FIELD OF THE INVENTION
The present invention relates to an induction based automatic preheating device for
preheating of tube-tube joints preceding welding of high pressure boiler tubes in
straight tube butt welding (STBW) machines.
BACKGROUND OF THE INVENTION
Straight tube butt welding machine is used for joining two straight steel tubes by
metal arc welding process, wherein a stationary torch welds two ends of steel tubes
rotating in a lathe type mechanism. Various sizes of tubes, with various grades of
steel can be welded in the machine. In certain steel grades, before start of the
welding, the steel tubes require pre-heating to a set temperature depending on the
grade of steel. The preheating is done on said steel tubes to reduce cooling rate
after welding, thus minimizing embrittlement of the heat affected zone (HAZ) of the
tubes due to martensite formation.
According to prior art, preheating of the steel tube joints is done by a hand held
flame torch, by the operator, while the tubes are rotating. Once the preheating is
completed, the welding is started without affecting the fit-up of the tube-tube joint.
This flame torch preheating method has several disadvantages for example,
inaccuracy, irregular pattern of heating; non-uniformity in heating; safety hazards of
hand held torches; high cost of fuel; high time consumption; atmospheric pollution
and deterioration in the chamfered edges of tubes due to carbon pick-up which
affects the quality of weld.
Replacement of the pre-heating process by a hand-held frame torch, with induction
heating process, many of the disadvantages may although be overcome, but in the
induction heating system, the draw backs are mainly the mechanism to approach the
tube-tube joints without affecting their alignment and level. Because in a tube-tube
joint, once the alignment and leveling are done, it should not be altered; however, in
an induction heating system, a closed induction coil is used in the tube-tube joint,
which requires removal after each phase of heating.
OBJECT OF THE INVENTION
It is therefore an object of the invention to propose an automatic tube preheating
device for straight tube butt welding machines.
SUMMARY OF THE INVENTION
Accordingly, there is provided an automatic tube preheating device comprising a
high frequency power supply unit to resonate an induction coil, a 'C' shape induction
coil, a water chiller to circulate cool water to the induction coil, at least one power
converter, a linear motion (LM) guide way to mount and guide the induction coil, a
ball screw mechanism to move the 'C' shape induction coil 'to and fro' to the tube-
tube joint, a servomotor for rotating the ball screw, a programmable logic controller
to control the servomotor based on the feedbacks transmitted by field devices.
When a preheating cycle is started, the tube rotates in a lathe chuck of the STBW
machine, and the 'C' shape induction coil moves towards the tube -tube joint to heat
the junction to a set temperature. After the set time lapses, the 'C' shape coil
retracts from the current position to the home position.
All the process steps are automated by the Programmable logic controller (PLC). One
key feature of the process is that the coil automatically determines the distance
between the tube junction and the 'C' coil by means of a proximity sensor. The same
cycle can be repeated any number of times till the tube outer diameter is changed.
The major benefits of the inventive device are that the process is faster than flame
torch preheating; operation of the device does not require any significant expense
on fuel gases; produces fewer fumes than prior art flame heating, hence very low
atmospheric pollution; only the work part becomes hot; uniformity in heating; and
no carbon deposit on the chamfered edges, assuring high quality joints.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 : Shows a schematic of an induction-based automatic pre-heating device in
Straight tube butt welding machines according to the invention.
Figure 2 : Shows a block diagram of the controller of the automatic device of the
invention.
Figure 3 : Shows a 'C' shape induction coil in side view used in the inventive device.
Figure 4 : Shows a front view of the 'C' shape induction coil of figure 4.
Figure 5 : Shows the 'C' shape induction coil and position of the work piece to be
preheated.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION.
Figure 1 shows the schematic of the automatic device used in STBW machine. A 'C'
shape induction coil [1] is connected to a high frequency power source by a power
cable [2]. A water chiller is used to cool the coil (1) by circulating sufficient cool
water [3] by means of hoses. The movement of the induction coil (1) is supported
by a sliding means [4] connected to a ball screw [5] driven by a servomotor [6]. The
servomotor (6) is controlled by a programmable logic controller. The programmable
logic controller transmits signals to the servomotor (6) based on the inputs given by
an inbuilt servo encoder, a plurality of proximity sensors and a limit switch. The
control wiring and the programmable logic controller and associated switchgear
means are assembled inside the control panel.
The sliding means [4] of the 'C' type induction coil [1], consists of a plate fitted on a
linear motion guide way [7]. The 'C' type induction coil (1) is mounted on the top of
the plate. Two proximity sensors (8, 9) are kept on either ends of the means (4) to
detect the reference positions of forward and reverse movement of the 'C' type
induction coil [1] respectively through feedback cables [10]. The movement between
the two reference points is ensured by the servomotor's [6] inbuilt encoder.
The 'C' type induction coil [1] comprises a fin[11] to read the distance between the
tube ends to be preheated and the 'C' type induction coil [1]. When the 'teach' logic
is selected in the programmable logic controller (PLC), the servomotor [6] moves the
'C' type induction coil [1] forward till the fin [11] touches the tube-tube joint to be
preheated. Once the fin [11] touch the tube (work piece), the fin [11] activate a
third proximity sensor [12] and the servomotor [6] stop to drive the ball screw [5]
and the 'C' type induction coil [1] stops leaving a minimum air-gap between the
induction coil and work piece. The current position of the 'C' type induction coil [1] is
stored in a memory of the programmable logic controller through the position
feedback given by the servomotor's inbuilt encoder. A moment later the 'C' type
induction coil [1] reverses to the reference position till it reaches the sensor [9] and
the difference between the reverse reference position and stored position is
calculated by the controller to determine the distance to forward the 'C' type
induction coil [1] upon start of a preheating cycle. The 'C' type induction coil [1] will
always moves the same distance every time till the next 'teach' logic is selected. This
system enables the automatic operation of preheating over repeated cycles.
Figure 2 shows the control schematic of the automatic preheating device.
When the preheating cycle is started by means of an electrical push button switch,
the 'heating' logic is selected in the programmable logic controller. The PLC checks
the current position of the 'C' type induction coil [1] through the servomotor's [6]
inbuilt encoder and calculates the distance to reverse till reference sensor [9] is
activated. From the reverse reference position the 'C' type induction coil [1] moves
forward towards tube-tube joint and stops leaving a small air gap in between. The
forward distance to be moved is preset in the PLC by means of 'teach' logic as
explained in the previous paragraph. Once the 'C' type induction coil [1] reaches its
final position, the PLC gives signal to switch on the high frequency power supply.
The high frequency generated resonates the 'C' type induction coil [1] through the
power cable [2]. Due to resonance high current flows in the tube-tube joint which
generates a large amount heat in it, till the preset count in the timer of PLC lapses.
The heating time is predetermined by the user depending on the size of the work
piece to be heated. After lapse of the heating time, the PLC switches off the high
frequency power supply, thereby stopping the passage of current to work piece and
stops further heating of the work piece. The 'C' type induction coil [1] retracted back
by reverse movement of the servomotor (6) and thereby enabling the welding torch
to approach on the tube-tube joint for welding the joint.
During the heating process, the 'C' type induction coil [1] is cooled by circulating
cold water through hoses [3] to avoid overheating and damage of the coil. For
different sizes of tubes different sets of coil assembly [13] are to be used. To ensure
a minimum air-gap between the 'C' type induction coil [1] and the work piece,
corresponding coil assembly [13] is used by replacement of the earlier coil simply
removing the fasteners in the assembly joint [14]. A protection cover [15] is used to
protect the 'C' type induction coil [1] from weld spatters during welding.
Figure 3 shows the 'C' shape induction coil in side view. The work piece is shown in
dotted circle.
Figure 4 shows the 'C' shape induction coil in front view.
Figure 5 shows a general set up of the work piece and the 'C' shape induction coil
[1]. The work piece consisting of two tubes [16] & [17] held by two parallel lathe
chucks [18] & [19]. By moving the chucks the two tubes' edges can be butt together
[20]. By energizing a rotating means both the chucks [18] & [19] can be rotated
together. While starting the heating cycle by electrical push button means, the
rotating work piece's butt end and adjacent area is heated by 'C' shape induction
coil [1].
WE CLAIM :
1. An induction-based automatic preheating device in straight tube butt welding
machines for preheating of the joint preceding welding of high pressure boiler
tubes, the device comprising a high frequency power supply, a 'C' shape
induction coil in electrical connection with the high frequency power supply, a
sliding means to mount the 'C' shape induction coil, a servomechanism to provide
the 'C' shape induction coil a to and fro motion and a programmable logic
controller to actuate the servomechanism, wherein the 'C' shape induction coil
moving to determine a distance between the 'C' shape induction coil and work
piece, to be preheated, by a teach logic incorporated in the programmable logic
controller, wherein the 'C' shape induction coil moving forward to the work piece,
to be preheated, and halting at a position calculated by the programmable logic
controller, wherein the 'C' shape induction coil starts heating the rotating work
piece till lapse of a preset time in the programmable logic controller, and
wherein the 'C' shape induction coil reverses back to a reference position as
sensed by sensor means and halting in that position.
2. The device as claimed in claim 1, wherein the 'C' shape induction coil is
constructed of a hollow metal tube, of known cross-section, spirally wound
around a box opening, and having two lead outs for enabling electrical
connection with the high frequency power supply.
3. The device as claimed in claim 1 or 2, wherein the shape of the induction coil in
the side view appearing as English alphabet 'C' and rectangular pancake shape in
the front view.
4. The device as claimed in claim in any of the preceding claims, wherein cold water
is circulated to avoid damage of the coil due to overheat and to reduce the
electrical resistance developed due to high temperature.

ABSTRACT

The invention relates to an induction-based automatic preheating device in straight
tube butt welding machines for preheating of the joint preceding welding of high
pressure boiler tubes, the device comprising a high frequency power supply, a 'C'
shape induction coil in electrical connection with the high frequency power supply, a
sliding means to mount the 'C' shape induction coil, a servomechanism to provide
the 'C' shape induction coil a to and fro motion and a programmable logic controller
to actuate the servomechanism, wherein the 'C' shape induction coil moving to
determine a distance between the 'C' shape induction coil and work piece, to be
preheated, by a teach logic incorporated in the programmable logic controller,
wherein the 'C' shape induction coil moving forward to the work piece, to be
preheated, and halting at a position calculated by the programmable logic controller,
wherein the 'C' shape induction coil starts heating the rotating work piece till lapse of
a preset time in the programmable logic controller, and wherein the 'C' shape
induction coil reverses back to a reference position as sensed by sensor means and
halting in that position.