E1. Boiler Tube Failure Part 1

8/10/2019 E1. Boiler Tube Failure Part 1

1/54

1

Boiler Tube Failure

Indonesia Customer Seminar

June 13 & 14 2012

Jakarta Indonesia


2/54

Purpose, Process & Pay Off

Purpose: To share proper identification of tube failure mechanisms & root cause of

Boiler Tube Failure

Process Presentation & discussion

Pay Off Higher plant reliability & availability

2


3/54

Topics

Tube Failure - EPRI Survey

Road Map for Analyzing Tube Failure

Tube Failure Mechanisms & Root Causes

Determine extend of damage Feature

Mechanisms

Location

Root cause and action to confirm

Case history

Recent boiler tube failure in the region

3


4/54

Tube Failure


5/54

Mechanisms, Root Causes & Solution

5

Mechanisms Root Causes Solution


6/54

The Guide Line

6


7/54

EPRI: Road Map for Analyzing HRSG Tube Failure


8/54

Boiler Tube Failure Mechanisms

Fatigue Corrosion Fatigue

Mechanical/Thermal Fatigue

Flow Accelerated Corrosion

Under Deposit CorrosionAcid Phosphate Corrosion

Caustic Corrosion

Hydrogen Damage

Overheating Short term overheating

Long term overheating

8


9/54

Confirm the Mechanisms

Location

Fracture

Deposit Analysis

Mechanical, Operation &Chemical related factors

Metallurgical analysis

9


10/54

Fatigue

Fatigue damage occurs when tubing is subjected torepeated cyclic loading that produces nominal stress level

Boiler tubes may be subjected to cyclic stresses resultingfrom:

Pressure fluctuations Temperature transients and restriction of expansion

Fluctuating mechanical loads

Forces induced vibration

10


11/54

#1 Corrosion Fatigue

Result of a combination of both repeated cyclic stress and acorrosive environment

Characteristic or rate is influenced by corrosiveenvironment

11


12/54

#1 Corrosion Fatigue: Features

Cracks Initiation from inside surfaces

Multiple, parallel cracks

- Tube-to-header: circumferential

- Bends: axial

- Attachment: multidirectional

Often associated with pits

Not specifically related to thepresence of weld discontinuities

12


13/54

#1 Corrosion Fatigue Mechanisms

Break down of magnetite film

Pitting

Crack-like-pits Crack growth through repeated

mechanical disruption or chemicaldissolution and reforming of theoxide

13


14/54

#1 Corrosion Fatigue - Location

Water touched tubes but may occur in all other sectionsof tubing including steam-touched tubing that, duringoperational transients, contains condensate.

Most likely locations:

Welded connections Bends

Attachment

14


15/54

#1 Corrosion Fatigue Location

May also occur in steam touched tubes that duringoperational transients, contain condensate Superheater/Reheater, frequently off-line

Not implementing proper lay up

15


16/54

#1 Corrosion Fatigue: Location & Crack Type

16

Source: EPRI, Heat Recovery Steam Generator Tube Failure Manual, 2002


17/54


Root Causes & Action to Confirm

Influence of Excessive Stresses/Strain Visual examination

Field test with thermocouple

Infinite element stress

NDE, selective tube sampling

Influence of Environmental Factors Low pH situation

High dissolved oxygen (operation-startup)

Pitting corrosion (tube sampling)

17


18/54



Improper chemical cleaning Selective tube sampling

Improper shutdown/start up and lay up procedure Follow the EPRI/VGB guide line

Excessive DO not happened during start up

Influence of Unit Operation Operating hours and starts

Service hours

No of start/stop and characteristic

18


19/54

#1 Corrosion Fatigue: Case History

Case History

Industry: Pulp & Paper CogenerationLocation: Superheater near outlet header

Orientation: Vertical

Tube metallurgy: Low alloy steel

Drum pressure: 86 bar

Treatment Program: Coordinated Phosphate

First superheater failure in the plant.

Microstructural examinations of the tube wallconfirmed the presence of families of un-branchedtransgranular crack near the fracture indicatingcorrosion fatigue mechanisms.

The circumferential orientation of the cracks

reveals that the stresses responsible were cyclicbending stress, possibly caused by thermalexpansion and contraction of the tube.

In-proper start/stop operation and lay up couldinitiate the corrosion fatigue mechanisms.

Source: R.Port, The Nalco Guide to Boiler Failure Analysis, Mc Graw Hill, Inc.,1991

20


20/54

#2 Thermal-Mechanical Fatigue

Occur when the thermal expansion or contraction oftubing or parts are sufficiently restricted

The magnitude of thermal expansion (& correspondingstrains) in tubes and pipes at connection to headers is

influenced by the rate of heating and cooling

20

21


21/54

#2 Thermal-Mechanical Fatigue: Features

Cracks Initiation from gas side (outsides)

Single cracks are most common

- Tube-to-header: circumferential

- Bend: circumferential/axial:

- Oriented to tensile stress

Often associated with surfacediscontinuities as weld undercut

21

22


22/54

#2 Thermal-Mechanical Fatigue: Mechanisms

Thermal expansion or contraction isrestrained sufficiently to producelocalized yielding of the material

When these cycles are repeated,crack initiation and growth will occur

The magnitude of the local stressrange is the dominant attribute that

determine if and when thermal-mechanical fatigue cracks will occur

22

23


23/54

#2 Thermal-Mechanical Fatigue - Location

All sections of Boiler (water &steam touched) Most likely failure locations:

Welded connection

Attachment

Bends

23

24


24/54


High Thermal Transient in Horizontal HRSG

Temperature difference of HP SH/RH leading row tubescompared with the trailing rows attached to the sameheader

Failure to remove all the condensate from lower sections ofSH/RH prior the start up

Air or steam vapor builds in the upper return bends ofeconomizer (wit upper return bends)

24

25


25/54


Tube to Tube Temperature Difference in RH

25


26


26/54


Failed to Remove All Condensate

Firing boiler too fast resulting in uneven boiling out of SH

tubes during start-up. Especially after performing a hydro

Uneven boiling out of condensate from RH tubes.

26

Source : F.Starr, HRSG System and Implication for CCGT Plant Cycling, OMMI (Vol 2, Isue 1), April 2003

27


27/54



Excessive stresses/strain factors Visual examination

Field test with thermocouple

Infinite element stress

NDE, selective tube sampling

Influence of Unit Operation Operating hours and starts

Operating procedures high stress

- Start up/shut down procedure- Particularly cold start

27


28/54

#3 Flow Accelerated Corrosion (FAC)

Mechanisms that has caused metal losses and failures in

piping due to dissolving of protective magnetite layer(Fe3O4)

Occur under specific conditions of: Flow

Water chemistry Geometry

Material

Relatively narrow temperature range

FAC is not a significant concern in mixedMetal system. Copper is considered a factor in

Reducing the FAC potential


29/54

#3 Flow Accelerated Corrosion

Location : Temperature Dependent

30


30/54

#2 Flow Accelerated Corrosion: Features

Thin-edged

Single Phase FAC Orange-peel appearance

Chevron or horse shoe toward the flow

Two Phase FAC Scalloped and wavy

Often black & shiny

30

Source: EPRI, Guidelines for Controlling Flow Accelerated Corrosion

in Fossil and Combined Cycle Plants

31


31/54

#2 FAC Single Phase Features

Source: EPRI, Guidelines for Controlling Flow Accelerated Corrosion

in Fossil and Combined Cycle Plants

32


32/54

#3 FAC Two Phase Features

Condenser wall & Tubes

33


33/54

#3 FAC Two Phase Features

Deaerator

Source: EPRI, Guidelines for Controlling Flow Accelerated Corrosion in Fossil and

Combined Cycle Plants

34


34/54

#3 Flow Accelerated Corrosion Mechanisms

Source: EPRI, Guidelines for Controlling Flow Accelerated Corrosion in Fossil and

Combined Cycle Plants

35


35/54

#3 Flow Accelerated Corrosion Mechanisms

Source: H.G. Seipp, Damage in Water/Steam Cycle-Often Matter of Solubility, PP Chem 2005 (7)

36


36/54

#3 Flow Accelerated Corrosion: Mechanisms

37


37/54

#3 Flow Accelerated Corrosion


High reducing condition ORP < -300 mV

DO < 1 ppb

Iron is high in LP Evaporator

Entrained water droplets (2 phase FAC)After 1 phase FAC is eliminated & high iron persist


38/54

#3 Flow Accelerated Corrosion: Case History

Case History

Industry: Power plant-HRSG

Location: LP Evaporator, riser


Tube metallurgy: Carbon steel

Treatment Program: All Volatile (ammonia +

hydrazine)

The failure developed in the bend of the riser tubenear the upper collector of the drum.

The failure was caused by stress rupture of theobviously thinned wall in the outer bend of thetube. The orange peel or scalloped, appearancetypical of single phase FAC is evident.

Water chemistry: Dissolved oxygen 50 ppb)


39


39/54

Deposit

Deposits are needed before many tube failure mechanisms

become active

Deposit characteristic may influence the rate of corrosion &extend of damage

Tube failure mechanisms which involve water side depositsare:Acid Phosphate Corrosion

Caustic Gouging

Hydrogen Damage

Short Term Overheating

Long Term Overheating

40


40/54

#4 Acid Phosphate Corrosion

Occur when tube deposits formed from feed watercorrosion products allow a concentration of phosphatesalts of low sodium-to-phosphate ratio

This leads to under deposit corrosion & eventually totube failure

Very much a potential problemPhosphate hide out problems

41


41/54

#4 Acid Phosphate Corrosion: Features

Thin edged fracture Ductile rather than brittle

Thick layer of deposits Distinctive layer of maricite

(NaFePO4) deposits

No microstructuraldecarburization

Unit using mono and/or di-sodium phosphate chemical

42


42/54

Acid Phosphate Corrosion Features


43


43/54

#4 Acid Phosphate Corrosion-Mechanisms

Phosphate Hide Out

44


44/54

#4 Acid Phosphate Corrosion- Mechanisms


#4 A id Ph h t C i L ti45


45/54

#4 Acid Phosphate Corrosion - Location

Water flow is disrupted

Welded join

Internal deposition

Thermal hydraulic flow disruption

- Local steam blanketing Overheating of the tube

#4 A id Ph h t C i46


46/54

#4 Acid Phosphate Corrosion


Excessive deposits High iron in BFW and evaporator dirty boiler systems

Selective tube sampling

Flow disruption


Gas side Tube temperature measurement

Improper cycle chemistry Phosphate hide-out

Disodium/Monosodium PO4 addition

47


47/54

#5 Caustic Gouging

Occur when caustic concentrate within tube deposits fromfeed water corrosion product resulting very high pH

Under such conditions, protective magnetite layer isdissolved and rapid corrosion of the tube is occurs

48


48/54

#5 Caustic Gouging

49


49/54

#5 Caustic Gouging: Features

Tube wall thinning Thin edged fracture Pinhole

Thick, layered deposits Distinctive crystals of sodium

ferroate (NaFeO2) and/or sodiumferroite (Na2FeO2)

No microstructuraldecarburization

50


50/54

#5 Caustic Gouging:Features

Source: B. Dooley, PPChem101-Boiler and HRSG Tube Failure: Caustic Gouging, PP Chem 2010 , 12(2)

51


51/54

#5 Caustic Gouging: Mechanisms


52


52/54

#5 Caustic Gouging : Mechanisms

#5 Caustic Gouging53


53/54

#5 Caustic Gouging


Excessive deposits High iron in BFW and evaporator excessive porous iron deposits


Flow disruption Selective tube sampling

Gas side issue Tube heat flux & temperature measurement

Excessive caustic concentration Pretreatment up set/contamination

Improper PO4or AVT or Caustic treatment

#5 Caustic Gouging: Case History


54/54

#5 Caustic Gouging: Case History

Case History

Industry: Power plantLocation: Back wall


Pressure:103 bar

Tube metallurgy: Carbon steel

Treatment Program: Coordinated Phosphate

Time in Service: 6 years

Numerous caustic attack on the ball wall of acyclone-fired boiler were all observed within amonth.

42% reduction in tube wall thickness.Microstructural examination disclosed moderate

overheating in the gouged region. Evidencerevealed that DNB, rather than deposits, wasresponsible for caustic corrosion in this case. Overfiring during start-up and low flow rate of the feedwater were suspected.

Documents

E1. Boiler Tube Failure Part 1