FIELD OF THE INVENTION
The invention relates to an in-situ process for stress relieving of heat transfer
surfaces inside a heat exchanger chamber without damaging the peripheral
refractory wall.
BACKGROUND OF THE INVENTION
Tube bundles in heat exchangers are fabricated by means of welding. Generally
when the tube material is of superior grade for example alloy steel or stainless
the steel, procedure for tube to tube welding requires a post weld heat treatment,
to remove any undue stresses that may build up due to the welding. This process
is called stress relieving operation. Like any other heat treatment process, if not
performed properly this simple operation can have disastrous consequences.
Tube failure at tube to tube joint location is the most common consequence.
Brittle failure of the tubes due to excessive hardness imposed during improper
stress relieving operation, is one of the most common reason behind this.
Therefore, the stress relief operation needs to be carried out very carefully by
following a proper procedure. On the other hand, if there is any tube leakage
during the operation, a repair work needs to be done, which needs to follow the
same criteria of post weld heat treatment for stress relieving. One method of
repair is to remove one side of the heat exchanger chamber and takeout the coils
for repair. But this requires a huge time and availability of a desired furnace for
heat treatment. Instead of that, if the heat treatment can be performed inside the
heat exchanger chamber itself, the same reduces infrastructural requirements
including process time. During in-situ heat treatment operation, there is a chance
that the input heat may cause damage to the surrounding refractory wall of the
heat exchanger.

US 6247231 refers to a Method for repairing heat exchanger tubing through
partial tube replacement. The method includes a step of removing a degraded
tube segment from an operative heat exchanger tube in a steam generator,
positioning a weld metal ring between the operative heat exchanger tube and a
new tube segment, welding the metal ring to the operative heat exchanger tube
including the new tube segment to form a linking tube section which is then heat
treated using any conventional electric heating technique. The new tube segment
is then expanded toward a tube sheet of the steam generator, welded to the tube
sheet.
US 8038931 B1 relates generally to induction heating, and particularly to a
method and apparatus for inductively heating a work piece using an induction
heating system located at a worksite. The induction heating system comprises a
portable power source, a power source controller, a fluid-cooled induction
heating cable, and a portable cooling unit. The induction heating system may be
used to perform a variety of induction heating operations, including: annealing,
surface hardening, heat treating, stress-relieving, curing, and shrink-fitting, etc.
US 3414697 A describes an apparatus for stress relieving of welded metal pipe
joints and more particularly to an induction heating apparatus for stress relieving
girth welded joints in large diameter pipe. In the construction of a large diameter
metal pipe line, such as a penstock for a hydro-electric power plant, the pipe
sections are joined together by girth welds which are generally made from inside
of the pipe. Due to high internal pressure to which the pipe is subjected in service,
it is essential that the welds be sound and that the welded areas be substantially
free of stresses.
Thus, the applicants admitted prior art (AAPA) describe various methods for heat
treating of tube to tube weld joints after welding. In all these disclosed prior art,
the method of induction heating is used for heat treatment, which is time
consuming and can be done for each tube one by one. Whereas in the present

invention, single source such as a burner is used to heat the entire coil bundle
inside the chamber without causing any damage to surrounding refractory. This
necessarily reduces the time required for heat treatment drastically.
Heat transfer surfaces like coils, are formed of a plurality of tubes which are
disposed inside an enclosure known as the heat exchanger chamber, where the
heat transfer takes place from hot flue gas to a fluid inside passing through the
tubes. Coils are formed by carrying-out tube to tube butt welding. When the tube
material is of higher grade alloy steels, it warrants implementation of a stress
relieving process after the welding in order to remove any stresses that may be
formed during the fabrication. Improper stress relief operation leaves the tubes
with undesired Hardness. Accordingly, an improved stress relieving operation
has to be carried out to eliminate possible defect formation in the welded
structure. When a repair work needs to be subsequently done at the tube to tube
joint location due to tube leakage, a procedure of post weld heat treatment is
carried-out. In other words, repetitive welding of tube to tube joint is a common
phenomenon which suggests that the hat treatment be carried-out inside the
chamber itself, to avoid time for repair and provision of a desired furnace.
OBJECT OF THE INVENTION
It is therefore an object of the invention to propose an in-situ process for stress
relieving of heat transfer surfaces inside a heat exchanger chamber without
damaging the peripheral refractory wall.
Another object of the invention is to propose an in-situ process for stress
relieving of heat transfer surfaces inside a heat exchanger chamber without
damaging the peripheral refractory wall, which reduces process time both for
initial welding as well as at the time of repair work.

SUMMARY OF THE INVENTION
Accordingly, there is provided An in-situ process for stress relieving of heat
transfer surfaces inside a heat exchanger chamber without damaging the
peripheral refractory wall, comprising the steps of: placing at least one burner (4)
inside a chamber (2) of the heat exchanger to carry out the heat treatment, the
in-situ heating being made through an opening created in the wall of said
chamber (2), the burner (4) being located on the chamber (2) leaving a gap from
tip of the burner (4) to the refractory surface inside the chamber (2), so as to
maintain a minimum length between burner tip and the coils which avoids flame
impingement on the coil tubes (1), and positioning one each dummy horizontal
wall (6) and vertical wall (7) made of insulation material inside the chamber (2) to
allow a focused heating of the coils (1), an opening (8) for exhaust vent
arrangement being provided in the horizontal dummy wall (6) for exhaust of the
flue gases, wherein the burner (4) is equipped with means for adjustment
including controlling the rate of heating and cooling, the process is characterized
by comprising: activating the burner and gradually heating the enclosure to about
760°C, the rate of heating being maintained at 50°C / hour; continuing, in case of
interruption during heating; heating up to 760°C at a rate of 50°C / hr; allowing a
soaking time of 60 minutes at 760°C in case of an interruption during the soaking,
the heating being re-continued to reach at 760°C, maintaining the rate of heating
at 50°C /hr and further soaking for the balance duration; gradually slowing down
the burner operation to cool the coil bundle to 250°C by maintaining a rate of
cooling at the rate of 50°C /hour; suspending the burner operation at 250°C
below which natural cooling is carried out; wherein when an interruption taking
place during cooling, re-starting the burner operation and heating again the
welded joint-upto 760°C, maintaining a maximum rate of heating at 50°C /hr and
soaking for 5 minutes and then cooling the joint.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
The following is the list of figures accompanying the patent specification and their
description.
Fig. 1: - Shows a vertical cross section diagram of the interior of a refractory lined
chamber of a heat exchanger in a side view.
Fig. 2: - Shows the horizontal cross section diagram of the interior of the
refractory lined chamber of the heat exchanger, when viewed from the top.
Fig. 3: - Shows a vertical cross section of the interior of the refractory lined
chambers of the heat exchanger, specifically showing the arrangement of one
each temporary horizontal and temporary vertical partition walls with vent
opening, viewed from the side.
Fig. 4: - Shows a Heating and cooling curve
DETAILED DESCRIPTION OF THE INVENTION
The attached drawing may be referred for better understanding the process of
the disclosed invention.
Coil tubes (1) are placed inside the heat exchanger chamber (2). The chamber
(2) wall is internally lined with refractory material (3), to provide sufficient
insulation to the chamber wall during heat transfer inside the chamber. A Burner
(4) is placed inside the chamber (2) to serve the purpose of heating the coil tubes
(1) during heat treatment, through an opening created in the chamber (2) wall.
The burner (4) is located on the chamber (2) leaving a gap from tip of the burner
(4) to the refractory surface inside the chamber (2), so as to maintain a minimum
length between the burner tip and the coils (1). The Burner (4) is placed in such a

way that flame impingement over the tube (1) can be avoided. The Burner (4) is
provided with means controlling the rate of heating and cooling. For a focussed
heating of the coil (1) and avoidance of damages of the coils disposed inside the
adjacent chamber (5), once each dummy horizontal wall (6) and vertical wall (7)
made of insulation material are provided inside the chamber (2). An opening (8)
for exhaust vent arrangement is provided in the horizontal dummy wall (6) for
exhaust of the flue gases. The stress relieving operation is done in the following
way (Refer Fig-4).The burner (4) is started gradually and the enclosure is heated
to 7600C. The rate of heating is maintained at 500C / hour, to satisfy the
requirements of the refractory. In case of any interruption during the heating,
heating up to 7600C shall be still continued at a rate of 500C / hr. A soaking time
of 60 minutes is maintained at 7600C .In case of interruption during the soaking,
the heating shall be continued to reach at 7600C maintaining the same rate of
heating of 500C /hr and soaking for the balance duration. Gradually, the burner is
slowed down and the coil bundle is cooled to 2500C by maintaining a rate of
cooling of 500C /hour (max). The burner shall be put off at 2500C, below which a
natural cooling is carried out. In case of interruption during the cooling, heating
again shall be done to reach at 7600C, maintaining said rate of heating at 500C
/hr (max) and soaking for 5 minutes and then shall be cooled as mentioned
above .Thermocouples are installed for measuring the temperature at the butt
joint locations during the entire process. Non Destructive Test is carried out on
any of the weld joints. Hardness values at top and bottom locations of the coil
bundle is checked in order to ensure whether proper stress relieving procedure
has been followed or not. The dummy horizontal wall (6) and vertical wall (7)
made of insulation material are dismantled and removed from the heat
exchanger chamber. Thereby, the in-situ heat treatment process is completed.
ADVANTAGES OF THE INVENTION
For post weld heat treatment for stress relieving of heat transfer tubes inside a
heat exchanger chamber is carried-out to eliminate the requirement of huge work

associated with bringing the coils back to workshop and finding appropriate
furnace to proceed with heat treatment. Even if the heat treatment is done at site,
in the vicinity of the chamber, each tube assembly where welding is carried out
locally, consumes several hours and to be done one by one by method of
induction heating. This inventive process is time consuming and expensive.
Stress relief operation of heat transfer tubes done inside the heat exchanger
chamber without damaging the tubes. Proper Exhaust vent was provided to avoid
entrapment of flue gas inside the chamber.
Heat treatment for the entire coil bundle can be done at one time, saving huge
time & manpower.
The stress relieving process is done at a rate to minimise damage to surrounding
refractory, the rate of heating and cooling are selected in an optimised way.

WE CLAIM
1. An in-situ process for stress relieving of heat transfer surfaces inside a
heat exchanger chamber without damaging the peripheral refractory wall,
comprising the steps of:
- placing at least one burner (4) inside a chamber (2) of the heat
exchanger to carry out the heat treatment, the in-situ heating being
made through an opening created in the wall of said chamber (2), the
burner (4) being located on the chamber (2) leaving a gap from tip of
the burner (4) to the refractory surface inside the chamber (2), so as to
maintain a minimum length between burner tip and the coils which
avoids flame impingement on the coil tubes (1), and
- positioning one each dummy horizontal wall (6) and vertical wall (7)
made of insulation material inside the chamber (2) to allow a focused
heating of the coils (1), an opening (8) for exhaust vent arrangement
being provided in the horizontal dummy wall (6) for exhaust of the flue
gases,
wherein the burner (4) is equipped with means for adjustment including
controlling the rate of heating and cooling, the process is characterized
by comprising:
- activating the burner and gradually heating the enclosure to about
760°C, the rate of heating being maintained at 50°C / hour;
- continuing, in case of interruption during heating; heating up to 760°C
at a rate of 50°C / hr;
- allowing a soaking time of 60 minutes at 760°C in case of an
interruption during the soaking, the heating being re-continued to reach
at 760°C, maintaining the rate of heating at 50°C /hr and further
soaking for the balance duration;

- gradually slowing down the burner operation to cool the coil bundle to
250°C by maintaining a rate of cooling at the rate of 50°C /hour;
- suspending the burner operation at 250°C below which natural cooling
is carried out;
wherein when an interruption taking place during cooling, re-starting
the burner operation and heating again the welded joint-upto 760°C,
maintaining a maximum rate of heating at 50°C /hr and soaking for 5
minutes and then cooling the joint.
2. The process as claimed in claim 1, wherein a plurality of thermocouples
are installed for measuring the temperature at the butt joint locations
during the entire process.
3. The process as claimed in claim 1, wherein non-destructive test (NDT)
wherein hardness values at top and bottom locations of the coil bundle is
checked to ensure a complete stress relieving has been achieved.
4. The process as claimed in claim 1, wherein the dummy horizontal wall (6)
and vertical wall (7) made of insulation material are dismantled and
removed from the heat exchanger chamber, after completion of stress-
relieving.