Intemational Jourmal of Plant Engineering and Management Vol 10 No. 3 September 2005The Mechanism and Influencing Factors of Corrosionin a gas Heating-furnaceYANG Zhi-gang, ZHANG Ning-sheng, WU Xin-min1. School of Energy and Power Engineering, Xi'an Jiaotong UniversityXi'an 710049. P. R. China2. Xi'an Shiyou University, Xi'an 710065, P.R. ChinaAbstract: Natural gas should be heated and throttled for the purpose of purification and transportationat the first gas production factory of the Changqing field. The safe use and heat-transfer efficiency of aheating-furnace affect the safe and smooth production of natural gas directly. At gas collecting stationsnow, no measures of anticorrosion have been adopted in heating furnaces which erode and scale badly.In order to solve the corrosive problem of heating-furnaces, prolong operating life of heating-furnacesassure safe and smooth production of natural gas, the mechanism and influencing factors of corrosion ofthe heating-furnace were analyzed and some corresponding measures were brought forward based on afield investigation ofbehavior and present operational status of heating-furnaces at the first gasproduction factory. The results show that the corrosive ion and soluble Co2 and 0, in water erode metalbadly at the condition of being heated. Corrosion of a heating furnace are mostly oxygen corrosion, cor-rosie ion corrosion, acid corrosion, iron encrustation corrosion, dry and wet interface corrosion, causticcorrosion, etc; The influencing factors of corrosion mainly include soluble 02 and CO2 in water, pH val-ue, heat loading, corrosive ion, soluble solid (salinity )and non-flowing character of water,etc.Key words: heating-furnace, corrosion mechanism, influencing factor, oxygen corrosion, protective1 IntroductionThe first gas production factory of the Changqing oil field has 176 wells. For the purpose of purification and transportation, natural gas should be heated and throttled which make the pressure dropform 15-22MPa to below 0. 4MPa. Heating-furnaces used in the Changqing oil field are multip-wellgas heating-furnaces made by Beijing Chaotuo Science and Technology Development Company. Oneheating-furnace has four heating coils, which can heat and throttle the gas of four wells at the sametime. Safe use and heat-transfer efficiency of the heating-furnace affect the safe and smooth produetion of natural gas directly. But at the gas collecting station now, heating-furmaces have eroded andscaled badly, heat transmission efficiency has reduced; further development would cause an emer-gency(] In order to solve the corrosive problem of heat山中國煤化工, the mechanismd influencing factors of corrosion were analyzed baseCNMHGf usage behaviorand present operational status of the heating-furnaces at the first gas production factorReceived 28 February 2004184International Journal of Plant Engineering and Management Vol. 10 No. 3 September 20052 Field investigationFume outlet Makeup water2. 1 Usage behavior of the heating-furnace Gas inlet /t-Source gasHeating-furnaces use gas as the fuel and use waPre-throttle gaster as heat carrier to heat pre-throttle gas. In theheating-furnace, gas goes in inner pipes and wa.HearthFurnace mantleter is in the furnace mantle. The structure ofFigure 1 Structure of heating-furnaceheating-furnace is shown in Figure 1Processing parameter of the heating-furnace( case of the heating-furnace at middle 13 gas collectingstation is as followsRated load is 400k W; inner pipe pressure is 25 Mpa; heat efficiency is 82% operating pressure isnormal atmosphere; material of furnace is mainly carbon steel; volume of the furnace mantle is about8.5 m: operating temperature is about 90C; makeup water is about 17.0 m per year; enrichedmultiple is 2.0-3.0(estimation); temperature of the gas inter station taken is 0-60C,mean annual temperature is 30C. After being heated, the temperature is more than 50c before beingthrottled)and 15-25 C( after being throttled)2.2 Corrosion status of the heating-furnaceHeating-furnaces were checked after they had operated for a long time, it found that (1)Corrosicwas serious at the upper part of waterside of the wall; (2)Corrosion was inside of the heating coil(gas side), but corrosion was serious outside of the heating coil; a thick reddish and black rustyscale attached on the pipe wall; (3)Some of the deposit appears on the whole wall of the furnacemantle and the coil pipe was black burr; some was black, rusty and knotty. The waterside of thefume pipe has attached scale layered corrosion products and some granular black viscositybuildup; ( 4) The surface of metal under deposit buildup appeared pitted with a rut and groove ofcorrosion after it was cleanedAfter observing the corrosion shape of heating-fumaces, it was found that the main corrosion of heating-furnaces were general corrosion( uniform corrosion), ulcer corrosion, dot etching,etc.Ulcercorrosion and dot etching were mainly factors of generation burr rust. The damage of ulcer corrosionto the metal surface was deeper, and the wound was bigger. Dot etching made the surface of themetal produce round pitting corrosion, which depth was inequable and closeness was unequal. Thesetwo kinds of corrosion belong to local corrosion, by comparison; the damage of them to equipmentand the hazard degree of them are far heavier than uniform corrosion3 Mechanism and influencing factors of corrosion of heating-furnaceCorrosion and its form were different with different parts of the waterside of the heating-furnaces. Themajor reason of these phenomena relates to many factors such as different heat engineering state and difaferent corrosion reason, etc. The water for the heating-fuml中國煤化工 on factory is lifewater(groundwater)of the Jingbian base station. The qualitCNMH Gty and low hardness. The corrosive ion and soluble CO, and 0, are in the water, which could erode metal badly at thecondition of being heated. By study of the corrosion form of different part of the heating-furnaces, itcan be concluded that the water side corrosion mainly include oxygen corrosion, corrosive ion corrosionThe Mechanism and Influencing Factors of Corrosion in a Gas Heating-Furmaceacid corrosion, iron encrustation corrosion, dry and wet interface corrosion, caustic corrosion, etc3. 1 Oxygen corrosion0H→Fe(OH2+Fe2Oxygen corrosion is the mainly and the worst corro-sion factor of heating-furnaces; oxygen acts as a de203+H20polarizer in the process of the corrosion. It belongs tome Anode regionoxygen absorption corrosion. The mechanism of theoxygen corrosion is shown in Figure 2Anode region Fe+Fe+2e,igure 2 The mechanism of oxygen corrosionNegative polarity zone:022H20+4e=+40HIn the water: Fe++20H-Fe(Oxygen corrosion relates to manysuch as temperature, current velocity, pH value, salt con-tent, etc(3). For the closed system, the oxygen corrosion rate is a straight climb with temperature;for the open system, it goes up before down with temperature. Temperature, current velocity and saltcontent affect oxygen corrosion mostly by the influence of solubility of oxygen in the waterIn the gathering and transportation process, source water 0.75of the heating-furnaces was always exposed in the air;The dissolved oxygen was basically in a saturated state. 5050In the operational process, heating-furnace is connected gwith air only by a very fine pipe(used for water replenishing), so the heating-furnace could be0.25considered asOpen systemclosed system. In addition, the water of the heating-furnace was in dead state, it made the dissolved oxygen bereleased a hardly The increase of the oxygen corrosion020406080100120Temperature,℃rate was in direct proportion to the raise of the temperature of the furnace water. The influence of furmace water Figure 3 Influence of furnace water temperaturetemperature on oxygen corrosion is shown in Figure 3on oxygen corrosionOxygen corrosion occurred at any part of the heatingurnaces, especially at the upper part of theheating coil and the gas pipes. The solubility of oxygen becomes diminiohed because of the water beg heated; dissolved oxygen is released and pours up, but it cannot be discharged from the heating-furmace. It makes the dissolved oxygen of the upper part of the furnace water reach the saturationstate, the oxygen adheres to the coil pipes with the form of a bubble too. On the other hand, in thewhole microcirculatory system of the furnace water, the water temperature of the upper part of theheating coil and the gas pipes are higher then other parts of the heating-furnace, so oxygen corrosionof the upper part of the heating coil and gas pipes is the worst.The main form of oxygen corrosion is mostly ulcerative and leal corrosion and uniform corro-ion. With increase of temperature of furnace water, the中國煤化工and make theHvalue rise gradually, and these make corrosion productsCAMHn. no measuresof anti-scaling were adopted to furnace water which scaled, the adherence of rust and encrustationhave made carbon steel produce oxygen concentration cells and corrosion under the encrustation. AInternational Journal of Plant Engineering and Management Vol 10 No. 3 September 2005the condition of slight alkalinity, the corrosion products accumulated more and more, and appearwith the shape of a burr and rust knob with exacerbated oxygen concentration corrosion also. So thiskind of corrosion can make serious damage to metal machine and its structure strength3. 2 Corrosive ion corrosionBecause the macromolecules (such as CI, S04, ect.)in the furnace water can penetrate protective foil and produce corrosion to the metal, hinder the formation of the passivating film, the furnacewater produces spot corrosion easily at the condition of non-flowing. This kind of corrosion is noteasily found commonly, once produced, the hazard degree of it is very large. This kind of corrosionoccures at any part of the waterside of the fumace jacket, especially at the higher heat loading gaspipes and the ektexine of the waterside of the hearth. When the temperature of the ektexine is higher, it makes Cl and So2 produce local concentration and enlarge the probability of spot corrosion3. 3 Acid corrosionAcid corrosion is mostly hydrogen liberation corrosion. In the heating-furnace, it mainly occurred atthe ektexine of the waterside of the gas pipes and the upper part of the furnace jacket. The cause of(1)The water the heating-furnaces used is groundwater, generally speaking, some soluble CO2 dis-solves in it. After the furnace water is being heated, the soluble cO2 will be released quickly andadsorbs at the surface of the metal and produces CO, corrosionThe corrosion process of CO2 is as followsAnode reaction:Fe→FeCathode reaction:CO2+H2O→H2CO3,2H2CO3→2H'+2HCo,2H’+2e→H2,Fe’+2HCO+2H,0-Fe(HCO,)2+HOverall reaction: Fe +2C0 2, +2H, 0-+Fe(HCO,)2+H2, Fe(HcO,)2+FeCO, +H20+cOThe solubility of CO, in the water relates to system pressure, temperature and the element of the water4, which increases with pressure increase, and drops with temperature rising; and some of mineral substance in the water can affect it. Therefore, the corrosion of CO, is affected by multiple factors, for example, temperature, pressure, etc(2) The carbonate in the water decomposes after being heated, which causes the drop of the ph val-ue at the local area and produces corrosion2HCO3→CO2+CO32+H2OAcid corrosion is mostly uniform corrosion, the degree of corrosion for metal and the influence forstrength of component are less, but the corrosion products(Fe,Fe)of it can produce iron encrustation and corrosion under the encrustation easil3. 4 Iron encrustation corrosion中國煤化工Iron oxide encrustation not only affects heat transfer(the什HCN MH Gr less than general encrustation), but also produces corrosion under the encrustation, and makes the metal becomethin and sunken so much as punch a hole. This kind of corrosion mostly occured at the watersidesurface of the hearth and the gas pipesThe Mechanism and Influencing Factors of Corrosion in a Gas Heating-Furmacel87The existence of the iron ion in the water and the other kinds of corrosion make the surface of thecarbon steel produce an iron oxide encrustation. The existence of the iron oxide at the high heatingload areas produces iron oxide encrustation corrosion easily.The cause of iron oxide encrustation corrosion is as follows(1)Iron oxide acts as the cathode of electrochemical corrosion, at the condition of heat-up, the sullunder the encrustation is destroyed, make the naked carbon steel become an anode and produce cor-rosion in the waterAnode reaction: Fe-+Fe++2eCathode reaction: Fe( OH),+e-Fe( OH) 2+OH, Fe,04+H,0+2e-3Fe0 +20HCorrosion produces iron oxide encrustation, which can accelerate the development of this kind of cor-rosion. They are the cause and effect of each other and accelerate each other which makes the metalproduce bad corrosion under the encrustation.(2)At the higher heating load areas, the water under the encrustation concentrates rapidly and produces corrosion caused by factors such as alkali et(3)Steam-water corrosion due to metal overheat under the encrustation 3Fe+H20-Fe,O2+H,The main shapes of iron oxide encrustation corrosion are major encrustation corrosion, dark brownfish scale encrustation with most of the metal under the encrustation being corroded and presentingaccidented pockmark. This kind of encrustation is hard and cannot be removed easily3.5 Dry and wet interface corrosionDry and wet interface corrosion is one of the local corrosions, occurs at the interface of the air andwater,produces oxygen concentration cells( due to dry and wet alternating at the up-down of the in-terface)and corrosion. This kind of corrosion occurs generally, the form of it is creating a corrosiongroove at the dry and wet interface3.6 Caustic corrosionThe locale and probability of caustic corrosion in the heating-furnace were less. The main reasons forcaustic corrosion are that the alkalinity concentrate at the higher heating load part, oxide protectivefoil is dissolved and loses protection, makes the metal corroded[51Fe,O4 +4NaOH-NaFe02+ Na,Fe02+2H20Fe+2NaOH→Na2FeO2+H2This kind of corrosion mostly occurs at the stress concentration part such as the higher heating loadwaterside of the hearth, the waterside of the gas pipe, elbow bend, etc. The form of it is groovelessor sunk local ulcerative corrosion4 Anti-corrosive measures of the heating-furnaceSome reasons and forms related above are the main of the corrosion of the heating- furmaces, they willnot occur by one shape commonly, but more. Some of thsome of them accelerate each other. In order to resolve tlH中國煤化工 of each other;CNMHeating-furnacesome measures are brought forward as follows(1)Heating-fumaces should be cleaned or boiled, give the encrustation and rust in the heating-furnacesa thorough cleaning, and give the heating-furnaces a preformation film and passivating treatment.International Joumal of Plant Engineering and Management Vol 10 No. 3 September 20052)Give the feed water of heating-furnaces a thorough deoxygenization or use a deoxidizer, and de-carbonation(3)Control the ph value and alkalinity of the furnace water; prevent corrosion products from mixingin the heating-furnaces(4) Give the feed water of the heating-furmaces a corrosion and scale inhibition treatment; add thecorrosion and scale inhibitor(5)When the heating-furnace stops work, prevent the air from entering in the out-of-service heating-furnace, keep surfaces of the out-of-service heating-furnace dry; use a corrosion inhibitor to produce a protective foil on the surface of the metal and release the corrosion of the metal5 ConclusionsIn this paper, mechanism and influencing factors of corrosion of the heating-furnace were analyzedand some corresponding measures were brought forward based on field investigation of usage behaviorand present operational status of heating-furnaces at the first gas production factory.The results show that the corrosive ion and soluble CO, and 0, in water erode metal badly at the condition of being heated; Corrosion of heating-furnace are mostly oxygen corrosion, corrosive ion corro-sion, acid corrosion, iron encrustation corrosion, dry and wet interface corrosion, catcorrosion,etc; The influencing factors of corrosion mainly include soluble 02 and CO2 in water, pH value, heatloading, corrosive ion, soluble solid (salinity) and non-flowing character of water,etcIn order to resolve the corrosive problem of heating- furnaces, some measures are brought forwardfollows: cleaning, passivation, preformation film, corrosion and scale inhibitionReferences[1] S P Zhan, Chemical cleaning of boilers. Chemical Cleaning, Vo. 15, No 4, pp. 16-18, 19992] P. Zhang and Y. LJi, Oil field corrosion and prevention technology. Published by Anticorrosion Center of Zhongyuan Petroleum Exploration Bureau, 1994(In Henan)[3]L Shen, Oxygen corrosion of low pressures hot-water boiler and its protective measure. QualityWorld, No. 7, pp 63, 2003[4J.S Zheng and Z P Lv, A study of mechanism and influencing factors of carbon dioxide corro-sion. Journal of Petroleum, Vo. 16, No 3, pp 134-138, 1995[5 H. P. Li, analysis and solutions of scaling and corrosion causing of boiler. Metallurgy PowerNo.3,pp.34~36,2002Brief BiographiesYANG Zhi-gang is now a Ph. d candidate in Xi'an Jiaotong University. His research includes envi-ronmental protection in oil-gas fields and oil field chemistryZHANG Ning-sheng is now a professor in Xi'an Shiyou中國煤化工 ludprotection and environmental protection in oil-gas fieldsCNMHGWU Xin-min is now a associate professor in Xi' an Shiyou University. His research includes oil fieldchemistry and drilling