湿硫化氢环境下管线的腐蚀及防护

湿硫化氢环境下管线的腐蚀及防护
刘海滨　莫少明
中国石化股份公司齐鲁分公司胜利炼油厂(山东省淄博市255434)
摘要:文章对胜利炼油厂蜡油加氢装置的临氢高压空冷器出口管线及重油加氢装置低压酸性气体脱硫系统胺液再生塔顶空冷器出口管线的腐蚀进行了分析,指出H2S含量、介质流速、温度及材质是影响腐蚀的主要因素,并提出控制介质流速在0.4～0.61m/s、合理选材、加注缓蚀剂和加强监测可避免腐蚀事故的发生。

关键词:加氢装置　湿硫化氢环境　管线　腐蚀　防护
对于湿硫化氢环境下压力容器等设备而言,目前已积累了比较丰富的选材、制造、使用等经验,并纳入了强制性的标准规范中;而对该环境下的工艺压力管道而言,则还没有引起人们足够的重视。

下文以胜利炼油厂蜡油加氢装置的临氢高压空冷器出口管线及重油加氢装置酸性气体脱硫系统的胺液再生塔顶空冷器出口管线的腐蚀事故为例,提出该部位湿硫化氢环境下工艺管线的防护措施和对策。

1　胺液再生塔顶空冷器出口管线的腐蚀
胜利炼油厂重油加氢装置酸性气体脱硫系统胺液再生塔顶空冷器的操作条件如下:
操作介质:H2S、水蒸气、DE A、NH3,C O2;
操作温度:入口102℃,出口60℃;
操作压力:0.2MPa
空冷器出口管线原设计采用20号无缝钢管(G B8163—87),规格为Φ89mm×5mm。

自1990年装置投产以来,该段管线多次发生腐蚀穿孔泄漏事故,阀门亦经常因锈蚀而失效。

1994年检修时将上述管线全部更换为S US321不锈钢管线,使用5年多未发生泄漏事故。

1999年11月装置扩建改造时因酸性气量增大,这些管线设计扩径更新(规格为Φ108mm×6mm),材质又改回20号无缝钢管。

1999年底改造投产后,从2000年6月至2001年1月该段管线共先后发生13起腐蚀泄漏事故,只好采取带压堵漏等措施维持运行。

2000年2月该管线一DN300mm立管弯头穿孔,大量硫化氢泄漏,为此进行了停工处理,将上述管线、管件及阀门全部更换为S US321材质。

通过对割除的旧管线现场调查及测厚发现:有三个弯头(Φ108mm×6mm)的外弧顶部位被腐蚀穿孔,形成拳头般大小的孔洞,还有两个弯头外弧顶部位减薄到不足1mm,这些弯头因外壁包了盒子才维持使用;有三个出口弯头外弧顶侧最小壁厚分别为2.9、2.8、3.0mm,马鞍口立管的最小壁厚为3.1mm。

但所有直管线的壁厚正常,只有轻微减薄;另外对胺液再生塔顶挥发线、E1703入口管线的测厚也很正常,几乎没有腐蚀。

2　临氢高压空冷器出口管线的腐蚀
蜡油加氢装置临氢高压空冷器为反应生成物的高压空冷器操作条件如下:
操作压力:7.0～6.6M pa
操作温度:入口120℃,出口60℃
操作介质:焦化腊油、减压腊油、H2、H2S、H2O、NH3、和氯盐等
2001年7月E103/C南侧出口弯头发生破裂,装置被迫紧急停工。

E103/C、D出口管线于1988年投用,规格为Φ89mm×7mm,材质为20号无缝钢(G B8163-87)。

1999年装置改造扩建时,设计在原有二台高压空冷器(工艺编号:E103/C、D)的基础上,并列新增了两台高压空冷器(工艺编号E103/A、B),两台旧空冷器及出入口工艺配管均属利旧移位。

为保证利旧管线的可靠性,施工时进行了检测,测厚结果为E103/C南侧出口弯头外弧顶侧最小壁厚为5.3mm,其余三个出口弯头最小壁厚在5.7～6.5mm。

根据SH3059-94《石油化工企业管道设计器材设计通则》规定的管子壁厚计算公式(已考虑了腐蚀裕量及壁厚负公差):
s=P D0/(2[σ]tΦ+P)+C
式中:D0—管外径,mm;(89mm)
P—设计压力,MPa;(7.6MPa)
[σ]t—在设计温度下材料的许用应力,
装置・设备 石油化工腐蚀与防护
2001,18(5)・30・
MPa;(130MPa)
Φ—焊缝系数,无缝钢管取1;
C—管子壁厚的附加余量(包括腐蚀裕
量及壁厚负偏差),mm;(此处取3mm,偏
于保守)
得出s=5.53mm。

设计人员按当时弯头最小壁厚为5.3mm、使用10年、局部最大腐蚀率为0.15mm/a计,该弯头的设计计算厚度仅需5.53mm(还包括了2.54mm的腐蚀裕量),因此确定这部分管线可以利旧。

通过对破裂的3号弯头测厚,测得其外弧顶最薄处为3.1mm,按运行时间20个月计(1999年12月至2001年7月),其局部最大腐蚀率竟达到1.56mm/a,E103/C、D其余三个出口弯头局部最大腐蚀率分别为0.94、1.12、0.946mm/a,新投用的E103/A、B的4个出口弯头(Φ89mm×10mm)局部最大腐蚀率分别0.5、0.81、0.87、0.56mm/a,而所有入口管线及弯头、出口管线均减薄较少,腐蚀率不大于0.1mm/a。

3　腐蚀原因分析
3.1介质的影响
管线腐蚀损坏的根本原因是加氢反应中生成了大量硫化氢,与再生塔内的水蒸气或高压空冷器的注入水经过冷却后形成了H2S-NH3-H2O-C O2的腐蚀环境。

对空冷器出口20号钢内表面腐蚀产物进行X射线衍射定性相和能谱分析表明,腐蚀产物主要是含铁的硫化物FeS、FeS2和铁的氧化物FeO(OH)、FeO、Fe2O3,说明腐蚀主要是由H2S、C O2引起的。

蜡油加氢装置改造前主要加工焦化蜡油,由于含硫量低,加工量稳定、操作平稳,因此该管线腐蚀率较低且呈现比较典型的均匀腐蚀状态(出口最大腐蚀率为0.15mm/a)。

装置改造后,掺炼了50%硫含量很高的减三线蜡油。

据定期分析统计,2000年第二加氢装置混合蜡油的原料硫含量平均为1.23%,其中9月份竟达到1.72%,远远超出了对原料硫含量的最高设计要求(1.08%)。

从而引起腐蚀加剧。

日常分析循环氢脱后H2S为1%～2%,工艺指标仅为0.1%～0.5%,冷高分酸性水分析含硫量为6.61%、5.61%、6.67%,pH值为3～5。

3.2流速的影响
根据设计核算结果,在正常操作条件下,蜡油加氢E103/C、D出口管线(Φ89mm×7mm)的介质流速为9.6m/s,重油加氢E1703出口管线的介质流速为31m/s,而美国腐蚀工程师学会(NACE)在对几十套加氢装置反应馏出物空冷器的腐蚀情况进行调查、分析后,提出在H2S-NH3-H2O-C O2的腐蚀环境下,介质流速要控制在4.6～6.1m/s的范围内,介质流速若超过6.1m/s则腐蚀+冲蚀速率将急剧增大[1]。

在重油加氢装置E1703出口管线部位安装的挂片、探针腐蚀监测表明:当介质流速小于5m/s时,20辆钢的腐蚀率小于1mm/a;流速大于5m/s时,腐蚀率显著上升,当流速大于7m/s 时,腐蚀率高达9mm/a。

,见图1。

图1　20号钢腐蚀率与介质流速关系曲线
E103/C、D出口管线的流速为9.6m/s,远远超出了NACE规定6.1m/s的临界流速,如果考虑到介质偏流的影响,则超出的范围更大。

由于介质高速流动,在突然转弯或缩径处形成涡流,携带能量大且具有腐蚀作用的小液滴会与管壁激烈碰撞,使得管壁表面形成的FeS保护膜不断被去除,从而再次发生腐蚀,形成了腐蚀—冲刷—腐蚀的循环破坏形式。

重油加氢装置E1703出口管线的介质流速远高于腊油加氢E103/C、D出口管线的介质流速,因此前者的腐蚀率比后者大得多;而直管段介质流速虽高,但仍处于环状流动,冲刷并不严重,因此腐蚀率较低。

3.3温度及溶解氧的影响
对于低温硫化氢腐蚀而言,当操作温度高于水的露点温度以上时,由于水以蒸汽形式存在,构不成硫化氢的电化学腐蚀状态,而气相硫化氢的腐蚀性很弱,上述空冷器的入口温度工艺指标均大于100℃,因此入口管线腐蚀轻微;当操作温度低于水的露点温度时,硫化氢则溶解在水中,空冷器的出口温度为60℃,硫化氢浓度之高已达到饱和溶解状态,发生强烈的阳极溶解型电化学腐蚀,而且高压空冷器前的注入水及再生塔内的汽提蒸汽,均含
・
1
3
・
　第5期 刘海滨等.湿硫化氢环境下管线的腐蚀及防护
有一定的溶解氧,会以FeS 为阴极形成闭塞电池,产生坑蚀。

3.4材料的影响 根据重油加氢E1703出口挂片探针腐蚀试验,当流速为7.1m/S 时,20号钢腐蚀率达9mm/a ,因此操作条件下20号1钢是不耐腐蚀的(见表1)。

表1　空冷出口挂片探针腐蚀试验数据
挂片材质
腐蚀率
/mm ・a -1
腐蚀形态介质流速
/m ・s -120号0.81减薄并有蚀坑 4.4ND 0.66减薄并有蚀坑 4.120号9.00减薄至0.5mm 7.10Cr13 2.20减薄7.108Cr2AlM o 1.54减薄7.41Cr18Ni9T i 0.30均匀减薄7.01Cr18Ni9T i
0.21
均匀减薄
5.0
在此操作条件下,ND 钢的耐蚀性与碳钢差别不大,0Cr13不锈钢与碳钢相比耐蚀性提高了4倍以上,但不如抗硫化氢腐蚀新钢种08Cr2AlM o 。

1Cr18Ni9T i 耐蚀性最好,但随流速的增加,腐蚀率也有一定程度的升高。

各种钢抗蚀性能的顺序为:1Cr18Ni9T i >08Cr2AlM o >0Cr13>ND >20号。

3.5其它因素的影响
空冷器设计的介质流向应是对称分配的,其设
计准则是靠对称的布局及工艺配管的对称分布保证介质流量均衡。

而蜡油加氢装置改造时,新增的E103/A 、B 出口管线规格变为Φ89mm ×10mm ,利旧的E103/C 、D 出口管线规格仅为Φ89mm ×7mm (若考虑倒壁厚减薄因素则管内径更大),破坏了工艺配管对称的设计准则,造成了E103/A 、B 出口管线内径小、阻力降大,E103/C 、D 的介质流量要大于E103/A 、B ,致使E103/C 、D 的出口弯头产生严重的
涡流冲蚀减薄。

从测厚计算出的腐蚀率情况来看,
E103/A 、B4个出口弯头的局部最大腐蚀率分别为0.5mm/a 、0.81mm/a 、0.87mm/a 、0.56mm/a ;而E103/C 、D4个出口弯头本周期局部最大腐蚀率分别为0.94mm/a 、1.56mm/a 、0.94mm/a 、1.12mm/a ,说明由于介质流量发生了较大偏流,E103/C 、D 出口弯头腐蚀率远高于E103/A 、B 的弯头腐蚀率。

4　湿硫化氢环境下工艺管线的防护措施4.1控制介质流速
设计配管时应借鉴NACE 有关规范要求,注意控制介质流速在0.4～0.61m/s ,并据此确定合适的管线规格;生产中在满足工艺操作条件下要尽量控制介质流速不超标,以免发生严重的冲刷腐蚀。

4.2合理采用耐蚀材料
对加氢装置低压酸性气脱硫系统湿硫化氢环境下的管线应尽可能选用奥氏体不锈钢材质。

根据使用经验,此系统管线采用奥氏体不锈钢材质是经济的,可大大降低装置的腐蚀事故率,而且没有发生过硫化氢应力腐蚀开裂事故。

高压系统湿硫化氢环境下的管线,考虑到介质中含有部分氯离子,其管线均选碳钢材质。

国内20号无缝钢(G B8163-88)与国外A106G rB 钢质量指标相当,按NACE 的规范,只要控制合理的介质流速可减缓冲刷腐蚀,但管线焊缝必须进行焊后消除应力热处理,否则容易发生应力腐蚀开裂事故。

4.3注缓蚀剂 胜利炼油厂曾于1994年在重油加氢装置溶剂再生塔顶挥发线进行注缓蚀剂试验,通过E1703出口管线20号钢挂片腐蚀监测情况看,缓蚀效果很好(结果见表2)。

表2　再生塔顶空冷出口注缓蚀剂试验结果
试验部位平均腐蚀率
/mm ・a -1S 2-/M g ・L -1
Fe 总
/M g ・L -1NH 3-N /M g ・L -1
pH 值
缓蚀率
/%
空冷出口(空白)
　7.5
5.35×105　3.49
　1.58×105
　9.2
空冷出口(注剂)
　0.005 4.29×105
　1.95　1.56×105
　9.3
　99.9
从试验结果可以看出,20号钢的腐蚀率仅为
0.005mm/a ,系统中的总铁含量下降,缓蚀率达到99.9%。

此外,试验还证明缓蚀剂不会引起DE A 溶液的降解和发泡,对装置正常操作无不利影响。

高压系统湿硫化氢环境下可以注多硫化纳缓蚀剂,胜利炼油厂重油加氢装置及加氢裂化装置高压空冷器入口管线一直间歇注软化水和多硫化钠缓蚀剂,上述管线已使用10年,未发生明显腐蚀减薄。

4.4加强腐蚀监测
对低压湿硫化氢环境要加强易冲蚀部位的挂片探针在线腐蚀监测以及下游分离设备的冷凝水分析,并及时反馈结果指导生产。

高压系统的湿硫化氢环境,因探针和挂片监测法难以实施,因此应加强易冲蚀部位的在线定点定期测厚,以防腐蚀加剧发生突发事故。

(收稿日期:2001-08-22)
・
23・ 石油化工腐蚀与防护 第18卷　
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K eyw ords fluid catalytic cracking,salt depositing,measure C ause Analysis of Corrosion B locking of Crude Oil Stabilizer and Countermeasures
Zhao Qingping,Yan Weiyou
The analysis of the corrosion blocking of the crude oil stabi2 lizer has found that the corrosion and blocking were mainly caused by HCl,H2S,NH3,H2O and O2etc.The corrosion blocking could be eliminated by improving”one rem oval and tw o injec2 tions”,injecting amine instead of amm onia and enhancing sulfur rem oval rate,etc.
K eyw ords blocking,,corrosion,fouling H S-2000neutraliz2 ing corrosion inhibitor
Corrosion of Piping in Wet H ydrogen Sulfide E nvironment and Protection
Liu Haibin,Mo Shaoming
The corrosion of the outlet piping of high-pressure air cooler of the hydrotreating unit and the outlet piping of the overhead air cooler of amine regenerator of the low-pressure acid gas desulfur2 izer of the heavy oil hydroprocessing unit was analyzed.It was concluded that H2S content,fluid velocity,temperature and con2 struction of material were the major factors for the corrosion.The corrosion could be prevented by controlling the flow velocity within 0.40.61m/s,selecting proper materials,injecting corrosion in2 hibitors and strengthening corrosion m onitoring.
K eyw ords hydroprocessing unit,w et hydrogen sulfide envi2
ronment,piping,corrosion,protection
Corrosion Cracking of R egenerator and S afe Operation Wang Yujue,Zhang Xiaolu
Corrosion F ailure of R efinery Process Piping and Material Se2 lection
Rao Xinghe
S ince the fluid in m ost of the refinery process pipelines is flammable hydrocarbons,the corrosion failure of the pipeline can easily cause fire emergency.4corrosion failure cases(s ome were the results of inappropriate selection of materials and s ome were the outcome of misusing welding materials)were introduced and the principles of material selection for refinery operation were dis2 cussed.
K eyw ords refinery,process piping,corrosion,failure,m ate2 rial selection
C ause Analysis of Rupture of Pressure G auge N ozzle of Q uench H ydrogen Pipeline
Li Yahong
The pressure gauge nozzle of quench hydrogen pipeline of the hydroprocessing unit in T ianjin Petrochemical C ompany’s refinery ruptured in June20,2001.The chemical,metallographic and SE M testings con firmed that the rupture was the result of stress corrosion cracking in the wet hydrogen sulfide environment because of inappropriate material selection.The selection of austenitic stainless steel and proper protection in unit shut-down could pre2 vent the incident.
K eyw ords hydrotreating,nozzle,w et hydrogen sulfide,stress corrosion cracking,1Cr18Ni9Ti
BC-2072N eutralizing Corrosion I nhibitor for Condensing System of H ydrotreating U nit
H e Qiuxia,Zheng Wenlong,Wang Jianjun
The trial of BC-2072neutralizing corrosion inhibitor in the condensing system of the hydrotreating unit showed that the corro2 sion inhibition efficiency could reach90%and the corrosion rate of the carbon steel was only0.1mm/yr if the dosage was150μg/g. K eyw ords hydrotreating,neutralizing corrosion inhibitor, corrosion rate
Application of GX-195H igh-temperature Corrosion I n2 hibitor in Atmospheric-vacuum Distillation U nit
Wang Wenting,An Hui
When10μg/g G X-195high-temperature corrosion in2 hibitor was injected into the high temperature sections of the crude distillation unit of Jinxi Petrochemical C ompany,the corrosion rate of equipment was markedly reduced and the operating cycle was extended by over tw o times.G reat economic benefit has been achieved.
K eyw ords atmospheric-vacuum distillation unit,corrosion, GX-195corrosion inhibitor,application
Optimization of Process Conditions for E lectrostatic Desalting Luo Wanming,Li Jun,Liu Changqing
The FC-9302emulsion breaker was selected for Y an’an blended crude which contained higher salt and water for the elec2 trostatic desalter operation.A fter the optimization of the process parameters,the salt content of the desalted oil was reduced to less than3mg/L.
K eyw ords high-salt crude oil,electrostatic desalting,FC-9302emulsion breaker,process p arameter Some Problems in C athodic Protection Operation
Wang Zhifang
The filed investigation has found several problems in cathodic protection operation,such as proper conditions for power supply facilities,high-efficiency reference electrode and the sacrificing anode etc.These deserve detailed discussion.
K eyw ords cathodic protection,impressed current,problems in sacrificing anode operation.
Corrosion Protection Design for Large Steel Crude T ank Guo Mengfei
The corrosion protection design options including coating de2 sign and cathodic protection design for large steel crude tanks were described.The combined coating and cathodic protection option was the m ost cost effective measure of corrosion protection.
K eyw ords crude oil tank,corrosion protection,coating,ca2 thodic protection,option
Application of RG W ater-proofing Coating in H ouse R oofing Wang Wei,Shi Wenjie
RG water-proofing coating was suggested to replace the ex2 isting felt paper and other materials in roofing of the civil and in2 dustrial buildings.The application proved that this coating had very g ood performances and theoretical life of the coating could be as long as20years.The cost of unit area of the coating was com2 parable with that of other conventional materials.
K eyw ords building roofing,w ater leak age,RG w ater-proof2 ing coating
Study on the Corrosion of16MnR,316L Steels in Sulfur-containing Crude
Dong Shaoping,Yuan Junguo,Zhang Kangda,Fang Deming The study on the corrosion of16MnR,316L stainless steels in sulfur-containing crude showed that the equipment manu factured with16MnR material could continue to operate at a total sulfur content of less than2.0%and the uniform corrosion rate was about1.6mm/yr.The slow tensile stress corrosion testing dem on2 strated that the initial concentration of Cl-1when the stress corro2 sion cracking of316steel began to appear was250mg/L while H2S and pH content had little effect.
K eyw ords sulfur-containing crude oil,16MnR,316L,cor2 rosion,testing
Assessment of Static Corrosion of Sour W ater in Visbreaker Zhou Junfang
The static corrosion assessment of s our water in visbreaker was performed by both probe and coupon testings.It was found that the corrosion of s our water was mainly the pitting corrosion. The error of these tw o testings was only1.2%.
K eyw ords sour w ater,static corrosion assessment,corrosion probe,coupon testing,pitting corrosion
Construction of F Class Corrosion Pipeline in HF Alkylation U nit
Peng Jin
The codes&standard,welding specifications and keeping, installation and inspection of the materials were introduced.The measures for ensuring the construction quality of the F class pipeline were discussed.
K eyw ords pipeline construction,stand ard,processing,in2 spection,acceptance。