T-TYPE NAPHTHA PUMP STRAINER MESH FAILURE ANALYSIS

299 Muhammad A. Butt et al.DiscussionStress Corrosion CrackingTable 6 presents a large number of environmentsthat have caused stress corrosion cracking(SCC) of various metals and alloys under certainspecific conditions. A classic example of this typeof localized corrosion is chloride-SCC ofaustenitic (300 series) stainless steels whichresult in brittleness and usually transgranularfracture of an otherwise ductile alloy, oftencausing a catastrophic equipment failure. Thepresent failure of the strainer occurred in about5 years.SCC failures are macroscopically brittle inthe sense that the ductility and load bearingcapacity of the material are impaired.Microscopically, the cracks are either intergranularor transgranular and cleavage-like. Induplex microstructures, one phase often cracksmore easily than the other, leading to characteristiccracking patterns. Plastic deformationalways accompanies crack growth and plays akey role in most cracking mechanism. Sometimesthe fracture surfaces show evidence of stepwise(discontinuous) crack advance, especially intransgranular SCC. Stress corrosion crackinginitiation at smooth surfaces shows a thresholdstress (σ th) that varies from 20% to more than100% of the yield stress (σ y), depending uponthe metal and the environment. Thermal stressrelief of welded or cold-worked components canprevent SCC if σ th/σ yis failrly high, e.g., 0.7.SCC is usually nucleated by some form oflocalized corrosion. In chloride-SCC or stainlesssteel or aluminum alloys, cracks start from areasof pitting, intergranular corrosion, or crevicecorrosion that creates the stress concentration andthe acidity required for cracking. Intergranularcorrosion occurs along segregated zones rich incarbon, nitrogen or phosphorous and provide astress concentration [2].Role of H 2SPresence of H 2S or its oxidation productS 2O 32-(thiosulfate) greatly enhances localizedcorrosion and chloride-SCC of austenitic andduplex stainless steels. Concentration of H 2Srequired to start SCC is called critical concentrationand it is 0.01M for SS; below this concentrationH 2S is depleted in deep pits and cracks.Nonpropagating cracks are found around H 2S,while crack tips are H 2S run out sites. Enhancementof SCC by H 2S is often ascribed to a hydrogeneffect as given in the literature [2].Selection Criteria for Stainless SteelSelection of a proper stainless steel fortypical environment requires several considerations.The first consideration is corrosionresistance. For mild atmosphere, type 430 gradesare recommended. For industrial atmosphere,type 430 grades are considered. In case of manyfood processing environments and mildcorrodents, type 304 are available, chemical andsevere corrodents require type 316 or the highalloyed type such as 20Cb-3. The secondconsideration is design criterion of mechanicalproperties such as yield strength. It may be notedthat high strength stainless steels often sacrificeresistance to some form of corrosion especiallySCC. The third consideration is fabricability, e.g.ability of the stainless steel to be machined,welded or formed. The corrosion resistance ofthe fabricated article to the environment must beexamined in the light of chemical species, pH,aeration, flow rate, impurities such as chlorideas well as temperature and heat transfer andcrevice effect. [3]SensitizationSeveral of the high-nickel, high molybdenumsteels are quite satisfactory with regardto stress corrosion attack in most industrialapplications. Both 316, 317 steels contain about

Naptha pump failure 302It is clearly indicated in Figure 8 that type304 stainless steel is used for mild corrodents.Under the prevailing conditions of naphthacirculation, the presence of H 2S in wet conditiongreatly impairs the suitability of 304 stainlesssteel in the naphtha pumps.microstructure. The inferior corrosion andmechanical properties of the welded componentsin comparison with the base metal are due to thepreferential corrosion attack at the alloy depletedregion, segregated interfaces, dendritic cores andaustenite/delta-ferrite and other secondaryprecipitate interfaces in the weld metals. [5].Normally, SCC will not occur if theequipment/part is in compression. Failure istriggered by a tensile stress that must approachthe yield stress of the metal. The stresses may befrom faulty installation or represent residualstresses from welding, straightening, bending oraccidental denting of the component. Pits, whichact as stress concentration sites, will often initiatestress corrosion cracking.Figure 9. Yield Strength and corrodent action classify amultitude of stainless steels into 11 families [7].It is shown in Figure 9 that type 304 SS has loweryield strength than type 316 SS which contains2-3% molybdenum. Type 316 SS has highercorrosion resistance in reducing environments.Welding of austenitic stainless steel usuallyresults in weld metal with a dendritic andinhomogeneous microstructure having a smallamount of delta-ferrite, M 23C 6carbides, sigmaetc., and significant segregation of major alloyingelements at the phase interfaces. The presenceof delta-ferrite leads to preferential corrosionattack in the weld metal in certain environments.Pits have been shown to nucleate preferentially,depending on the alloy composition, either at theaustenite/delta-ferrite interfaces or inside thedendrite cores of austenite. The ageing of welddeposits, either during stress-relieving operationsor during exposure to high temperature in service,leads to the formation of complex precipitateIf austenitic steels are exposed to heattreatmentof less then 870° C, they can besensitized. For this reason, local stress relief ofunstabilized austenitic stainless steel is usuallyimpractical, since the runout areas immediatelyadjacent to the region being heat-treated will besensitized [6].Conclusions and RecommendationType 304 SS is susceptible to stresscorrosion cracking when used for naphthafeedstock. The choice of type 304 SS was notcorrect due to its susceptibility to SCC (StressCorrosion Cracking) and IGC (IntergranularCorrosion) due to sensitizing. Future failure canbe avoided by design modification, fabricationchanges, material changes and environmentalcontrol. The change of material was recommendedconsidering the present observations.Selection of 316 L (low carbon) stainless steelor carbon steel can reduce SCC & IGA problem.It is also suggested that regular cleaning ofstrainer after 20-25 days can improve theefficiency and reduce the chances of blockage orcontaminations.

303 Muhammad A. Butt et al.References1. A-lab Corporation. The American Association Forlaboratory accreditation 2005. Chemical testing.Dayton, OH.2. Philippe, M. 2002. Corrosion mechanism in theoryand practice. In: Stress Corrosion Mechanisms. Ed.Newman, R.C., pp. 402-406. Marcel Dekker. NewYork.3. Khan, I.H. 1989. Corrosion technology. Vol-2., pp.442. Institute of Chemical Engineering &Technology, Punjab University, Lahore.4. Roberge, P.R. 1999. Hand book of corrosionengineering, pp. 734-736. McGraw Hill Company,New York.5. Khatak, H.S. and Raj, B. 2002. Corrosion ofaustenitic stainless steels; mechanism, mitigationand monitoring. In: Sensitization and testing ofintergranular corrosion. Ed. Parvathavarthini, N.,pp. 123, 129-130. Narosa Publishing House NewDelhi.6. Philip, A.S. 2004. Encyclopedia of corrosiontechnology. 2 nd ed., pp. 569, 572. Marcel Dekker,New York.7. Kenneth J.M. 1980. Materials engineering-1,selecting materials for process equipment, chemicalengineering. McGraw-Hill Publications Co., NewYork, N.Y., pp. 15-19 and pp 36.