Figures/Tables 13

爆管次数	氧化膜堵塞占管子通径比例		超标管子根数	爆管位置
爆管次数	30%～60%	60%以上	超标管子根数	爆管位置
第一次	18-2、19-1、63-2、64-1、66-5、67-1	68-1、69-2、79-1、83-1、86-1、96-2	12	80-1
第二次	27-1、42-2、49-6、95-5、137-2	61-2、69-5、80-1、81-3、82-6、83-3、85-5、87-6、90-6	14	82-5

编号	管号及位置	规格	材质
1号	82-5，爆口处	$?$ 50.8mm×11mm	12Cr1MoV
2号	84-1，顶棚下2m处	$?$ 50.8mm×11mm
3号	84-6，顶棚下2m处	$?$ 50.8mm×11mm

编号	管号及位置	规格	材质
1号	82-5，爆口处	$?$ 50.8mm×11mm	12Cr1MoV
2号	84-1，顶棚下2m处	$?$ 50.8mm×11mm
3号	84-6，顶棚下2m处	$?$ 50.8mm×11mm

编号	C	Si	Mn	P	S	Cr	Mo	V
1号	0.12	0.26	0.58	<0.010	<0.010	1.02	0.28	0.23
2号	0.11	0.26	0.58	<0.010	<0.010	1.02	0.29	0.24
3号	0.11	0.26	0.58	<0.010	<0.010	1.02	0.28	0.23
标准值	0.08～0.15	0.17～0.37	0.40～0.70	≤0.025	≤0.010	0.90~1.20	0.25~0.35	0.15~0.30

编号	检测位置	实测硬度值（HBW）	硬度标准值
1号（爆口远端取样）	爆口侧	177.5	135～195
	爆口背侧	152.2	135～195
2号	向火侧	168.1	135～195
	背火侧	169.1	135～195
3号	向火侧	171.3	135～195
	背火侧	168.0	135～195

样管	氧化膜成分（原子分数）/%
样管	C	O	Si	S	Fe
1号	13.13	65.54	0.41	0.52	20.41
3号	19.25	55.66	—	—	25.09

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