Abstract This paper is concerning with the state of corrosion caused the failure of carrier pipe elbow of three steam boilers where one of them run periodically to one of the production line units & it is evaluated by microscopic examination. The steps and actions that have been taken to study the document related to operating line and the date, duration and time for failure occur, where samples taken from the failure part and analysed for water input and output of boiler examined as well. The analysis of microscopic examination indicates that the corrosion caused by the dissolved oxygen in steam, which caused an intergranualr corrosion& stress corrosion cracking as well as.
This case represented ina failure apart ofthesteampipeline (elbow) connectstheboiler(1)with threesteamboilersthat would beoneboileroperationperiodicallyand tube metal is acarbon steeldiameter of150mm and athickness of5 mm andoperating pressure20atmospheric pressure andat a temperature of200 ° Cand failure occur on04/08/1997and Figure(1) illustrates a diagram for failure part in the steam line.
Fig (1) shows the failure position
thecrack existsintheleftsideof the elbowintheformofa fish’s mouthandthere isabulge
in place of the crack is not regular. And after cutting theelbowsectionwe find pitting corrosion near theopen crackandsomeof them are deepandtherearesomecracksinthe end oftheopen crack, as showninFigure (2).
Fig (2) shows the shape of failure
Measurements & laboratory tests
Thickness of elbow
Length of rift
Section thickness was measured by ultrasound and the results were as shown in Figure (3).And Figure (4) shows the length of the crack
Figure (4) shows the length of the crack
Sample wastaken itfromthe elbow partandanalysisby metals spectrometer analyser andthe results are shown inTable (2).
Samples weretakenfromtheelbow sections shown in FigureNo. (5) itwasnext tothecracksamplesandgraduallyawayto notehowspreadthecrack. Microscopicexaminationofthesamplesshow thatthestructureof the metalisaferrite andperlite.
the pettingcorrosioneffect ontheinternalstructureofthemetal andfoundthat there isanextensionofthepetting DeolattackPerlitebodywasextendtoferrite as on fig (6).Corrosion caused by the dissolved oxygen in steam, which caused an intergranualr Corrosion&Stress Corrosion Cracking as well as.
Fig (6)a Fig(6)b
Discussion of Results
Through field visits and examinationof the place ofthefailure it is close tothe boilerNo(1).
It is out of worksince thethreeyears and thisindicates there is no flow ofsteam inthispart and there is valvecontrolfor the passage ofsteam from theboiler(1)tothe main lineand thecrackon the sideof the elbowand thiscauses thefailure is not from erosion.
It is by measuring thethicknessof the sampleinseveral different locationsnext tothe crack away from itshowing thatthere is a differencein the readingsatmany locationsranging from(4.1 to 5.3mm) theoriginallythickness5mm increaseStatistics ofa layer ofoxidationon the sample.When cutting thesampleand makechemical analyses and microscopic examination to study themicro structureOf the metal anddeterminethecauseofthefailure also we noted that therethepitting corrosionindeferent places.Within thissegmentmaking a difference readingsthethickness found insome placesthedepthofpettingto(0.9 mm) as notedthepresenceofpettinginthe centre ofthecrackandthisshowsitdoes notreduce the thickness of the sampleresulted in thefailure.At study ofthe micro structureof some ofthe samplestaken from thefailure part shows thatthe micro structure of metal is a perlite the baseofferrite with no difference on grain size.It was noted thatcorrosionpettingextends to theperlite phase thenferritecorrosioncaused by the dissolved oxygen in steam, which caused an intergranualr corrosion& stress corrosion cracking as well as.Throughchemical analysisthat themetallow-carbon steelto beconsideredonthemetalcaused by the presenceof dissolved oxygen in the steam which cause thecorrosion.
Thefailure ofpart of themain steam pipe line fromboilers to productionunite
Is a present of theoxygengashas resulted inthepresenceofpetting which thereason fortheweakness in themetal withan increase intemperature and pressureresulted inthisfailure.
There is no changein the thickness ofthemetal, except at the point of petting.No change on microstructureof the metal.Throughchemical analysis of thesamplethepipesusedinsteamlines according the specification standard.