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, 2011Source: Petaflop Blog WaterSteam Steam + Water Droplets Water Vapor Water Droplets + Vapor Heat 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
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of a cross-section through a failed Aluminum Tube reveals that a localized area of the coating on the ID surface has been replaced by oxide scale (arrow) Source: Metallurgical Technologies Inc.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, 2 nd Edition by Thomas C. Eliott, Kao Chen and Robert C. Swanekamp