FORM 2
THE PATENT ACT 1970
(39 of 1970)
&
The Patents Rules, 2003
PROVISIONAL/COMPLETE SPECIFICATION
(See section 10 and rgte13)
1. TITLE OF THE INVENTION
A System for Heat Treatment of a Long Welding Pipe using Multiple Asynchronous Induction
Heaters and method thereof
2. APPLICANT (S)
(a) NAME: Electronics Devices Worldwide Private Limited
(b) NATIONALITY: Indian
(c) ADDRESS: 31, Mistry Industrial Estate, Cross Road A, MIDC, Andheri (East), Mumbai -400093
3. PREAMBLE TO THE DESCRIPTION

PROVISIONAL
The following specification describes the invention.

COMPLETE
The following specification particularly describes the invention and the manner in which it is to be performed.
Attached Separately

4. DESCRIPTION {Description shall start from next page.)
5. CLAIMS (not applicable for provisional specification. Claims should start with the preamble — "l/we Claim" on separate page)
I 6. DATE AND SIGNATURE (to be given at the end of last page ot specification)
7. ABSTRACT OF THE INVENTION (to be given along with complete specification on separate page)
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| *Repeat boxes In case of more than one entry.
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TITLE
A System for Heat Treatment of a Long Welding Pipe using Multiple Asynchronous Induction Heaters and method thereof.
FIELD OF INVENTION
The present invention relates to a system and method of heating long welding pipe using multiple asynchronous induction heaters to heat up each segment of long welding pipe identified as heating zone. Electrical engineering.
BACKGROUND OF INVENTION
An essential step in welding and manufacturing metal constructions and equipment is the heat treatment process. During heat treatment, the material is heated up and cooled down, using predefined methods to achieve the desired mechanical properties like metallurgical structure, residual thermal stress, hardness, toughness, strength, etc. Heat treatment of welding pipes is prerequisite for long service life of the pipes. During welding of seam joints (joint formed by uniting two edges), a high-intensity energy is produced in the welding arc which is used for the purpose. The associated thermal cycle of this high-intensity energy causes distortion, produces residual stress around the joint and because of all these, there could be a prospect of hydrogen cracking. Hydrogen cracking which is also called cold cracking or delayed cracking, is a type of crack that occurs in ferritic steels, most often immediately on welding or a short time after welding. This renders the welding defective. Thus to avoid Welding defects along with the physical, metallurgical and mechanical properties of steel structures it is important to use of proper heat-treatment procedures. A way to avoid hydrogen cracking is to make the prospective joint undergo pre weld heat treatment, and for stress relieving post weld heat treatment.

From the view point of the design of heat source for this purpose, the Post Weld Heat Treatment (PWHT) plays most important role because the heat input in this phase is maximum. Moreover, post weld heat treatment of pipes used in oil and gas industry is a pre-requisite in certain caustic and sour environments. In Post Weld Heat Treatment (PWHT) process, the heating cycle starts from room temperature till 650 0C in 2 hours, soaking between 595 to 6500C for 2 hours and cooling from 650 to room temperature within 2 hours. There are multiple options to do this. A typical temperature profile of post weld heat treatment of welding pipe is shown in Figure 1.
One of the process is to perform PWHT inside a large furnace. But this is a clumsy process as putting an entire pipe of, say, length 12m, diameter 1.1m and thickness 50 mm (weight: 15.4 Ton) into a furnace and taking it to PWHT temperatures of 600 0C, holding it there for an hour; also it requires a large furnace. Some of the shortcomings of this process are that the furnace needs long start up time, high cost is involved for heat treatment of a number of pipes to be heated up to 600 0C, long time is taken to reach the temperature of 600 0C even then the rate of expected heating (300 0C/hour) would be difficult to achieve. Further, in this process there is significant energy loss from the walls of the furnace, which makes the process further expensive. For example, considering the approximate cost per kg at around ₹3/kg, the cost of PWHT of one, say, 15.4-ton pipe would be ₹37,000. Moreover, performing PWHT for 100 to 200 pipes per day would need a very large furnace. Finally, handling of the pipe after soaking to cooling will also pose additional challenges and costs.
One other way is to use Gas burners to heat the welded seam and do localized PWHT. However, this process too suffers from multiple shortcomings, like, the gas consumption for heating the seam would be high, temperature gradient throughout the thickness would be large, high chances of having ‘hot spots’ just below the burners while there would be ‘cold zones’ between two

overlapping burners, the loss of heat into the atmosphere will be high, making it difficult for the operator to work on the shop floor near the area. These makes the process highly inefficient and expensive. Further it is difficult to meet the desired heating rate as well as temperature uniformity throughout the length or thickness of the pipe.
Another method to perform PWHT is heating through electrical heating pads i.e., resistance heating. In this process, multiple pads would be needed to cover the length and width of the weld seam. Alongside, multiple low voltage transformer controllers would also be needed. Here, the desired temperature profile in each zone could be achieved through use of PID controller. The temperature uniformity could as well be obtained using the multi pad control. However, this process needs very high setup time, insulation present between the pads and the pipe would slow down the response time, there would be significant heat loss. It takes longer time to achieve the temperature as it works on thermal conduction principles. The power consumption for heating the entire pipe up to 600 0C would be high too.
Multiple options are available to transfer requisite energy to the pipe, amongst all, due to several benefits induction heating is preferred to transfer requisite energy to the pipe. However, making an arrangement for performing heat treatment of long-seam joints is still remains a challenge. Long seam joints need long coil with large inductance value where the voltage could be unsafe for human operator. Also there is a need that the coil should be flexible to cater different applications for easy handling. Hence there is a need of novel method and system to address these shortcomings.
Thus there is a need for a process to perform PWHT which addresses all this issues and provides an effective solution. A process which is low cost, low or no energy loss, low setup time, fast response time, uniformity in temperature

profile across the length of the welding pipe, low power consumption and like advantages.
SUMMARY OF INVENTION
The present invention discloses a system and method of using multiple induction heating systems for pre and post weld heat treatment of large welding pipes. In this method, the length of the welding pipe divided into multiple zone and each zone have coil segment driven by an independent ZVZCS (zero-voltage switching at turn-on and zero-current switching at turn-off) series resonant inverter assisted by a power control circuit to achieve near resonant frequency operation where the power loss in each converter is minimum. Each coil segment is heated by independent non-synchronized power converters to a desired current and transfer of energy to load/ zone through induction heating.
Through this process a superior uniformity in temperature rise achieved through separate zone-wise power control boosts the energy efficiency as overheating in any zone is avoided.
This invention uses a novel flux bypass circuit which avoids noise coupling of one system from the adjacent coil heads and hence the converters which otherwise would create problem for equipment malfunction. The noise coupling from adjacent asynchronous inverter could disturb the phase locked loop for resonance frequency tracking.
Here the coil is air-cooled coil for which the power transfer efficiency to the pipe is high, and overall energy efficiency of the whole system is improved. Also because of this system being air-cooled, any external arrangement for conduction cooling is avoided making the system compact. Here the flexibility

of each air-cooled litz-wire based coil helps the controller could be moved close to the pipe.
For low power applications, say for pre weld heat treatment (<200 C), only desired number of controllers could be made active instead of all controllers, making the arrangement more energy efficient. The use of induction heating system for energy transfer converts the system friendliest to people in the shop-floor, it makes the system eco-friendly for energy use at the load end. The induction heating system does not cause ambience heating; and the use of smaller coils for each controller makes the system comfortable to the operators for easy handling.
BRIEF DESCRIPTION OF DRAWINGS
Figure 1: Typical post weld heat treatment temperature profile of a steel pipe
Figure 2: Power controller for induction heating system
Figure 3: Layout arrangement of air-cooled coil placed on cerawool blanket for PWHT.
Figure 4: Air-cooled induction heating system to perform PWHT of longitudinal seam.
Figure 5: Power control of PWHT uses two-loop control.
Figure 6: Schematic diagram of arrangement of magnetic shield from an adjacent coil driven by different asynchronous power controllers.
Figure 7: Actual construction of a magnetic shield using multiple ferrite core slabs.
Figure 8a: Placement of an actual magnetic shield between two adjacent coils in a pipe
Figure 8b: Placement of four coils separated by ferrite magnetic shunts for a 12m pipe
Figure 9: Schematic diagram of arrangements of complete induction heating system.

Figure 10: Photo of four coils fed by 4 asynchronous induction heaters.
Figure 11: Undisturbed waveforms of currents of four coils demonstrated that there was no coupling between any two adjacent coils, the use of magnetic shunts was demonstrated.
Figure 12: Waveforms of one 40 kW inverter with 250A coil current, at, a) no load and, b) full load.
Figure 13: PWHT of zone 1 of a 12m long steel pipe using four-zone heating by independent power converter perfectly matches to the reference profile of Figure 1.
Figure 14: PWHT of zone 2 of a 12m long steel pipe using four-zone heating by independent power converter perfectly matches to the reference profile of Figure 1.
Figure 15: PWHT of zone 3 of a 12m long steel pipe using four-zone heating by independent power converter perfectly matches to the reference profile of Figure 1.
Figure 16: PWHT of zone 4 of a 12m long steel pipe using four-zone heating by independent power converter perfectly matches to the reference profile of Figure 1.
DETAILED DESCRIPTION OF INVENTION
The detailed description of the present invention should not be read as to limit the scope of the invention.
The present invention discloses a system and method of using multiple induction heating systems for pre and post weld heat treatment of large welding pipes. In a preferred embodiment, the length of the welding pipe divided into multiple zone and each zone have coil segment driven by an independent ZVZCS (zero-voltage switching at turn-on and zero-current

switching at turn-off) series resonant inverter assisted by a power control circuit to achieve near resonant frequency operation where the power loss in each converter is minimum. Each segment of pipe is heated by an independent non-synchronized power converter by feeding a desired current to coil and to facilitate transfer of energy to load/ zone through induction heating. Through this process a superior uniformity in temperature rise achieved through separate zone-wise power control boosts the energy efficiency as overheating in any zone is avoided. It boosts energy efficiency.
In another preferred embodiment, the system uses a novel flux bypass circuit which avoids noise coupling of one system from the adjacent coil heads which otherwise would create problem for equipment malfunction.
Post weld heat treatment of a long seam joint requires large power. The power requirement for post weld heat treatment is more than pre weld heat treatment of the same area. To perform post weld heat treatment of long seam welding, this invention uses multiple air-cooled coil-heads fed by independent power converters. To avoid any cross-coupling, each coil is magnetically shielded from the adjacent ones. Conventional air-cooled induction heating controllers are normally for 40 kW applications. This invention boosts the power capacity where wide range heat-treatment solutions on a large area could be obtained. Multi-converter multi-coil air-cooled controller is equipped to handle wider applications efficiently. Due to smaller size coil-heads, handling of new jobs/smaller jobs becomes easier.
Here the coil is air-cooled coil for which the power transfer efficiency to the pipe is high, and overall energy efficiency of the whole system is improved. Also because of this system being air-cooled, any external arrangement for conduction cooling is avoided making the system compact. Here the flexibility of each air-cooled litz-wire based coil helps the controller could be moved close to the pipe.

For low power applications, say for pre weld heat treatment (<200 C), only desired number of controllers could be made active instead of all controllers, making the arrangement more energy efficient. The use of induction heating system for energy transfer converts the system friendliest to people in the shop-floor, it makes the system eco-friendly for energy use at the load end. The induction heating system does not cause ambience heating; and the use of smaller coils for each controller makes the system comfortable to the operators for easy handling. Through induction heating this method transfers requisite energy contactless efficiently while the coil remains cold. The heat is directly transferred to the object to be heated although the heater is not heated.
In a preferred embodiment, the induction coil carries current of desired magnitude and frequency that generates electromagnetic field on its surrounding area. Under this method multiple options are available for controlled energy delivery to the section of the pipe where welding is either to be performed (pre-weld heat treatment) or joint just made (post-weld heat treatment). In the first case, moisture is removed before welding while the second case is for releasing the stress around the joint. The energy required for pre weld heat treatment is much less.
In typical case, the energy demand for circular seam is less than that of long seam joint. Thus to take temperature to 650 0C, the power demand by a 12m long, 1.1m dia. and 50mm thick pipe could go up to 120 kW. A power controller suitable for heat treatment of pipes is shown in Figure 2 comprising chopper control circuit, inverter control circuit. This power controller operates on series resonant principle. However, it is difficult to feed such a large power of 120 kW using a single coil head. In a preferred embodiment, the length of the pipe is divided into multiple sections, each section identified as heat zone, wherein One coil is proposed to heat one section fed by independent (non-

synchronized) power converters. For example, a 12m long pipe (shown in Figure 3) is divided into 4 sections, that is, 4 heating zones, each 3m long. This is for the post weld heat treatment. For low power demand such as in pre-heat treatment, single coil may be used (see Figure 4).
In preferred embodiment, multiple induction heating controllers feeding energy to the desired zones separately of a joint using smaller coils so that voltage across each coil is safe. For reliable operation of multi-controller system, one preferred embodiment uses ferrite concentrators to avoid any cross-talk between adjacent coil heads to avoid disturbance in the operation of the controllers.
In this method the power loss by convection and radiation is absent and the start/stop time of the process is negligible. The distribution of transferred power is achieved by coil configuration. This air-cooled induction heat treatment process is energy-efficient and environment friendly process. The present invention could be used in wide range heat-treating applications because the number of coils with associated power controllers could be selectively enabled/disabled.
In another embodiment, the induction heating the coil is energized to a desired current. The coil could be fed in multiple ways. For an application where power delivered is controlled over a wide range, there the controller configuration as shown in Figure 2 is used. Moreover, the power controller achieves zero voltage at turn-on and zero voltage at turn-off of power devices Q2-Q5, thereby the controller achieves high efficiency. This application needs flexible coil because the job is not fixed. Also the power density (W/m2) needed is moderate where air cooled coil is preferred. Compared to conduction-cooled coil, the power loss in litz-wire based air-cooled coil is much less. The power PL delivered to the pipe by the coil through induction effect depends on several factors as listed below:

PL = ilReq=f(Kc,L1,iL,fs) (1)
L1 is inductance value of the coil, fs is frequency of coil current IL and parameter Kc depends on coupling between coil and the pipe. The distribution of PL in the pipe is important factor. The volumetric distribution of PL should be uniform. However, the major part of it is concentrated on pipe's surface whose depth 6 is decided by skin effect, as,
8 = , 1 P^ (2)
Where µo is permeability of free space and µr and p respectively are relative permeability and resistivity of the metal. Due to large µr, even at moderate fs, the value of 6 for a pipe constructed of magnetic metal such as steel is small. The design involves a compromised choice of Li, IL and^s. Distribution of heat through thermal conduction plays important role to achieve desired temperature profile, it increases the effective mass m.
The resistance of eddy current path Rjob in the pipe consisting of diameter D and coil width W is expressed as
Rjob =^ = ^1 =^V10-7(P^/S) 0)
L is the length of the pipe to be heated. The value of both Rjob and p changes when temperature of the job rises. In PWHT the total width of heating zone W (mm) on either side of welding may be approximated as [1], [2],
W = SjR~Ts (4)
Rin and ts respectively are inside radius and thickness of the pipe, their units are in mm. The area of the induction coil Acoil for heating the specified joint area of pipe of length Lpipe is
•^coil — ^pipeV irc s (*-*)
The power density Pa needed for heat treatment use is

Due to large value of Acoil and theat the value of both Pa and fs are moderate where litz wire based air-cooled coil is preferred.
In another preferred embodiment, the power controller uses two-loop control (see Figure 5). The external loop is for control of process parameters; it generates the reference power for the inner loop. The coil LI facilitates the energy transfer to load. To improve the energy efficiency air-cooled litz-wire based flexible multi-coil arrangement is used to transfer power to different segments of the pipe. Considering the ideal situation of zero radiation loss, etc., the power PL needed to raise the temperature of pipe to 7°C from ambient Tamb in time duration theat could be expressed as,
PL = mC^T~Tamb\ and PL = ^ (7)
theat Req
Cp is specific heat, m is mass of steel in the pipe, VCH is chopper voltage, Req is effective load resistance and K1 is constant.
PWHT applications of 12m long pipe needs large power (around 120 kW) to be transferred to the pipe, particularly, particularly because both the diameter (1.1m) and thickness (50 mm) of the pipe are large. However, the capacity of a single air-cooled is not large, it is restricted by the safe voltage limit of each coil head i.e., the value of inductance in LI cannot be increased to any limit. To overcome this problem, in this invention, the heating of long pipe is converted into a multi-zone heating where there is no restriction on characteristics of power inverter.
The advantages of the present invention are that the handling of 3m long pipe and associated cerawool insulation is comparatively much easier, placement of pre-set coils of 3m length can be done quickly on the welded seam, fast heating rates can be achieved, each zone of heat zone here 3m can be independently controlled respectively by separate self-tuning PID controller, also quick removal of coil is possible because the coil itself doesn't get heated.

In the present invention uniform temperature can be easily achieved across length and thickness of the pipe, the Cycle time can be optimized as low setup time, fast heating rate, quick change over possible. It has very low cost of operation (15 Ton Pipe, long seam PWHT over 2 hours heating, 2 hours soaking, no power required for cooling, approximately 7500 per pipe), the process is operator friendly, environment friendly, it does not cause any ambience heating. There is no polluting gas in the surrounding.
Multi-zone induction heating using asynchronous power controller provides wider scope because for applications where PWHT for different size pipes needs to be performed then the power controllers could be selectively enabled/disabled. The efficiency of energy transfers by non-contact means using an air-cooled coil-head is maximum. Due to the small current density used, the power loss in the air-cooled coil-head is small. Finally, because the coil is flexible, the air-cooled system could easily adapt to new applications of different dimensions.
In another embodiment, the arrangement to isolate one coil (also the power converter) from the adjacent one is shown in Figure 6. The coupling of one coil from other disturbs the functioning of the affected inverter. The phase-locked loop gets affected. The electromagnetic noise decoupling is done by using ferrite core assembly. One such assembly, used in practical demonstration, is shown in Figure 7. The actual physical arrangement of decoupling coils/converters magnetically is shown in Figure 8a, and Figure 8b shows the arrangement of shielding four coils. Each coil is fed from a 40 kW power converter. The schematic diagram of the complete system is shown in Figure 9. For feedback of temperature, thermal probes are used in each zone. Four number of actual 40 kW power converters and the associated coil assemblies are shown in Figure 10.

Test Result:
For experimental validation the details of system parameters are listed in the
following Tables:
Date:03/08/2024 • Coil Parameter:

From the Tables it is evident that the coils inductances are different and Coil current without load and with load across the heating zone are different indicating the asynchronous system.

To conduct post weld heat treatment to achieve the temperature profile of Figure 1. The physical dimension of the pipe is as follows- diameter: 1.1m, length: 12m and thickness: 50mm. The total weight of the pipe is 15430kg (please see the Test Report). The power required to heat the total mass to 600 0C needs 120 kW power to be transferred to the welded section. Figure 11a shows the current in each coil at no load condition. Figure 11b shows the coil current while performing post weld heat treatment of a 12 m long pipe. It is clear from waveforms under no load and full load conditions that there was no noise coupling from adjacent energized coils. The complete system could deliver close to 150 kW. Figure 12 shows the operation of one power converter – both at no load condition and loaded condition.

The present invention provides a solution of complete heat-treatment of longitudinal seam joint of long pipes, post weld heat treatment consumes much more energy than pre weld heat treatment. The present embodiment provides for independent zone-wise temperature control through use of
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multiple air-cooled coil segments each fed by independent ZVZCS based power controller makes the system energy efficient. Independent power control helps achieves uniform temperature throughout the desired range. Use of multiple small-size coil segments each fed by an isolated power converter makes the system safe for operators around. Use of magnetic shunt or flux bypass circuit consisting of ferrite cores simplifies the system design because the converters need not be synchronized, each could be independently driven. Through use of air-cooled induction system, this invention converts the heat-treatment of a massive long pipe eco-friendly at the process end, causing zero harm to the workforce present around.

CLAIMS
1. A system [900] for heat treatment of a long welding pipe [910] using multiple asynchronous induction heater [920], the system comprising:
a. a welding pipe acting as load [910] is divided into one or more number of
zone or heating zone, wherein each heating zone [930] is of length 3m;
b. one or more flexible heating coil [610] wherein each coil is of length 3m
placed on each zone of the welding pipe along with cerawool insulation;
c. one or more independent power converter [200] of 40kW to energize the
coils [610], wherein each independent power converter energizes one coil,
and each power converter comprising a ZVZCS series resonant inverter
assisted by two-loop power control [500] circuit with a programmable
logic controller circuit [510], chopper control circuit [220], inverter
control circuit [230] and power devices [Q2-Q5],
wherein the power converter [200] achieves zero voltage at turn-on of the power devices [Q2-Q5] and achieves turn off of the inverter power devices [Q2-Q5] at near zero current, making the power controller highly efficient; wherein an external loop [530] of the two-loop controller [500] is responsible for generation of power reference for an inner loop [520]; and
d. ferrite core to function as magnetic shield or shunt [620] or flux bypass
circuit with high magnetic permeability and low electrical conductivity
preventing crosstalk by coupling of signals or magnetic field of one coil
from influencing the magnetic field of the adjacent coil and prevents any
sensing issues for proper power control and phase-lock between the
inverter voltage and coil current of each inverter and to ensure the
functioning of internal loop remains unaffected;

all operatively connected.
2. The system [900] for heat treatment of a long welding pipe [910] using multiple asynchronous induction heater [920] as claimed in claim 1, wherein the number of power converter [200] is equal to the number of coils [610], and number of coil [610] is equal to number of heating zone [930].
3. The system [900] for heat treatment of a long welding pipe [910] using multiple asynchronous induction heater [920] as claimed in claim 1, wherein each power converter [200] is having different phase or working at different frequency or both compared to remaining power converters making the system asynchronous.
4. The system [900] for heat treatment of a long welding pipe [910] using multiple asynchronous induction heater [920] as claimed in claim 1, wherein each flexible heating coil is air-cooled litz-wire based flexible coil [610] having low power loss, moderate power density, suitable for independently heating a zone [930] when zone length is less than 3m or equal to 3m.
5. The system [900] for heat treatment of a long welding pipe [910] using multiple asynchronous induction heater [920] as claimed in claim 1, wherein the power delivered to the welding pipe through the coil is a function of inductance value of the coil, coil current, frequency of the coil current, coupling coefficient (kc) between coil and welding pipe.
6. The system [900] for heat treatment of a long welding pipe [910] using multiple asynchronous induction heater [920] as claimed in claim 1, wherein the power density for heat treatment is a function of Power delivered (PL), length of pipe (Lpipe), inside radius (Rin) of the welding pipe and thickness of the welding pipe (ts).

7. A method of heat treatment of a long welding pipe using multiple asynchronous induction heaters, the method comprising:
a. dividing of a welding pipe acting as load [910] into multiple number (n)
of zone, wherein (n-1) number of zone [930] is of length 3m and one last
zone is less than or equal to 3m depending on the actual length of the
welding pipe based on divisibility of length of pipe by 3;
b. one or more flexible heating coil [610] wherein each coil is of length 3m
placed on each zone of the welding pipe along with cerawool insulation,
wherein some portion of 3m coil is hangs if zone length of the last zone
is less than 3m portion of coil;
c. separating each heating zone length with ferrite core to function as
magnetic shield [620] or flux bypass circuit with high magnetic
permeability and low electrical conductivity for low core loss in flux
bypass circuit preventing crosstalk through coupling of signals or
magnetic field of one coil from influencing the magnetic field of the
adjacent coil and prevents malfunction of phase-lock loop circuit
between the inverter output voltage and the coil current of each system;
d. energizing each coil with one independent power converter [200] of
40kW, wherein each independent power converter energizes one coil
[610];
e. varying and controlling of heating rates of each coils independently in
asynchronous manner and cycle time, set-up time, and quick
changeover is achieved through programmable logic controller; and the
controller [500] achieves zero voltage at turn-on of the power devices
[Q2-Q5] and achieves near zero current at turn off each of the power
devices [Q2-Q5], making the power controller highly efficient; and
f. removal of coil [610] post operation.

8. The method of heat treatment of a long welding pipe using multiple asynchronous induction heaters as claimed in claim 7, wherein the method comprising activation of only such number of power controller to attain desired temperature.
9. The method of heat treatment of a long welding pipe using multiple asynchronous induction heaters as claimed in claim 7, wherein the method activates only such number of power controller for the preheat treatment of the long welding pipe requiring temperature less than 200°C.