FIELD OF THE INVENTION
The present invention relates to a method of fixing Heat retaining shell in
fabrication of high pressure boilers. More particularly the invention relates to a
holding device for positioning a assembly Heat retaining shell assembly for
uniform post heating of pipe with pipe butt joints.
BACKGROUND OF THE INVENTION
In boiler production, lines & links are manufactured to connect different parts
of a boiler and these links are made up of number of butt joints, which are
welded in SAW machine, to reduce cycle time of the product. During welding of
a job, following three welding temperature need to be monitored to have a
defect free welding :
1. Preheating temperature for minimum 2 hours
2. Interpass temperature to be maintained throughout welding.
3. Post heating temperature for minimum 2 hours.
Before commencing a welding the job is pre-heated for 2 hrs. (,) then the
welding is done. After completion of the weld, a post heating is started. Since
heating is done by using a producer gas burner, after completion of the welding
(i.e.) the welder starts the burner for post heating which may exceed the post
heating for more than 2 hours. Moreover, the burner flame is focused at a
particular area during the post heating, which further results in non-uniform
heating throughout the circumference of the butt joint.
OBJECTS OF THE INVENTION:
It is therefore an object of the present invention to propose a device (to retain)
uniform post heating (in) pipe with pipe butt joints (welding).

Another object of the invention is to propose a device for a heat retaining shell
for uniform post heating of pipe with pipe butt joints to propose, in which the
heat retaining shell is disposed within the work piece.
SUMMARY OF THE INVENTION:
Accordingly, there is provided a holding device for positioning a Heat retaining
shell assembly for uniform post heating of pipe with pipe butt joints. In
accordance with the invention, the heat retaining shell is mounted on a circular
pipe exactly on the butt joint, where post heating is required. This method of
fixing the Heat retaining shell assembly over the butt joint, immediately upon
unloading from the Auto welding machine, is as follows. First, the two
hemispherical shells are lifted by a lifting hook and bolted using a locking pad to
form a holding device positioned over the butt joint for which the post heating
to be done. Then, the temperature is monitored using thermocouples, which
are attached at the surface of the butt joint to check whether the desired post
heating is done. After about 2 hours of post heating, the shell can be either
removed or placed over the surface of the butt joint until it goes to furnace.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1 shows the constructional features of a Heat retaining shell assembly in
front view.
Figure 2 shows the plan view of the constructional features of the Heat
retaining shell assembly.

DETAIL DESCRIPTION OF A PREFERENCE EMBODIMENT OF THE
INVENTION
According to the present invention, after completing the SAW welding of the
butt joint (,) two hemispherical shells (10) (of the device) are mounted on the
butt joint by lifting hook (2) provided with a locking pad (1). This locking pad
(1) is attached to the two hemispherical shells (10) to position a so-assembled
holding device (on pipe) during post heating of the butt weld. The movement
can be arrested using bolt and nuts (9). One of the two hemispherical shell has
a Wool packing chamber (3), in which glass wool material (4) is packed tightly
to avoid heat being transferred to outer hemispherical shell and there by heat
loss by convection and radiation (is avoided). At the sides of the hemispherical
shell (10), a gripping pad (6) is provided to arrest the rotation of holding device
along the circumference of the pipe joint and also to avoid heat conduction loss
at the gripping pad (6). Two slots are provided to accommodate an insulation
rope (7), which acts both as a gripping rope as well as an insulation medium to
heat conduction to the gripping pad (6). To avoid heat conduction to the inner
hemispherical shell, a hollow chamber (8) is provided which having insulation
material along its inner surface area. So, the device also acts as an isothermal
barrier for heat, hence post heating is done without usage of the heating
burners or by induction resistance heating.

WE CLAIM:
1. A holding device for positioning a assembly Heat retaining shell assembly
for uniform post heating of pipe with pipe butt joints, the heat retaining
shell (10) assembly consisting of an inner and an outer hemispherical
shell, the device comprising :
a locking pad (1) to position and hold the device along the circumference
of the pipe to pipe butt joint.
- a lifting hook (2) to lift the device including the locking pad;
- a wool packing Chamber (3), in which glass wool material (4) is packed
to act as an isothermal barrier to prevent heat transfer from the inner to
the outer hemispherical shell including heat loss by conduction,
convection and radiation;
a cover plate (5) to close the wool packing chamber (3);
- a gripping pad (6) to arrest the rotational movement of the device
disposed along the circumference of the butt joint;
- an insulation rope (7) to avoid heat conduction to the gripping pad (6);
and
- a hollow chamber (8) with insulation material to avoid heat conduction to
the inner hemispherical shell (10).