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Showing posts with label power plant maintenance. Show all posts
Showing posts with label power plant maintenance. Show all posts

What is the significance of post weld and Pre-weld heat treatments (PWHTs)?

 













Post weld Heat Treatment (PWHT):

Post Heating in welding is a process where heat is applied to a welded joint after welding to control the cooling rate and reduce the risk of cracking.

When to carry out post weld heat treatment?

It is commonly used in materials that are prone to hydrogen-induced cracking, such as high-carbon steels, alloy steels, and cast iron.

What is the significance of post weld heat treatment?

Importance of post weld heat treatment

Prevent Hydrogen Cracking: Slows down cooling to allow hydrogen to diffuse out, preventing cracks.

Improve Mechanical Properties: Enhances toughness and ductility.

Reduce Residual Stresses: Helps relieve stresses induced by rapid cooling.

Improve Metallurgical Structure: Minimizes hardness variations and prevents brittle micro structures.

What actually is being done in post weld heating process?

In post weld heat treatment, the welded component is heated to a specified temperature say 450 deg C to 700 deg C. This temperature is maintained for a certain duration up to 6 to 8 hours based on material composition and its thickness, later slow cooling is ensured, often using insulation or controlled cooling methods.

What are the different methods of post weld heat treatment (PWHT)?

Torch/Gas cutter Heating: Using an oxy-fuel or propane torch for localized heating.

Furnace Heating: Placing the component in an oven or furnace for uniform heating.

Electric Resistance Heating: Using heating pads or ceramic heaters.

Induction Heating: Applying electromagnetic induction for controlled heating






















What is the criteria for post weld heating or heat treatment (PWHT) or heat treatment of carbon and alloy steels as per IBR regulations?

Actually, arc welded butt joints should be post weld heat treated effectively except in the following cases;

For high pressure alloy steel post weld heat treatment (PWHT ) is not done

1-If 0.5% Molybdenum steel having thickness & Tube OD are < 13 mm and < 127 mm respectively

2- If 1 Cr & 0.5 % Molybdenum steel having thickness & Tube OD are < 13 mm and < 127 mm respectively and it is been pre-heated to 125 deg C

3-In case of 2.25 Cr & 1 Molybdenum steel, PWHT is not necessary in following conditions;

a-Maximum Chromium content is 0.3%

b-Maximum Nominal out side diameter is 102 m

c-Maximum thickness of the alloy steel is 8 mm

d-Maximum specified carbon content 0.15%

e-Minimum pre-heated temperature is 150 deg C

For Carbon steel post weld heat treatment (PWHT ) is not done if;

1-Maximum Carbon percentage is 0.30%

2-Maximum thickness is 9 mm

Pre- weld heat treatment

What is the significance of Pre- weld heat treatment?

Pre-weld heat treatment, often referred to as preheating, is a critical process applied to the base material before welding.

Its main goal is to prepare the metal for welding by ensuring that temperature gradients are minimized, thereby reducing the likelihood of thermal stresses, cracking, and other welding defects.

Importance of Pre-weld heat treatment

To reduce or remove moisture content in the materials: Preheating helps evaporate moisture from the base metal, which is particularly important for materials prone to hydrogen embrittlement.

Reduce Thermal Stress: Preheating helps in minimizing the rapid temperature changes that occur during welding. This gradual change reduces residual stresses in the weld and surrounding base metal.

Prevent Cracking: By slowing down the cooling rate, preheat treatment decreases the risk of hydrogen-induced or cold cracking, especially in materials with high carbon content or high strength.

To get quality weld: A controlled preheat ensures that the weld area is at a uniform temperature, improving the fusion between the base material and the weld metal, leading to a more sound joint.

When to use Pre-weld heat treatment process?

Generally applied for Alloy steels, high carbon steels & Cast irons where there is a possibility of cracking due to rapid cooling.

Pre-heating is also done during rainy seasons or cold climate conditions to remove moisture from the steel.

What is the temperature for Pre-heating?

The required preheat temperature varies with the material type and its carbon or alloy content. For example, mild steels might require preheat temperatures of around 100–150°C, while high-carbon or high-strength steels might need 200–300°C or higher.

 

What are the different methods of Pre heat weld heat treatment ?

Torch/Gas cutter Heating: Using an oxy-fuel or propane torch for localized heating.

Electric Resistance Heating: Using heating pads or ceramic heaters.

Furnace Heating: Placing the component in an oven or furnace for uniform heating.

Induction Heating: Applying electromagnetic induction for controlled heating

In most of the applications gas heating is done.
























Tips for best Pre-weld heat treatments:

1-Uniform heating: Select the weld area to be pre-heated, apply the heat uniformly thought the selected area to avoid the development of localized temperature gradients.

2-Proper controlling of Temperature: Ensure pre-heating temperature is within limit. Make use of calibrated temperature sensors.

3-After completing the welding: Allow the metal to cool slowly. Faster cooling again induces stresses and even form cracks.

For conducting pre & post weld heat treatments skill of welder & welding team is also plays a vital role.


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How to calculate the quantity of oxygen required for gas cutting operation?


 We know that, for gas cutting operation generally we use combination of oxygen and LPG or Oxygen and acetylene.But now a days for some industry LPG is banned for safety point of view.

 





















Following table gives the difference between Oxy-acetyle and Oxy-LPG gas cutting

COMPARISON BETWEEN ACETYLENE AND LPG FUELS FOR GAS CUTTING OPERATION

SL No.

Acetylene

LPG (Propane)

1

Highest flame temperature up to 3100 Deg C

Flame temperature up to 2800 Deg C

2

Flame speed up to 7.5 m/sec

Flame speed up to 3.3 m/sec

3

Most of the heat released is in inner cone

Most of the heat released is in outer cone

4

Higher flame GCV of inner cone (4500 kcal/M3)

Lower flame GCV of inner cone (2500 kcal/M3) as compared to acetylene

5

Stoichiometric air fuel ratio1.2:1 (Requires 2.5 to 3 Oxygen cylinders for burning one Acetylene cylinder)

Stoichiometric  air fuel ratio 4.3:1 (Requires 7 to 8 Oxygen cylinders for burning one LPG cylinder)

6

Can be used in gas welding, as acetylene when burning with air creates reducing zone that cleans the steel surface

Cannot be used in gas welding as it does not create reducing zone

7

Acetylene has Specific gravity 0.9 kg/m3, so if it leaks it will raise in air without harming much

Propane  has Specific gravity 1.6 kg/m3,which is heavier than air.So if it leaks it will concentrate in deck level or any other closed/corner area

8

Acetylene requires less air for complete combustion

Propane requires more air for complete combustion, so there may be chances of formation of carbon monoxide (CO) in case of incomplete combustion. This incomplete combustion may result into poisoning of working area, as CO is poisonous gas

9

Can be used for cutting & welding applications in industry

Used only for domestic applications

 

Calculate the number of Oxygen cylinders required to consume 1 no.of industrial LPG cylinder for gas cutting operation

Commercial LPG (C3H8) has 19 kg weight that is 19 kg of propane

Combustion equation of propane

C3H8 + 5O2 = 3CO2 + 4 H2O

44 + 160 = 132 + 72 (Molecular weight of C = 12, O = 16, H = 1)

 Divide equation by 44

1 + 3.63 = 3 +1.63

 From above result it is clear that 3.63 kg of Oxygen is required to burn 1 kg of Propane to achieve 100% combustion.

So for burning 19 kg of commercial LPG, need 19 X 3.63 = 68.97 Kg of oxygen

 Volume of oxygen cylinder in cylinder = 6.9 M3 compressed at 140-150 kg/cm2

Convert 6.9 to kg by dividing oxygen density, we get weight of O2 in cylinder = 9.1 kg

 So total O2 cylinders required = 68.97 / 9.1 =7.58 Nos for consuming 1 LPG cylinder

Calculate the number of Oxygen cylinders required to consume 1 no.of dilute acetylene cylinder for gas cutting operation

DA (C2H2) cylinder has 8 m3 of acetylene

 Convert volume to kg by multiplying the density of the gas

8 X 0.899 = 7.192 kg

Combustion equation of propane

2C2H2 + 5O2 = 4CO2 + 2H2O

52 + 160 = 176 + 36 (Molecular weight of C = 12, O = 16, H = 1)

 Divide equation by 52

1 + 3.07 = 3.38 +0.69

So for burning 7.192 kg of DA, need 7.192 X 3.07 = 22.07 Kg of oxygen

So total O2 cylinders required = 22.07 / 9.1 =2.42 Nos

 Read Powerplant maintenance calculations

Note:

DA (Dilute Acetylene): OD 265 mm X Height 1 meter (Appx) and thickness 4.0 mm.Volume of acetylene in cylinder is 8.5 m3

 Oxygen cylinder size : OD 235 mm X Height 1.34 meter (Appx) and thickness 4.0 mm.Volume of O2 in cylinder is 6.9 m3

 

For more read>>>power plant and calculations

 

IBR STANDARD INSPECTION PROCEDURES

 








A-Standard Inspection procedure for Dry & thorough inspection

  • Checking the registration number of the Boilers
  • Carryout thorough inspection of Boiler from both inside and out side
  • Check for defects like corrosion, erosion, bend, bulging, pitting, deformation, thermal expansion etc of pressure parts
  • Check thickness of pressure parts
  • Check the conditions of mountings & fittings
  • Witness non destructive tests if required

B-Standard procedure for ground inspection of pressure parts under erection

  • Verification of documents of pressure parts with relevant certificates
  • Verification of approved drawings
  • Checking pressure parts makers stamp & other identification marks with form no-II
  • Checking of leading dimension of the parts & comparing with approved drawings
  • Checking general condition of the pressure parts like dent marks, pitting, bend etc
  • Checking of fittings & mountings with relevant drawings

C-Standard procedure for material inspection

  • Verification of the approved drawings corresponding to the materials & documents
  • Checking of the pressure parts materials with relevant IBR certificate and  approved drawing.Check name of the material, its specification, heat no, cast no.class, size, identification number & stamping etc
  • Checking of leading dimension of the parts & comparing with approved drawings
  • Checking general condition of the pressure parts like dent marks, pitting, bend etc
  • Selection of samples for physical and chemical analysis/testing

D-Standard Procedure for weld set up inspection

  • Verification of approved drawing
  • Verification of Welder’s certificate
  • Verification of the certificates of welding consumables
  • Verification of the approval of contractor for particular job
  • Verification for the procedure of welding procedure
  • Verification for the site satisfactory  simulation test results
  • Verification of test results of pipe, tube or plates
  • Checking of root gap,weld groove profile and alignment of the pressure parts to be welded as per approved drawing
  • Ensure weld joint area to be welded is free from dust, dirt, oil & grease.And also ensure it is crack free
  • Check weld joint identification number.

E-Standard Procedure for welding joint inspection

  • Visual inspection of general condition of the weld joint like, slag, under cut, finish, surface crack, leg length etc
  • Check alignment of the pressure parts
  • Witnessing Dye penetrant test, magnetic particle inspection test & hardness tests if required
  • Selection of weld joints for NDT test like ultrasonic & radio graphic tests

F-Standard Procedure for Boiler Hydraulic tests

  • Verification of the satisfactory non destructive tests of the welding joints
  • Verification of PMI (Positive Material Identification) report of the weld joints
  • Verification of pressure parts calculation approval
  • Verification of all previous inspection reports and Post weld heat treatment (PWHT) charts
  • Check the calibration reports of pressure gauges using for hydraulic test
  • Witnessing Hydraulic test carried out as per IBR-1950
  • Checking of deflection, distortion and extension of pressure parts during hydraulic test
  • Thorough inspection of pressure parts for any leakages and sweating

G-Standard Procedure for Boiler steam tests

  • Verification of the provisional order of the Boiler
  • Witnessing the steam test carried out as per IBR-1950
  • Check, popping pressure, reset pressure, blow down, accumulation, chattering, lift
  • Checking of the performance of the mountings and fittings

 

Read more >>>> Power plant and calculations

 


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