26 September 2023

Boiler Pressure Parts Welding Joints Pre and Post weld Heat Treatment and Required equipments

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Boiler Pressure Parts Welding Joints Pre and Post weld Heat Treatment and Required equipment’s..



When a weld is made in a structure, between pipes, on vessel joints or between two plates, between a flange and pipe, heat exchanger tubes etc. stresses are induced in and adjacent to the joint region. In case these stresses are not removed, then there are chances of cracks induced due to these stresses resulting in the failure of the joints.

In order to relieve these stresses, the joints and its surrounding areas are heated to a certain temperature to make the crystal structure homogenous. The joints are to be slowly heated to that temperature( which is decided upon depending about type of material/ its chemical composition) . The joint is kept at that temperature for a certain period and then slowly cooled down. Generally the joints are heated up to about 300°C without any control. Thereafter the rate of heating is controlled ( may be 50°C/hr or 75°C/hr or even 100°C/hr as per the procedure adopted. )

After the temperature reaches the soaking temperature (as stated – decided based on type of material) – it is held at that temperature for a certain period- 1 hour/ 2 hours or so( depending on wall thickness of the joint). The joint is then allowed to cool down- once again the at a certain decided rate of cooling. Normally after the temperature reaches 300°C, rate of cooling does not matter. For most of the carbon and ally steels, soaking temperature between 600°C to 700°C.

Stress reliving can also be carried out mechanically- by subjecting the job to mechanical vibrations – where in the grains of the weld as well as surrounding area re-align themselves and stresses are relieved. But this is not a very well accepted method.

Stress reliving can also be carried out mechanically- by subjecting the job to mechanical vibrations – where in the grains of the weld as well as surrounding area re-align themselves and stresses are relieved. But this is not a very well accepted method.

Stress reliving can also be carried out mechanically- by subjecting the job to mechanical vibrations – where in the grains of the weld as well as surrounding area re-align themselves and stresses are relieved. But this is not a very well accepted method.




Equipment :

We shall discuss here Heat treatment by thermal methods only. Heating the joints can be carried out by wrapping the joints with heater elements/ Pads or by Induction heating coils. Heating can also be done by oil fired or gas fired burners.

Large spheres or vessels are heated by burners.

Eastwest Engineering & Electronics Pvt. Ltd., generally provides equipment for Stress relieving using normal resistance heating elements in various shapes and sizes.

We shall require suitable power sources to provide power to heater coils or heater pads. Nickel/ Chromium ( Nichrome) resistance wire is used in making heater coils. For making heater coils, single core Nichrome wire is used whereas multi-strand twisted wire rope is used in making heater pads. The size of heater pads/ coils is decided on several factors such as its resistance ( based on wire diameter) per unit length. When connected across Power source – care should be taken to have a certain minimum length (minimum resistance) – lest the heater is over loaded and melts or the power source trips. Normal ohm’s law can help in avoiding such mishaps. Complete data giving details about the resistance/ unit length, current carrying capacity etc are available. The power out put per heater can be calculated by applying ohm’s laws as given under:

Watts = Volts x Current

V = I x R,     W = V x I   or W = V ²/ R

W         : Power in watts

V          : Output voltage from Power source in volts

I           : Current passing through the heater coil/pad in amperes

R          : Resistance of Heater coil/ Pad in ohms.

Suitable size heaters can provided sufficient power to heat a job even up to 1200°C as that is about the melting temperature of Nichrome wire. Depending upon the mass of job to be heated, suitable size/ design of heaters are required to be selected.

Power source

The normal power supply available is 110 Volts or 220 Volts. But they are designed to supply low or moderate power. Power for industrial purpose is available at 380Volts, 415 Volts or 440Volts. All the are termed medium voltages and are dangerous to handle supply power to heaters. In case while heating, the heater accidentally touches the test piece/ job, it can be fatal for people working around there.

It is normal to use the heaters designed to operate at safe 60 volts or maximum 85 volts. Thus power sources are designed to provided out-put voltage options such as 32.5V, 42.5 Volts, 60/65 Volts and 80/85 Volts. Heater elements and Heater pads are also designed to operate at these voltages. The input voltage to the Power Sources can be 110/230/380/415 or 440 Volts. In most industries   380/415 or 440 Volts – 3 Phase supply is available and that is the most efficient to use for Power sources.

A 3-phase transformer converts high voltage to the lower desired voltage. There are out lets in the power source. that permit the use of various types of heaters that can be connected to the designed voltage.

After the job/ weld joint has been fitted with suitable heaters, Thermocouples are fitted on or adjacent to the weld joint. These thermocouples are required to measure and monitor the temperature at the joint location. Usually a K type ( Chromel / Alumel) thermocouple is used as it can measure temperature up to 1200°C. One or several pairs of Thermocouple wires ( a pair of Chromel and Alumel wire of 22 swg – 0.77 mm dia. ) are tack welded to the weld joint . The thermocouples are connected to a Temperature recorder to monitor the temperature and record the same. Generally, a strip chart recorder is used and rate of heating, soaking and rate of cooling are plotted. The recorders are generally 6 point or 12 point – which can take up to 6 or 12 nos. thermocouples and plot the Stress relieving curves for each of the thermocouple. The number of thermocouple are calculated size of pipe, its wall thickness etc. This is to ensure that temperatures at different location of the same joints, are with in prescribed limits and rate of heating are also same.

The rate of heating/ cooling is controlled on Power source manually or by automatic control. Power source has several out-put pairs of terminals, where heaters can be connected using power cables. Some time up to 3 nos. heaters (in parallel) can be connected to one pair of terminals. The supply to these terminals is controlled using timer controllers.

Temcon Control Panels Vs. Power Sources
Temcon 30 or Temcon 60 Control panel

Output at 230 Volts

( Mostly used in India)

Temcon Power Source TR 50, TR 60, TR 80

Output at safe 32 V, 40 V ,60 V & 80 Volts

(Used all over)

Power supply Input supply :

415 Volts 63 Amps 3 Phase

Output supply :

a)      230 Volts 63 Amps 1 Phase x 3 channels

b)      230 Volts 32 Amps 1 Phase x 6 channels

Input supply :

415Volts 3 Phase 60kVA

Output supply:

60 Volts 1 Phase 200 Amps x 6 channels

Price Inexpensive , Portable and easy to use. Expensive , Haevy duty and on Wheels
Power supply to Heater Coils is at 230 Volts. Chances of shorting with the pipe and / or electric shock. Powe supply to heater pads operating at 32 Volts, 40 Volts and 60 Volts ( safe).
Safety Earth leakage Circuit Breakers are provided which will trip the supply in the event of short circuiting or accidently touching the wire in order to avoid electric shock. Still need to be careful in fitting heater coils. No need to worry as the connecting power is at safe voltage.
Heater elements. Heater Coils are less expensive Heater Pads are expensive, but provide concentrated heat energy to the location.
Portability Compact and portable Heavy and large in size




Temcon Power Sources are designed to provide output power at low and sage voltage. The Power Sources are available in several designs and various power outputs as stated under..

Manual System : In Manually controlled system,   the heating / cooling cycle is manually controlled with the help of Energy Regulators. There is one energy regulator for each channel. This system can also be connected to an external Portable PID controller for automatic operation. Each of the six output channels can be programmed independently.

Automatic System : Automatic Power source can be used in Manual Mode of operation as well as in Automatic Mode with the help of a selector switch. In the Manual mode, the rate of heating etc. can be controlled by Energy regulators. With the built-in PID controlled programming system each Channel can be independently programmed.

Power Rating : 50 kVA, 65 kVA ,75 kVA & 100 kVA

Input Voltage : 380 / 415 / 440 Volts   Out-put Voltage: 32.5 V, 42.5 V, 60 and 80 volts.





Temcon Power   Control Panels   supply power output at 230 Volts. The Heater pads/ Heater coils are connected across 230 Volts in each of the three channels. Since operation at 230 volts may be dangerous, the Control Panels are provided with ELCB ( Earth Leakage Circuit Breaker). The ELCB trips the supply when heater coils are accidentally shorted of grounded.

Temcon Power Control Panels are light weight and easy to use. They are economically priced and require inexpensive Heater coil for operation. The use of such panels involve high voltage and caution.   Stress Relieving Operation with such panels is not permitted outside India.

Temcon Power Control Panels are designed to have 3 channels output. They work with manually operated Energy regulators.   These control panels can also be connected to external PID controller for automatic operation. Special Temcon Power Control Panels with built in Automatic PID Controller are also available.

Models available : Temcon 60, Temcon 60A , Temcon 100 and Temcon 100A.


Thermocouple welding unit TAU-90 is compact and is used for welding thermocouple wires to the test piece. The equipment works on electric discharge principal. It works on 230volts mains supply or on built –in chargeable Battery.

Model available : TAU-90




The PID controller is used to control the Automatic Out-put energy of the Power Sources and Power Control Panels. With this the Rate of Heating, Soaking and Rate of Cooling can be controlled automatically. Models are available with 3 or 6 nos. PID controllers.   Each PID controller can be independently programmed. Thus each channel output can be controlled independently.

The controller can be fitted to any   Power Source or Control Panel internally. The controller is also available as an external unit which can convert any Manual system to an Automatic system.

Models available : PID- 3 & PID- 6



It will be important to have complete information about the test object for Hear treatment. Information such as Shape, Size, Thickness, Material type , Rate of Heating, Soaking Temperature, Soaking time, Rate of cooling etc. must be recorded befor selection of equipment, Heating coils / Pads etc.

In case of pipes in the plant, we suggest the use of Temcon 60 control panels. We do not recommend Temcon 30 as the output power per channel is maximum 230 volts x 32 Amps = 7.36 KW

This may be useful for pipes of diameter up to 10”. You can use only 12 SWG ( 2.4 mm dia) 14 SWG( 2.0 mm dia.) and 16 SWG ( 1.6 mm dia.) Nichrome heater wires.

When using TEMCON- 60, it is possible to connect heater coils of 8 gauge ( 4.0 mm dia.), 10 gauge ( 3.15 mm dia) heater wires . The output power is per channel is huge : 230 Volts x 63 Amps.= 14.5 KW .You may use Temcon- 60 for heating pipes up to 36” or 48” dia. ( Using tow or more heater coils in case wall thickness is high).

We give below more information on Hear Coils and Heater pads for your information.



Selection of Heater Pads

For Stress reliving of an object we should have Heater pads or Heater elements that have the following capabilities:

  1. Sufficient capacity (Heat output) to heat the object up to the required temperature. This has to be calculated based on Mass of material to be heated. For example a 24” Sch 40   pipe will need less heat input that a 24” Sch 80 pipe- which has a higher wall thickness – thus more mass of the same size pipe.
  2. Capable to cover the area requiring Stress Relieving. When localized heating is done- such as in case of weld on pipes, there is some areas ( heat affected zones) adjacent to the weld that also needs heating. This is generally 3 times wall thickness on either side of the weld.
  3. Design of Heater element shall be such that it can be safely connected to required out-put voltage of Power source. Some of the Heaters are connected to Power control panels with out-put voltage of 230 Volts. Considering this voltage is a little dangerous to handle,   Heaters pads are used that can be connected to Power sources that have Voltage out-put of 60 volts and 80 volts.

In your case we have pipes of 24” dia ( 600 mm) . At the moment we do not know the wall thickness and as such we can not really define as to how much mass has to be heated. Further,

in order to heat a pipe, the heater pad has to cover the entire weld length and a certain width of the pipe. The weld length is the circumference of the pipe.

This circumferential length of the weld is calculated by Multiplying Pipe diameter x 3.142( 22/7). While calculating, we may add little bit of weld reinforcement thickness (say 3mm on each side). Thus for pipe of   diameter 24” ( 610 mm + 3mm + 3mm) = 616 mm –the Circumference is = 616 x 3.142 = 1935 mm.

Now the width to be heated, is minimum 3 times the pipe wall thickness, on each side of the weld.  Say we have a 12mm thick pipe.  Thus we should cover minimum of 3 x 12 = 36 mm on each side of weld. Let us add width of the weld as 25mm.This the coverage will be 2 x 36 = 72 + 25= 97 mm or say 100 mm. 

Thus we must have a pad of width( Y ) –approximately 100 mm. We shall require several pads – where the length ( X )  will be added to cover entire Circumference.



We have the following heater pads to select from. In case we use 60Volts rating pads, we can choose:

1) Stock no. 209.          Width : 100 mm x Length : 535 mm.

Four such pads fitted adjacent to each other will have sufficient length requirement (535 x 4=2140 mm)

2) Stock no. 211.          Width: 85 mm x Length : 610 mm

Three pads will cover 610 x 3 = 1830 mm. This will be more suitable for pipes of

thickness say 6 to 10 mm. Though length wise coverage fall short – in case of low

thickness pipes, and proper insulation cover over the heaters, the gap between two

adjacent pads will be just 25 mm and that may be OK.

In case the wall thickness is more, say about 20 mm, we need more heat for higher metal mass. Besides we shall also need pads with more width. As such we may use stock no. 208 ( 5 pads)or   209 ( 7 pads).

Heater Pads                   60 Volts   &   30 Volts
Stock no. X   ( mm) Y ( mm) Volts kW
201 75 660 60 2.7
202 100 495 60 2.7
203 150 330 60 2.7
204 205 250 60 2.7
205 255 205 60 2.7
206 305 165 60 2.7
207 280 145 60 2.7
208 405 125 60 2.7
209 535 100 60 2.7
211 610 85 60 2.7
212 1220 45 60 2.7
213 85 255 30 1.35


                   Heater Pads                                                         80 Volts     & 40 Volts
Stock no. X   ( mm) Y ( mm) Volts kW
251 75 910 80 3.6
252 100 680 80 3.6
253 150 450 80 3.6
254 205 350 80 3.6
255 255 290 80 3.6
256 305 230 80 3.6
257 380 185 80 3.6
258 430 165 80 3.6
259 535 145 80 3.6
260 840 85 80 3.6
261 1675 55 80 3.6
262 45 760 40 1.8


In case we use 4 pads of 2.7 kW working at 230 Volts, the input energy to the pipe shall be 2.7Kw

x 4= 10.8 kW or   2.7 x 5= 13.5 kW.

In case we opt for 80 Volts heaters ( You may do that in case wall thickness is very high and requires more energy input per Heater pad. )

We are sure with the above, you will be able to decide on a suitable Heater pad.

We suggest that you choose Heater pads of one voltage only and start the work. In future depending on your requirements, you may buy and stock different size Pads which will enable you to utilize them from stock for what ever size pipe you require to heat.

We may further state that the Temcon Power source has six channel out-put. On each channel you may fit 3 to 4 pads. Thus you will be able to Heat one pipe with one or channel or two pipes with 3 channels. Thus your Pad requirements shall be large. We do not know as to how much job you have in hand. Please advise so that we can make our suggestions.

We have prepared specifications for Nichrome wire used for making heater coils in the following table. Although the table is for the specific use with our Control Panel Temcon-60, but the information available is useful for designing the heater coils for general purpose.

In the Temcon -60 or Temcon-100 control panels, the Heater coils are connected across 230 Volts output of Control panels. As such – to avoid passing of excess current, which may result in fusing/ melting of Heater coils, a minimum Length of Heater coil is recommended.

   Specifications of Nichrome Wire

Wire Gauge











Ohms/ mtr.

Maximum current carrying Capacity (Amps)


Minimum       Wire Length in each coil (mtr.) Length

Mtrs / Kg

Coil weight

( Kgs)

Resistance per coil (Ohms) Current passed in coil at 230V




Kilo Watts



6 4.88 .0591 90    46 6.365 7.23 2.541    90 20.7
8 4.06 .848 70    42 9.166 4.6 3.731    62 14.2
10 3.25 .133 50    35 14.32 2.45 4.655    49 11.2
12 2.64 .201 37    32 21.69 1.48 6.432    36    8.3
14 2.03 .339 26    26 36.67 0.710 8.814    26    5.9
16 1.63 .538 20    22 57.27 0.39    11.8 19.5    4.5

In case due to some reasons a shorter length of wire is required to be used, we can reduce the output voltage to

which the Heater coil is to be connected. This can be achieved by putting a step down transformer.





Example :

Using wire gauge 12 ( Diameter 2.64 mm) : the minimum length recommended is

32 meters when put across 230 volts. This is decided in following fashion.

Current Carrying capacity of 12 gauge Nichrome wire : 37 Amperes.

Thus we have to ensure that when the heater coil is connected across power point- current passing through the coil should be less than 37 Amps. Other wise the coil will fuse.

Now supply voltage is 230 Vols.

Max. current : 37 Amps.

From Ohms law : V= IR or R= V/ I

Resistance : 230/37 = 6.216 Ohms.

Now the resistance of the Heater coil made of 12 gauge wire must have minimum 6.216 Ohms to be safe.

Resistance of 12 gauge wite : 0.201 Ohms/ mtr.

Length of wire for 6.216 Ohms resistance : 6.216 / 0.201= 30.9 mtrs. Or 31 mtrs.

To be still on safer side Make coil length 32 mtrs.

Reverse Calculation

Length of Coil : 32 mtrs.    Resistance : 32 x 0.201= 6.432 Ohms. Output Voltage: 230 Volts

When this coil of 12 gauge and 32mtrs length is connected across 230 Vlos:

Current passing through wire will be : I=V/R= 230/6.432 = 35.758 Amps or 36 Amps.

This quite safe as the current carrying capacity of 12 gauge wire is 37 Amps.

Design your own coil

Now let us say- you wish to make a coil of 12 gauge wire and only 10 mtrs. length.

The resistance of this coil is : 10 x 0.201 = 2.01 Ohms.

Max. current it can carry: 37 Amps.

Safe voltage to connect: V= IR or = 2.01x 37= 74.37 Volts.

Or a very safe voltage should be no more than 73 Volts.

You will need to put in line a step down transformer to get the 73 Volts.

The minimum rating of the transformer should be : W=V x I =73x 37= 5452.69 watts or 5.45269 KW or KVA

The heat output of this heater coil will be : 73 Volts x 37 Amps = 5.45 Kwatts.


Pipe   Schedules

What is a Pipe Schedule?
Pipes are designed to carry fluid, therefore their internal diameter is their critical dimension. This critical dimension is referred to as the nominal bore, abbreviated as NB. Obviously, for pipes containing pressurised fluids the wall thickness, and by implication the pipe’s strength, is important.

Wall thickness is expressed in “schedules”, refered to as pipe schedules.

The wall thickness associated with a particular schedule depends on the pipe size as can be seen from the charts below for some of the more common sized carbon steel pipes encountered.
 Abbreviations used: NB – nominal bore, STD – Standard, EH – Extra Heavy, DBL EH – Double Extra Heavy.


 2″NB OD = 2.375 inch (60.32 mm)
Schedule 5 10 20 30 40
60 80
100 120 140 160 DBL EH
ID (ins) 2.245 2.157 2.067 1.939 1.689 1.503
ID (mm) 57.02 54.79 52.5 49.25 42.9 38.18


 3″ NB OD = 3.5 inch (88.9 mm)
Schedule 5 10 20 30 40
60 80
100 120 140 160 DBL
ID (ins) 3.334 3.260 3.068 2.900 2.624 2.300
ID (mm) 84.68 82.8 77.93 73.66 66.65 58.42


 4″ NB OD = 4.5 inch (114.3 mm)
Schedule 5 10 20 30 40
60 80
100 120 140 160 DBL EH
ID (ins) 4.334 4.260 4.026 3.826 3.624 3.438 3.152
ID (mm) 110.08 108.2 102.26 97.18 92.05 87.33 80.06






 6″ NB OD = 6.625 inch (168.275 mm)
Schedule 5 10 20 30 40
60 80
100 120 140 160 DBL EH
ID (ins) 6.407 6.357 6.065 5.761 5.501 5.189 4.897
ID (mm) 162.74 161.47 154.05 146.33 139.73 131.8 124.38
 8″ NB OD = 8.625 inch (219.1 mm)
Schedule 5 10 20 30 40
60 80
100 120 140 160 DBL EH
ID (ins) 8.407 8.329 8.125 8.071 7.981 7.813 7.625 7.439 7.189 7.001 6.813 6.375
ID (mm) 213.54 211.56 206.38 205 202.72 198.45 193.67 188.95 182.6 177.83 173.05 161.93


 10″ NB OD = 10.750 inch (273 mm)
Schedule 5 10 20 30 40
60 80
100 120 140 160 DBL EH
ID (ins) 10.482 10.42 10.25 10.136 10.02 9.750 9.564 9.314 9.064 8.750 8.500
ID (mm) 266.24 264.67 260.35 257.45 254.5 247.65 242.93 236.58 230.23 222.25 215.9



 12″ NB OD = 12.750 inch (323.85 mm)
Schedule 5 10 20 30 40
60 80
100 120 140 160 DBL EH
ID (ins) 12.42 12.39 12.25 12.09 11.938
11.626 11.376
11.064 10.75 10.50 10.126
ID (mm) 315.47 314.7 311.15 307.1 303.22
295.3 288.95
281.03 273.05 266.7 257.2


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