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 reduce the Specific Steam consumption (SSC) of a Steam Turbine??

 

        

Specific steam consumption is the amount of steam consumed by a Turbine to generate unit power.

Or else, Specific steam consumption is the ratio of steam entered into the Turbine to the power generated

SSC = Steam consumption / Power generation.

For example: A 50 MW steam turbine consumes 190 TPH of steam to operate on full load, then its Specific steam consumption (SSC) will be;

SSC = 190 / 50 = 3.8 TPH/MW or 3.8 kg/kwh

Specific steam consumption (SSC) of a Turbine depends on following factors;

1-Inlet steam parameters

2-Vacuum or exhaust pressure

3-Exhaust pressure

4-Bleed steam pressure, temperature & flow

5-Extraction steam pressure, temperature & flow

6-Turbine maintenance practice or preventive and corrective maintenance

Specific steam consumption (SSC) of a Turbine can be reduced by taking following corrective measures.

Maintaining inlet steam parameters: Maintaining Turbine inlet steam parameters play a vital role in improving Specific steam consumption (SSC). The steam pressure and temperatures lower than the design will lead to increase Specific steam consumption (SSC).

Therefore, always maintain Turbine inlet main steam temperature as per OEM recommended to reduce steam consumption.

Maintaining higher vacuum & lower exhaust temperature: Turbines operating at lower vacuum (exhaust pressure) and higher exhaust temperature call for more steam consumption to generate unit power as compared to Turbines operating at higher vacuum & lower exhaust temperatures.

So, it’s always recommended to maintain vacuum on higher side.

Ensure following to get more vacuum.

1-Sufficient cooling water for condenser

2-Condenser Tubes are being cleaned regularly

3-Enough temperature drop in cooling tower

4-Operation of the Turbine at optimum load. Operation of the Turbine at lower loads results into higher exhaust temperature.

5-Ejectors steam pressure & temperature are as per OEM

6-Use of Liquid ring vacuum pump (LRVP) saves steam energy & consumes around 40% of its power in terms of electrical power.

Bleed steam pressure, temperature & flow: Higher the bleed steam pressure, temperature & flow than design higher will be the Specific steam consumption (SSC) or lower will be the power generation at same quantity of inlet steam flow.

Hence, always try to maintain bleed steam pressure & temperature within limit.

Read>>>>Calculated reasons for more SSC

Higher bleed steam pressure & temperature carries away the heat from Turbine without doing any work in Turbine, hence steam consumption to generate specific power will increase.

Extraction steam pressure & Temperature: Similarly, steam extraction at higher steam pressure & temperature will lead into more steam consumption.

Wear out of labyrinth seals: If there is more clearance between internal seals & rotor, some amount of steam directly escapes into next stages of rotor without contributing work done. Hence in such case, Turbine draws more steam to generate desired power.

Therefore, always carryout the Turbine preventive maintenance regularly and replace worn out parts especially seals & gaskets

Steam leakages: Arresting all internal & external steam leakages.

Operation of the Turbine at optimized load: Operating the Turbine at lower loads will lead to more steam consumption to generate particular unit of power.

Hence always try to avoid the operation of the Turbine at partial loads.

Proactive operation & maintenance practice: Following operation & maintenance practices should be followed to reduce Specific steam consumption (SSC) of a Turbine.

Timely replacement of worn out bearings, ensuring precision alignments

Read more on>>>>Turbine major overhauling

Maintaining correct temperature of lube oil for bearings

Regularly servicing of actuators to avoid load hunting

Maintaining good quality for steam and water

Ensuring turbine blades cleaning during every major overhauling

Monitor the load & steam consumption on various loads & observe any deviations

Read more>>>>>Interview questions & answers on steam Turbine

Formulae for Boiler calculations

 

Boiler

1-Boiler efficiency direct method

Boiler efficiency = (Mass of steam flow X Steam enthalpy-Feed water flow at economizer inlet X Enthalpy-Attemperator water flow X Enthalpy) / (GCV of fuel X Fuel consumption)

2-Boiler efficiency by indirect method

Boiler efficiency = 100-Various losses

3-Theoretical air requirement for combustion

Theoretical air T_hair = ((11.6 X C% + (34.8 X (H2-O2/8)) + (4.35 X %S))/100

Where C = % of carbon in fuel

H2 = % of Hydrogen present in fuel

S = % of sulphur present in fuel

4-Excess air requirement for combustion

%EA = O2% / (21-O2%)

Where O2 = % of oxygen present in flue gas

5-Mass of actual air supplied

AAS = (1 + EA / 100) X Theoretical air

6-Mass of flue gas

Mfg = Mass of Air + 1

6-Mass of dry flue gas

Mfg = Mass of Co2 in flue gas + Mass of Nitrogen in fuel + Mass of Nitrogen in combustion air + Mass of oxygen in flue gas + Mass of So2 in flue gas

Mfg =(Carbon % in fuel X Molecular weight of CO2 / Mol.weight of Carbon) + N2 in fuel + (Mass of actual air supplied X % of N2 in air i.e 77/100) + ((Mass of actual air – Mass of theoretical air) X 23/100) + S2 in fuel X Mol.weight of SO2 / Mol.weight of sulphur)

7-% of heat loss in dry flue gas

Heat loss = Mfg X Cp X (Tf-Ta) X 100 / GCV of fuel

Where,

Mfg = Mass of flue gas

Cp = Specific heat of flue gas in kacl/kg

Tf = Temperature of flue gas

Ta = Ambient air temperature

9-% of heat loss due to moisture in fuel

Heat loss = M X (584 + Cp X (Tf-Ta))  X 100 /  GCV of fuel

Where,

M = Moisture in fuel

Cp = Specific heat of flue gas in kcal/kg

10-% of heat loss due to moisture in air

Heat loss = AAS X humidity X Cp X (Tf-Ta) X 100/ (GCV of fuel)

Where,

AAS = Actual air supplied for combustion

Cp = Specific heat of flue gas in kcal/kg

Tf = Temperature of flue gas

Ta = Ambient air temperature

11-% of Boiler water blow down

Blow down % = (Feed water TDS X % of makeup water) X 100 / (Maximum permissible TDS in Boiler water –Feed water TDS)

12-Steam velocity in line

Velocity of steam in pipe line,V = Steam flow in m3/sec / Area of pipe line (A)

Steam flow in m3/sec = (Steam flow in kg/hr / Density of steam X 3600)

Area of pipe, A = Pi X D² / 4

Where D is pipe internal diameter

13-Condensate flash steam calculation

Flash steam % = (H1-H2) X 100 / Hfg)

Where, H1 = Sensible heat at high pressure condensate in kcal/kg

H2 = Sensible heat of steam at low pressure in kcal/kg

Hfg = Latent heat of flash steam

14-Calculation of amount of heat required to raise the water temperature

Heat required in kcal=Mw X Cp X (T2-T1)

Where, Mw = Mass of water

Cp = Specific heat of water in kcal/kg (1 kcal/kg)

T1 = Initial temperature of water in deg C

T2 = Final temperature of water in deg C

15-Calculation of heat required to raise air temperature

Heat required in kcal=Mair X Cp X (T2-T1)

Where, Mw = Mass of water

Cp = Specific heat of flue gas in kcal/kg (0.24 kcal/kg)

T1 = Initial temperature of air in deg C

T2 = Final temperature of air in deg C

16-Surface heat loss calculation

S = (10 + (Ts-Ta) / 20) X (Ts-Ta) X A

S = Surface heat loss in kcal/hr m2

Ts= Hot surface temperature in deg C

Ta = Ambient air temperature in deg C

17-Dryness fraction of steam

X = Mass of dry steam / (Mass of dry steam + Mass of water suspension in mixture)

18-Heat content in wet steam

h = hf + xhfg

h= Heat content in saturated steam

x = Dryness factor of steam

Hfg =Enthalpy of evaporation

19-Heat content in dry saturated steam

h = hf + hfg

h= Heat content in saturated steam

Hfg =Enthalpy of evaporation

20-Heat content in superheated  steam

h = hf + hfg + Cps (Tsup - Ts)

h= Heat content in super heated steam

hfg =Enthalpy of evaporation

Cps = Specific heat of super heated steam

Tsup= Superheated steam temperature in deg C

Ts = Saturated temperature of steam in deg C

21-Calculation of Equivalent evaporation

Me = Ms X (h-hf) / hfg

Ms = Mass of steam

h = Steam enthalpy

hf= Feed water enthalpy

22-Factor of evaporation

Fe = (h-hf) / 539

23-Ash (Total) generation calculation

Ash generation in TPH = Fuel consumption per hour X % of ash in fuel / 100

24-Fly ash generation calculation

Fly ash generation in TPH = Fuel consumption per hour X % of ash in fuel X 80% / 100

25-Bottom  ash generation calculation

Bottom ash generation in TPH = Fuel consumption per hour X % of ash in fuel X 20% / 100

26-Calculation of ash generation in ESP

Ash generation in ESP in TPH = Fuel consumption per hour X % of ash in fuel X 80% X 80% / 100

27-Boiler safety valve blow down calculation

Blow down % = (Set pressure - Re seat pressure) X 100 / Set pressure

28-Calculation of attemperator water flow

Attemperator water flow  in TPH= Steam flow in TPH X (h1-h2) / (h2-h3)

h1 = Enthalpy of steam before desuper heating in kcal/kg

29-Economiser efficiency calculation

ηEco. = (Economiser outlet feed water temperature-Economizer inlet feed water temperature )  X 100 / (Economizer inlet flue gas temperature - Economizer inlet feed water temperature)

30-APH efficiency calculation

APH air side efficiency

ηAPHa = (Air outlet temp-Air inlet temp)) X 100 / (Flue gas inlet temperature -Air inlet temperature)

APH gas side efficiency

ηAPHg = (Flue gas inlet temp.-Flue gas outlet temp) X 100 / (Flue gas inlet temperature -Air inlet temperature )

31-Calculation of steam cost

Steam cost per ton = Steam enthalpy  in kcal/kg X Fuel price per ton/ (Boiler efficiency % X GCV of fuel used in kcal/kg)

32-Travelling grate Boiler heating surface calculation

Boiler heating surface (Appx) = Boiler capacity in kg/hr / 18

33-AFBC Boiler heating surface calculation

Boiler heating surface (Appx) = Boiler capacity in kg/hr / 22

34-Travelling grate slop fired Boiler heating surface calculation

Boiler heating surface (Appx) = Boiler capacity in kg/hr / 12

35-AFBC  slop fired Boiler (Low pressure up to 10 kg/cm2 WP) heating surface calculation

Boiler heating surface (Appx) = Boiler capacity in kg/hr / 8.2

36-Calculation of draught produced in Chimney

Hw = 353 X H (1/Ta – 1/Tg (Ma+ 1)/Ma)

H = Chimney height in meters

Ta = Atmospheric temperature in K

Tg = Flue gas temperature in K

Ma = Mass of air & Mass of flue gas = Ma+1

 

Also given as;

 P = 176.5 X H / Ta

Hw = Chimney height in meters

Ta = Absolute atmospheric temperature in Kelvin

Hw = Draught in mmwc

37-Calculation of mass of flue gas flowing through chimney

Mg (kg/sec)= Density of gas (kg/m3) X Area of Chimney (m2) X Velocity of flue gas in Chimney (m/sec)

38-How to calculate the quantity of De-aerator venting steam?

De-aerator vent rate = 10.98 X Absolute pressure in deaerator X (D X D) Diameter of venting line orifice….Kg/hr

Note: Pressure in PSI

Diameter in inches

Or.

Steam venting flow = 24.24 X P(absolute pressure in PSI) X D X D (Size orifice in inch)........Lbs/hr

Read more>>>>on>>>Powerplant and calculations