Boiler Pressure Parts & Tube Failure

By

Sri A.Prabhakar Rao

12.03.2010

BOILER PRESSURE PARTS

AND TUBE FAILURES

P > 1 Kg / cm2

EconomizerSuper HeatersRe-HeatersWater WallsSafety valvesDe-Super heaters andBoiler Drum

TO PREHEAT FEEDWATER

TO RECOVER HEAT FROM FG LEAVING BOILERFinned – staggered

Bare tube – inline

CARBON STEEL

Coal can be saved from 15% to 20%. Increase in 1% Thermal Efficiency for every

6 C change in temperature. Feed water changes to steam quickly. Increases Boiler Life. Decreases thermal stress of Boiler Internal

Parts. Decrease in combustion rate.

FUNCTION:It increases the temperature of Main

steam with the help of temperature of flue gases to get Saturated Steam

admitted to the HPT.

PENDANT SPACED SECTION-located behind the screen section – heat transfer by convection.

PLATEN SECTION- located above the furnace-heat absorption by radiation.

REAR HORIZONTAL SECTION- located in second pass- convective counter flow.

STEAM COOLED WALL- second pass enclosure.

ROOF SECTION- second pass roof

Super heats the steam from Boiler before admitting it to turbine.

Removes the moisture contents from the steam to avoid the corrosion and breaking of turbine blade tips.

FUNCTION:

It heats the temperature of steam outlet from HPT with the help of Flue gas

temperature.

Re-heats the steam to increase the thermal efficiency.

Increases the energy in the steam to perform additional work before exhausting into condenser from LPT.

FUNCTION:

Water walls carry feed water from ring headers to Boiler Drum through raiser

tubes.

Increase in efficiency.

Better heat transfer.

Easy and quick erection.

Increased availability of Boiler.

FUNCTION

These are used to safe guard the equipment in case of emergencies.

FUNCTION:It controls the main steam temperatures to

safe limit.

FUNCTION:

It separates the steam from steam-water mixture.

It houses all equipments used for purification of steam, after being separated from water.

Stores the DM Water.

Limits the Solid contents.

Facilitate in adding chemicals to maintain pH value.

To facilitate Blow down.

ASTM Max. Temp. C

(Oxidation Limit)

SA 210 Gr. A1 Carbon Steel C0.27% Mn

0.93%P.035 S0.10%

425

SA 209 T1 ½ % Mo steel C 0.10to

0.20%Mn

0.30to0.80%P0.025Si0.025

s

480

SA 213 T11 1 % Cr. ½ % Mo 550

SA 213 T22 2 ¼ % Cr. 1 % Mo 580

SA 213 TP 304 H 18 % Cr. 8 % Ni

(Stainless steel)

700

SA 213 TP 347 H 18 % Cr. 10 % Ni 700

By

Sri A. Prabhakar Rao

Very high expectations on performance

Fuel quality deterioration

New / imported fuels coming to use

Cheaper power to public – Govt

Availability based tariff

Achieving higher availability

Optimal performance of the units

Ability to vary load & meet varying targets

Optimizing combustion for varying fuel characteristic

Better heat rate and low cost power

Lower pollution levels

Better position in ABT

Low gestation period Enhancing life of the plant

Better heat rate and low cost power

Lower pollution levels

Better position in ABT

Low gestation period Enhancing life of the plant

Making the optimal plant heat rate

Handling the fuel quality change

Operational deviations

Reducing the damage mechanisms

•Availability and reliability of boiler

decreases with increased tube failures.

•Tube failure results in forced outages and

hence direct impact on availability.

Boiler Tube Failures - main cause offorced outages in electric utility steamgenerating boilers

Single tube Failure in a 500 MW Rs. 6to 7 Cores (replacement ,power chargesfor 3-4 days ,to repair) besides affectingPlant Morale.

Stress Rupture Short Term Overheating

High Temperature Creep

Dissimilar Metal Welds

Fatigue Vibration

Thermal

Corrosion

Water-side Corrosion Caustic Corrosion

Hydrogen Damage

Pitting

Stress Corrosion Cracking

Erosion Fly Ash

Falling Slag

Soot Blower

Coal Particle

Fire-side Corrosion Low Temperature

Waterwall -

Coal Ash -

Oil Ash

Lack of Quality Control Maintenance cleaning damage

Chemical excursion damage

Material Defects

Welding Defects

- indicates that such problems have not been reported in India

Steam / Water cooled tubes

Plugged by debris, scale etc.

High Heat Transfer / Improper firing

Low water/steam flow due to poor circulation / upstream leak

Corrective Action Prevent Blockage Maintain Drum

level Assure Coolant

flow Reduce over firing Redesign tubing

to promote flow Relocation of

horiz. / inclined tubes to avoid film boiling

Typical Locations

Steam cooled Tubes◦ Partially choked

◦ Radiant Heat Zone

◦ Gas Blockage

◦ Incorrect Material

◦ Material Transition

◦ Higher stress due to

◦ weld attachment

Corrective Action

RLA/ IOT

Fluid flushing

Material up-grades

Typical Locations

At SH / RH dissimilar weld joints :

Temperature / Stress excursions

Corrective Action Repair/Replacement

Relocating the weld

Use of Ni-base filler

Frequent inspection

Mechanism : 1. The formation of carbon depleted zone on the ferritic side of the

transition from the ferritic to austenitic structure is the initial step and any

treatment which enhances the formation of this zone will enhance the failure

probability.

2. The carbon depleted soft feerritic zone is constrained by the sorrounding harder

and stronger material and is subjected to strains induced by thermal expansion

mismatch, bending, vibration and pressure.

3. The strain accumulation in the carbon-depleted zone is relieved by creep at

elevated temperature.

4. Creep damage in the form of cavitation, grain boundary sliding and tearing

results in cracking in the carbon depleted zone along and adjacent to the weld

interface

Damage may result from high pH corrosion reaction.

NaOH removes protective magnetite iron oxide layer Fe3O4.

Iron react with water or NaOH eating away the parent metal.

It is also called caustic gouging or ductile gouging.

Typical Locations Water-cooled Tubes:

◦ At flow disruptions

◦ Horiz / inclined tube

◦ High Heat flux zone

◦ Flame impingement zone

Probable Root Cause Concentration of NaOH from

boiler water chemicals

Feed water system corrosion deposits

Condenser leakage

Temp. increase due to internal deposits

Corrective Action

Control Boiler Water Chemistry

Reduce corrosion product ingress

Chemical cleaning

Reweld irregular welds

Use T11 type steel or rifled tube

Hydrogen damage may occur where corrosion reaction results

in the production of atomic hydrogen. Damage may result from

Low pH corrosion reaction.

NaOH removes protective magnetic iron oxide layer Fe3O4

Iron react with water or NaOH liberating atomic hydrogen

Atomic hydrogen diffuses into Iron carbide producing methane

gas.

Methane or Atomic H2 cannot diffuse, it accumulates resulting in

cracks at grain boundaries.

Longitudinal burst occur with thick lip

Typical Locations Water-cooled Tubes:

◦ At flow disruptions

◦ Horiz / inclined tube

◦ High Heat flux zone

◦ Flame impingement zone

Probable Root Cause Concentration of acidic salts

and low pH water chemistry

Condenser leakage and ingress of corrosion products

Feed water system corrosion deposits

Chemical cleaning contamination

Corrective Action Control Boiler Water

Chemistry

Check corrosion product ingress

Chemical cleaning

Replace affected tubes

Pitting corrosion is a localized accelerated attack, resulting in the

formation of cavities around which the metal is relatively

unattached. Thus, pitting corrosion results in the formation of

pinholes, pits and cavities I the metal. Pitting is, usually, the result

of the breakdown or cracking of the protective film on a metal at

specific points. This gives rise to the formation of small anodic and

large cathodic areas

Typical Locations SH, RH-at regions of :

◦ conc. of chlorides, sulphates or hydroxide

◦ stressed in fabrication, service etc. like bends,attachment welds

Probable Root Cause

Corrosive conc. From drum carry-over or attemperator spray

SS Tube material sensitized

Stresses

Corrective Action Replacement

Surveillance for carry-over

Heat Treatment of bends

Care during chemical cleaning

Use of 347H tube

Typical Locations Gaps between tube banks

and duct walls.

Gas by-pass channels

Protrusions of rows.

Areas close to large ash accumulation.

Probable Root Cause Non-uniform, excessive gas

flow with fly ash particles.

High ash coal with -quartz.

Tube misalignment.

Corrective Action

Changing operating conditions like reduced load, low excess air etc.

Protections like shields, baffles etc.

Flow Model study

Information and data concerning the tube failure must be

gathered quickly before repair activities can begin.

Failure descriptions, operating conditions at the time of

failure, historical records, and tube samples must be acquired

and transferred to others who will conduct the investigation

while repairs are being performed.

Immediate corrective actions based on the initial results of the

investigation must be approved and implemented before repairs

are completed.

Follow-up corrective actions based on the complete results of

the investigation must be planned and implemented before

additional failures are experienced.

Design stage:

Selection of material,

• Compatible for working pressure / temperature

• Steam flow and the velocity / pressure

• Heat transfer characteristics/ surface effectiveness

• Metal temperature/Thermal expansion / constraints

• Radius of bends

• Attachments/ Weldments

• Manufacturing aspects

• Transportation / Handling

• Storage

• Erection

• Commissioning

• Operation and Maintenance

Lower flue gas velocity over tube banks

Plain tube in-line arrangement of heattransfer surface

Optimum end gaps to avoid preferentialgas flow

Erosion shields / cassette baffles

Erosion allowance for leading tubes

Higher flexibility in SH / RH nipples

Redesigned flexible connectors for pendant type SH coils

Improved supports for LTSH / Eco. Coils

Improved seal plate connection for bottom hopper

Modified LTSH inlet tube connection

FLUE GAS (FLY ASH) EROSION

Extensive inspection of Economiser / LTSH / screen tubes/

re-heater for erosion prone areas,Verifying the condition of

existing shields and baffles, LTSH supply tube refractory

conditions.

Mapping of thickness and identifying areas / locations for

repair / replacement.

Baffling / shielding at the points of erosion prone areas to the

maximum extent possible.

Changing operating conditions like reduced load, low excess

air etc.

Flow model studY

SB STEAM EROSION Ensure condensate free steam with a minimum

superheat of 15 C at blower.

Necessary gradient/downward slope of the SB pipingis to be ensured. 1 per meter length of pipe is to bemaintained.

Through thickness survey of WW in the SB location forthree meter radius and replacing the eroded tubes.

Ensuring wall blower nozzle alignment.

Temporary shielding / spraying.

Installing thermal drain system if not available.

FALLING SLAG EROSIONCheck the fuel characteristics for fouling.Change in fuel if warranted.Tuning the boiler air regime for optimised

combustion to avoid fouling.Welding of wear bars at the bottom

S-panel tubes to break the ash bouldersand to avoid direct hitting of the tubes.

Increase tube wall thickness.

LONG TERM OVERHEATING (CREEP) Maintaining & monitoring the metal

temperature within limits. Ensuring adequate flow through tubes. Following the start up curves for rate of

firing. Assessing the life thro' oxide scale thickness

/CRT by sampling to understand the extentof overheating.

Tuning of boiler viz. Excess air, temp. Etc. Mateiral up-grades. Strict quality control during tube

replacements to avoid foreign materialentry.

SHORT TERM OVERHEATING Ensuring adequate flow thro’ tubes.

Preventing blockage.

Maintaining the drum level.

Reduce over firing.

Avoid foreign material entry while

maintenance.

DMW JOINT FAILURE Replacement of DMW Joints with life >

1,00,000 hrs of operation.

Replacement of DMW joints with spool

pieces fabricated at shop.

Relocating welds away from highly

stressed points.

HYDROGEN EMBRITTLEMENT

WATER SIDE CORROSION / CAUSTIC GOUGING.

Maintaining / monitoring the water chemistry guidelines.

Avoiding corrosion products ingress.

Controlling copper deposition thro’ condenser leakage.

Chemical cleaning of the boier whenever deposit quantity is > 40 mg /cm2

Carrying out the in-situ hydrogen embrittlement survey.

Replacing the affected tubes

INTERNAL / O2 PITTING

Ensuring proper operation of de-aerator.

Control feed water oxygen levels to <10 ppb at CEP out let.

Preservation of the boiler during short /long outages by dry / wet preservation methods.

FRETTING / RUBBING. Ensuring proper expansion gapsAvoiding sharp corners of the support

lugs. Ensuring proper positioning of flexible

connectors. Giving adequate provision for relative

movement of pressure part tubing like steam cooled spacer and SH/RH tubes.

FATIGUE Ensuring the proper flexibility.

Removal of tie welding if it is done wrongly

Redesign of attachment to reduce restraints to thermal expansion.

Restriction on cyclic operation.

Heat treatment and contouring of welds.

WELDING DEFECTS

Ensuring the qualified welder.

Adopting proper quality control procedures.

Process controls.

Replacement of defective joints.

Preventive actions.

RLA of pressure parts / boiler.

Planning for replacement of pr. Parts sections in total for Economiser,LTSH,Re-heater, etc.

Carrying out IOT / CRT for SH & RH tubes.

H2- embrittlement survey of WW tubes.

Installing advance system like Smart wall blowing & Acoustic steam leak detection system etc. for reducing wall blowing frequency and to avoid secondary damages.

Determination of the correct failure mechanism is a complex

process which can involve many individuals and organizations.

Technical specialists in metallurgy, chemistry, combustion, and

boiler design are often called in to assist in a failure

investigation. The plant’s personnel must provide the initial

information on the failure and boiler conditions prior to the

failure. The plant’s operating records and failure histories

must be in order so that pertinent data may be extracted. The

plant’s management and technical staff must follow up on the

failure investigation and implement the corrective actions

required to correct the problem.

By incorporating joint Task force between the plant owner and

boiler designer / manufacturer/metallurgists/Experts the tube

failures can be prevented/reduced and the

availability/reliability can be increased.

Thank You