Limiting Factors for Steam Parameters

Presented by,Nithin Seenivasan, 859867

FH Aachen, Jülich, 15th December, 2011

Contents of the Presentation

• Introduction• Main Steam Parameters• Various Limiting Factors for Steam Parameters

• Corrosion • Material Limitations• High Steam Velocity Limitations • Impure Feedwater• Fuel Heating Value Limitations

• References

What is Steam?!

• Technical term for water vapor, the gaseous phase of water, which is formed when water boils.

What is Steam?!

Source: Alliance Knowledge, 2011 Source: Petaflop Blog

Water SteamSteam + Water

Droplets

Water Vapor

Water Droplets +

Vapor

Heat

Condensation Condensation

What is Steam?!

• Technical term for water vapor, the gaseous phase of water, which is formed when water boils.

• Dry Steam is INVISIBLE

Main Steam Parameters

• Pressure • Temperature• Mass Flow of Steam

Source: Exponent Consultants

Example of an Exploded Chemical plant in Nevada, USA

40 Mpa (400 Bar)900 °C (1173 K)600 Kg/s

Limiting Factors of Steam Parameters

• Oxidation Corrosion• Fuel Ash Corrosion• Carburization• Hydrogen Damage• Creep• Fatigue• Erosion• Increased Losses• Water Hammer• Impure Feed Water• Fuel Heating Values

Corrosion

Material Properties

Temperature Pressure Mass FlowOxidation Corrosion x x xHydrogen Damage x xCarburization x xFuel Ash Corrosion x x xCreep x xFatigue x x xErosion x xIncreased Losses x x xWater Hammer x x xImpure Feed Water x x xFuel Heating Values x x

Steam Parameters

Lim

iting

Fac

tors

Limiting Factors of Steam Parameters

Corrosion

Source: Checkthishouse Inc

Oxidation Corrosion

• Oxidation rates of metals increases with Temperature• Typical Value for tubes - .025-.051 mm / Year• Parts built below Oxidation Limit

A high magnification optical microscope view of a cross-section through a failed Aluminum Tube reveals that a localized area of the coating on the ID surface has been replaced by oxide scale (arrow)

Source: Metallurgical Technologies Inc.

Hydrogen Damage

• Caused by Excessive Deposition and low pH of feedwater

• Increases at higher Temperature and Pressure • Hydrogen ions from water are adsorbed into Metal

Matrix causing Hydrogen Embrittlement

Brittle failure in a pipe due to hydrogen damage

Source: Babcock and Wilcox – Boiler Tube Analysis

Fuel Ash Corrosion

• Wastage of tubing caused by chemical reactions with the fuel ash

• Chlorides, Sulfur corrode Oxidation resistant surfaces • Ash forms a corrosive liquid layer on surface

Fuel Ash Corrosion of OD of a Superheater. Very thick scale was observed on the OD, with a significant amount of chlorine detected in the deposit

Source: Maintenanceworld.com

Carburization• Occurs due to incomplete combustion, giving rise to

CO and un-burnt fuel in Flue Gas• Chromium from surface of steel reacts with C to form

Chromium Carbides• Causes severe Embrittlement and reduced Corrosion

resistance

Source: Lifechem

Excessive temperature in combination with high carbon activity in the Furnace caused changes in microstructure of the tube, which resulted in severe embrittlement and premature failure.

Limiting Factors due to Material Properties

Source: Infograph

Creep• Creep is the tendency of a solid material to deform

permanently under the influence of stresses for a prolonged period of time

• Stresses may be lower than Yield Strength• Creep always increases with Temperature• Even if no other failure occurs in the tube metal,

creep will definitely occur

Plastic Paper Clip Metal Paper Clip

Fatigue

• Fatigue is the progressive and localized structural damage (crack propagation) that occurs when a material is subjected to cyclic loading

• In Steam generation, stresses due to Temperature differences in Startup and Shutdown cause fatigue. (larger the temperature, larger the chance of fatigue failure)

Fatigue failure in a Train axle Source: Wikipedia

Versailles train crash!

Increased Losses

• High-pressure steam leaks from piping, components, and pressure reducing valves

• High-pressure radiation loss for steam piping and components

Source: Super Shakti Sealing Services

High Pressure Steam Leak being repaired by a technician

Erosion

• Erosion is a function of Ash content in Fuel, Ash Composition and Gas velocity

• Coupled with Stresses, can result failure

Source: INSA Lyon Source: Pair O Docs LLC

Melting!

Source: Hackaday.com

Limiting Factor due to High Steam Velocity

Source: CWLP, Illinois

Steam Hammer

• Steam hammer is a pressure surge or wave resulting when Steam in motion is forced to stop or change direction suddenly.

• Can cause noise, vibration or Pipe Collapse• Due to high Fluid Velocities resulting from high

pressure

Source: Wikipedia

Steam Expansion joints destroyed due to a Steam Hammer (magnitude up to tune of 5000 Psi = 34.4737864 MPa)

Limiting Factors due to Impure Feedwater

Source: Water Rhapsody

Effects of Impure Feedwater

Scaling (Deposition of impurities on boiler walls)

• Hinders Heat Transfer and Steam Production• Higher Temperature and Pressure increases rate of scaling

Source: Steam – The Babcock & Wilcox Company

Internal deposits resulting from poor boiler water treatment. These deposits, besides hindering heat transfer, allowed boiler water salts to concentrate, causing corrosion.

Effects of Impure Feedwater- Scaling

• Power Output- 450 MW• Boiler- Natural Circulation Boiler• Steam Production- 378 kilograms per second

• Feedwater Hardness – 1 ppm (minerals and oxides) • Deposits in Boiler - 11,340 Kg per year!!

Source: Steam – The Babcock & Wilcox Company

Effects of Impure Feedwater

Corrosion • Oxygen Pitting Corrosion (DO – Dissolved Oxygen)• Acid Corrosion

Source: Power Corrosion (Consultants) LTD

An Economiser showing the effects of Pitting corrosion, caused due to Feedwater impurities. The tube was damaged by oxygen pitting and poor feed water chemistry.

Limiting Factor due to Fuel Heating Values

Source: Texas Tribune

Heating value of Fuels Used for Steam Generation

Source: Steam – The Babcock & Wilcox Company

More Fuels Compared

Source: BioMass Energy Center

Conclusion

• A huge number of Limiting Factors exist for Steam Parameters

• All the Limiting Factors have to be taken into consideration while designing a plant

• Careful maintenance has to be done to avoid preventable breakdowns due to a few Limiting Factors

Book References

• Steam/its generation and use, Edition 41 by The Babcock & Wilcox Company

• Standard Handbook of Powerplant Engineering, 2nd Edition by Thomas C. Eliott, Kao Chen and Robert C. Swanekamp

Internet References1. http://www.maintenanceworld.com/Articles/coalpowermag/Role-Fireside-Corrosion-Boiler-

Tube-Failures-2.html2. http://met-tech.com/steel-heater-tube-analysis.html3. http://www.lifechem.co.id/CorrType.htm4. http://www.cdcorrosion.com/mode_corrosion/corrosion_erosion_gb.htm5. http://www.pairodocspro.com/Pair_o_docs_failure%20investigation%20page.htm6. http://www.biomassenergycentre.org.uk/portal/page?

_pageid=75,20041&_dad=portal&_schema=PORTAL7. http://www.checkthishouse.com/3258/musty-smell-from-heating-system-air-ducts-forced-air-

heat.html8. http://www.infograph.eu/galerie/galerie.htm9. http://www.watersense.co.za/2011/03/28/dirty-water-for-many-parts-of-sa/10. http://www.texastribune.org/texas-energy/energy/despite-pollution-worries-texas-builds-coal-

plants/11. http://www.seas.columbia.edu/earth/wtert/sofos/Lee_Them_Cast_HighTempCorrosion.pdf12. http://www.seas.columbia.edu/earth/wtert/sofos/Lee_Them_Cast_HighTempCorrosion.pdf13. http://www.structint.com/files/public/fossil-plant-services/Hydrogen-Damage.pdf14. ftp://ftp.eia.doe.gov/features/steamgen.pdf15. http://www.ias.ac.in/sadhana/Pdf2003JunAug/Pe1102.pdf (Hydrogen Embrittlement)

Thank you!