Basic STP (Sewage Treatment Plant) Formula Sheet 1 Flow Rate Calculation Determines the volume of sewage entering the plant per day. Formula: Q = V / t Where Q = Flow rate (m³/day), V = Volume (m³), t = Time (days) 2 BOD Load Calculates the organic load based on Biochemical Oxygen Demand. Formula: BOD Load = Q × BOD Q = Flow rate (m³/day) BOD = mg/L 3 COD Load Estimates chemical oxygen demand for treatment sizing. Formula: COD Load = Q × COD COD = mg/L 4 MLSS Concentration Mixed Liquor Suspended Solids concentration in aeration tank. Formula: MLSS = (Weight of solids / Volume of sample) Expressed in mg/L 5 F/M Ratio Food to microorganism ratio for biological treatment efficiency. Formula: F/M = (Q × BOD) / (V × MLSS) Q = Flow rate, V = Aeration tank volume 6 Sludge Volume Index (SVI) Indicates settling characteristics of sludge. Formula: SVI = (Settled sludge volume in mL/L × 1000) / MLSS 7 Oxygen Requirement Calculates oxygen needed for biological oxidation. Formula: O₂ Required = BOD...
WELDING OF CARBON STEEL 🔥
Welding of carbon steel is widely used in pressure vessels, pipelines, structures, fabrication industries, and power plants.
During welding, rapid heating and cooling significantly change the microstructure and mechanical properties of steel.
Understanding the Heat Affected Zone (HAZ), Fusion Zone, Fe–C Phase Diagram, and welding thermal cycles is essential for:
✅ Controlling weld quality
✅ Preventing defects
✅ Improving structural reliability
🔹 Key Regions in Weld Joint
📌 Fusion Zone (FZ)
• Completely melted weld area
• Highest temperature region
• Determines weld strength and fusion
📌 Partially Melted Zone (PMZ)
• Base metal partially melted
• Located between FZ and HAZ
• Highly sensitive to cracking
📌 Heat Affected Zone (HAZ)
• Heated but not melted
• Microstructure changes occur
• Affects hardness and toughness
📌 Base Metal (BM)
• Unaffected parent material
• Original properties retained
• Provides structural support
🔹 Important Metallurgical Concepts
📌 Fe–C Phase Diagram
• Iron-carbon transformation diagram
• Shows phase changes with temperature
• Crucial for welding metallurgy
📌 A1 Temperature (~723°C)
• Lower critical temperature
• Austenite starts forming
• Controls phase transformation
📌 A3 Temperature
• Upper critical temperature
• Complete austenite formation occurs
• Important for heat treatment
📌 Liquidus Temperature (TL)
• Metal becomes liquid
• Forms weld pool
• Controls penetration
🔹 Welding Thermal Cycle
• Rapid heating and cooling occur during welding
• Short time at high temperature creates localized changes
• Cooling rate directly affects hardness and microstructure
🔹 Important Phases in Carbon Steel
📌 Austenite (γ)
• High-temperature steel phase
• FCC crystal structure
• Influences hardenability
📌 Ferrite (α)
• Soft and ductile phase
• BCC crystal structure
• Improves toughness
📌 Cementite (Fe₃C)
• Hard iron carbide phase
• Increases hardness
• May increase brittleness
🔹 Residual Stresses & PWHT
📌 Residual Stresses
• Caused by uneven cooling
• May lead to distortion and cracking
• Requires stress control measures
📌 PWHT (Post Weld Heat Treatment)
• Controlled heating and cooling process
• Reduces residual stress and hardness
• Improves weld reliability and service life
🔹 Typical Welding Defects
⚠️ Cracking
⚠️ Porosity
⚠️ Lack of Fusion
⚠️ Slag Inclusion
⚠️ Undercut
⚠️ Distortion
🔹 Key Welding Takeaways
✔️ Control heat input and travel speed
✔️ Use proper preheat and interpass temperature
✔️ Follow approved WPS and PQR
✔️ Select suitable electrodes and consumables
✔️ Apply PWHT where required
✔️ Maintain cleanliness and joint preparation
Understanding welding metallurgy is not just about making a weld — it is about ensuring safety, reliability, and long-term performance of engineering structures.
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