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40 HYDROCARBON ASIA, JULY-SEPT 2010 Visit our website at: http://www.safan.comCorrosion

TECHNOLOGY

Flow Accelerated CorrosionFailures in Refineries

A mong the few identified failure mechanismsin condensate & steam services, Flow Ac-celerated Corrosion, or FAC, is establishedas the chief failure mechanism. This phe-

nomenon is also known as Erosion-Corrosion (EC).FAC is defined as a corrosion process in which theprotective oxide layer equipment in steam/conden-sate service is dissolved under the combined effectof corrosion and flow velocity. It is generally a slowcorrosion mechanism, but may attain a very highcorrosion rate (3-5 mm / year) if not taken care of.The FAC mechanism, first noticed in late 1960’s, isnow well understood. Following is an attempt tocompile the available information and mitigate theeffects of FAC in our refineries.

Prevalence of Flow Assisted CorrosionAt present, no reliable database exists for FAC-

related failure rates for refineries or any other con-ventional industries which deals with steam/condensate services and networks. However, it canbe inferred from databases of nuclear industries. It isexpected that effects of FAC in typical industry sce-narios would be similar or more than that of a typicalnuclear industry.

The prevalence of FAC in steam/condensate serv-ice can be understood from database of NuclearPower Plants (NPP). Even with utmost control inoperating parameters, FAC has affected more than50% of circuits in two Russian NPPs as observed inthe following survey results.

Among various deterioration modes in steam/condensate services flow acceleratedcorrosion (FAC) is established as the chief failure mechanism. This article focuses onvarious aspects of FAC from the standpoint of reliability and safety. It discusses areassusceptible to FAC, effects of different factors of FAC like flow velocity, water chemistry,material of construction, effect of dissolved oxygen etc., and possible solutions interms of water chemistry control, use of specific inspection program, selection andupgrading to suitable metallurgy and limiting flow velocity by proper design.

Both the survey records are from Russian NuclearPower plants. It may be noted that more than 55%piping have been effected and thinned down inabove two graphs.

It is expected that prevalence of FAC is more wide-spread in steam/condensate networks in a typical oilrefinery.

Corrosion

Figure 1

Figure 2

HYDROCARBON ASIA, JULY-SEPT 2010 41

Few well known cases of failures by FAC have beenincluded along with pictures in the article.

Susceptible ZoneEquipment / piping operating in the following

conditions are susceptible to FAC• Temp. : Between 100 C to 300 C• Service : Water or Water + Steam. Water as a

phase must be present.• Material : Chiefly CS & low alloy steel• Location : Chiefly located near bends, reducers,

downstream of orifice etc whereturbulence is created.

• Effect : a) General thinning of affected areasb) Deposition of oxides in equipment

– leading to under -depos i tcorrosion (caustic gouging/phos-phate attack/H2 attack in boilers)& inefficient heat transfer.

Types & Appearance of FACFAC may be from single phase (water) or two-

phase (water + steam).

Susceptible Zone:Single Phase : 100 C to 190 CTwo-Phase : Approx 130 C to 280 C, although

failures at 300 C have been reported.

LocationSingle phase FAC occurs at inside radius of bends/

fitting Two-phase FAC occurs at outer radius of bends /

fittings

Appearance

Factors Affecting FACThe effect of various operating factors can be

Figure 3. Location of singlephase & two-phase FAC.

Figure 4. Single phase FAC – orange peel appearance

Figure 5. Two-phase FAC - tiger stripes appearance


summarized as below:A) Temperature (Typical)

B) pH

C) Velocity

Salient Points1. FAC is temperature-dependent. Max corro-

sion observed at 150 C for single phase & at190 C for two-phase. Corrosion rate is less forCr-alloyed materials.

2. High pH reduces FAC. Above pH=9.0, corro-sion rate reduces drastically.

3. Velocity increases FAC. The rate of increaseis different for different material. CS is mostlyaffected. 2.25 Cr-1Mo is least affected by ve-locity.

4. Limiting velocity: Single phase – 5.2-6.1M/Sec, Two-phase – Approx 25 M/Sec (depend-ent steam fraction & pressure). Please notethat this is for straight piping. For U-bendsetc., a multiplication factor of 1.75 or more isto be used.

5. In two-phase FAC, sometimes, minor changesin operating pressure or feed-water flow candrastically change the FAC scenario. In loweroperating pressure, the steam fraction will bemore in volumes, which will increase thevolumetric flow rate. Similarly change infeed-water flow can change the steam frac-tion too.

Figure 6

Figure 7

Figure 8


D) Water ChemistryWater chemistry can be of three types

a) Water + Very low dissolved oxygen + Oxygenscavenger (ORP<<0 mv)

b) Water + Very low oxygen (O2 – 20ppb)(ORP> 0 mv)

c) Water + Low dissolved oxygen (O2- 20-80 ppb(ORP >> 0 mv)

For systems operating with conventional mixedmetallurgy (part CS/AS + Cu-based alloys like brass/Cu-Ni), only type (a) water can only be used as Cu-based alloys cannot tolerate oxygen at high tempera-tures. Type (b) and type (c) water chemistry, pioneeredby EPRI, is generally used in USA & Europe

The effects of different water types can be sum-marized as below: Figure 12 shows corrosion rates of different ma-

terials vis-à-vis dissolvedoxygen levels. Appreciablereduction in corrosion rate isobserved with dissolved oxy-gen level of 100 ppb or more.This can be utilized in sys-tems with no Cu/Brass com-

ponents (condensers etc.)HereSt 37.2 - SA 106 Gr. B15Mo 3 - C-0.5 Moly13 Cr Mo44 - P11/P12

Figure 9. The oxide structure for type (a).

Figure 10. The oxide structure for types (b) & (c).

Figure 11. Physical appearance of oxide structure.Black area – Magnetite (Type a water)Red area – Hematite (Type b & c water)

Figure 12. Effect of dissolved oxygen in type (b) and(c) water chemistry.


E) Effect of Material composition

Material composition has biggest effect on FACrates. Even traces of Cr, Cu, Mo can change thecorrosion rate drastically.

The resistance of material to FAC has been calcu-lated as follows :

RKEMA = 0.61 + 2.43 Cr + 1.64 Cu + 0.3 Mo

A R value of 0.92 and above provides satisfactoryresistance.

The report from Benson Group (now part of Sie-mens-Benson, Germany) gives practical guidelinesfor material selection.

For practical purpose, P-11/P12 and P-22 ap-pears to be the most suitable metallurgy. Thesematerials are less affected by temperature(Graph-A), pH level (Graph-B) and Velocity(Graph-C) as well as supported by Benson TestRig report (above).

Figure 13. Tubes from the high pressure preheater. FAC or Erosion-corrosion downstream from the butt welds:(A) Leakage (R = 0.81),(B) FAC (R = 0.84), and(C) No FAC (R = 1.01)

Figure 14

Surry Unit 2 (USA)18 inch elbow in a condensate line ruptured catastrophically in 1986.Four fatalities and several injuries

ConclusionA few well known examples of FAC are shown

below :

Mihama (Japan). A 22 inch condensate line ruptured Catastrophicallydownstream of an orifice in 2004. Five fatalities and several injuriesresulted


Failure in a high pressure extraction line at Fort Calhoun (USA) in 1997.No fatality.

Rupture of Feedwater heater shells at Point Beach 1 (USA) by FAC.

Some of the other reported failures on account of FAC or erosion-corrosion

Black Areas - Magnetite layers ….CorrodedRed Areas - Magnetite + Hematite – No corrosion

Troubles on Large Diameter Pipes in NPPsYear Unit /Company Location Pipe Diameter Consequence1986 Surry unit 2 USA 18” Pipe Rupture1987 Doel Beligium - (large dia) Rupture1990 Loviisa WER 1 Finland 12” pipe Rupture1990 Millstone Unit 3 USA 6” pipe Rupture1990 Millstone Unit 2 USA 8” pipe Rupture1993 Loviisa WER 2 Finland 12” pipe Rupture1993 Sequioyah Unit 2 USA 10” pipe Rupture1996 Maanshan Unit 2 Taiwan 12” pipe Rupture1997 Fort Calhoun USA 12” pipe Rupture1999 Point Beach 1 USA 4” Vessel Rupture1999 PWR Korea 16” pipe Severe Thinning2001 Callaway Unit 1 USA Vessel Nozzle Severe Thinning2001 Unknown USA 16” pipe 70% thinning2004 Ooi Unit 1 Japan 16” pipe 42% thinning2004 Mihama Unit 3 Japan 22” pipe Rupture

Appearance of Magnetite & Magnetite +Hematiteoxide layers in steam service

The possibility of cor-rosion of steam/conden-sate network by FlowAccelerated Corrosion(FAC) or Erosion-Corro-sion at Mathura Refineryis high. Repetitive failureshave taken place in vari-ous circuits.

To reduce the failurerate, the susceptible areasfor FAC to be identifiedand remedial measuresneed to be adopted. Fol-lowing remedial measuresare suggested.


a) Review of operating conditions, including veloc-ity, for steam and condensate network for locat-ing areas susceptible for FAC. Also look out forany changes in operating pressure or feedwaterflow which might change markedly change theFAC scenario.

b) The susceptible areas shall be checked specifi-cally for FAC related damage. This includes –inside & outside crowns of bends, downstream oforifice, valves etc.

c) Metallurgy upgradation with P-11/P12 and P-22(2.25 Cr-1 Mo) for the areas which are facingsevere FAC attacks and preparation of MoC ofthe same.

d) Reducing operating velocity in condensate net-work by increasing the pipe dia. in severely af-fected areas.

e) Review the possibility of use of oxygenatedwater in specific circuts where Cu-based al-loys are not in use. Oxygenated water can beobtained by adding DM water (2-5%) withBFW/condensate.

REFERENCES1. “FLOW ACCELERATED CORROSION” BY

R D. Port, Nalco Chemical Company, OneNalco Center, NapervilIe, IL, 60563,

2. IAEA Workshop on “Erosion-Corrosion in-cluding Flow Accelerated Corrosion and en-vironmentally assisted Cracking Issues inNPPs“ (21-23, April 2009, Moscow, RussianFederation).

3. Presentation by Jeff Anderson in 20094. NACE publications – 99346.pdf, 99347.pdf5. A r t i c l e s b y W . M . M . H u i j b r e g t s

(www.hbscc.nl)6. EPRI Publications7. Articles by Otaker Jonnas (steamcycles.com)8. Articles by Dooley et al.9. Presentation by Murat Bakirov Doctor, Gen-

eral Director of the “SCTC”CMSLM”, Russia,Moscow

10. Test-rig.pdf - Test results from Benson test-ing facility

11. B. Chexal, “Flow-Accelerated Corrosion inPower Plants”, TR-106611-R1, EPRI, 1998.

12. 2 J.S. Horowitz and D Munson, “Recommen-dations for an E_ective Flow Accelerated Cor-rosion Program”, NSAC-202L-R3, 1011838,

Enquiry Number 07/09-01HA

This publication thanks P L Chakraborty and M.Y. Bhave for providing this article.

P L Chakraborty is an InspectionManager for IOCL at Mathura Refin-ery. He holds a BE (Met) from VNIT,Nagpur and possesses 19 years expe-rience in health monitoring of equip-

ment, corrosion issues, root-cause analysis offailures, inspection and quality control of repairmaintenance/project job in oil refineries. He hasalso written and presented a number of papers.

M. Y. Bhave is presently DGM(M&I) at Indian Oil Corporation Lim-ited, Refinery Head Quarters, NewDelhi and holds a BE (Metallurgy).He has more than 28 years experience

in inspection and corrosion fields in oil refineriesand has hands-on experience in trouble shootingand failure analysis in metallurgical problems. Heis experienced in material selection and up-grada-tion suggestions, corrosion, welding, NDT andproject inspections. He has previously worked inIndian oil Corporation, at Gujarat, Guwahati,Mathura Refineries. He has presented many pa-pers at national and international conferences inIndia in the field of failure analysis, material selec-tion and corrosion. He is a member of IIW andISNDT and a fellow of Institution of Engineers

Have you readour other magazine?

see us on the web athttp://www.safan.com

EPRI Report, May 2006.13. 3 Wael Ahmed, “TRENDING FAC IN

CANDU REACTORS” COG-Report 07-4065,June 2007.

14. “Prediction of two phase erosion-corrosionin Bends” By Anthony Keating & Srdjan Nesic

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