Rapid
technological developments & economies of scale in process plant
industries has led to severe operating temperature and pressure
conditions for reactors, pressure vessels, and heat exchangers. In
the same way, all upcoming plants and equipment for nuclear, defense
and aerospace industries are also getting bigger and more complex. To
cope up with this trend, new generation materials are being developed
worldwide, design aspects are becoming increasingly complex with very
stringent quality and safety requirements. In addition, the delivery
time is being squeezed to minimize the project cost. All these
developments continuously pose new challenges to the welding
technologists connected with heavy engineering industries worldwide.
Till the advent
of the new century, Indian heavy engineering industries were mainly
engaged in catering to the needs of domestic customers for equipment
and accessories. In fact, many of the Indian customers were insisting
Indian heavy engineering companies have a tie-up with international
companies as a pre-request for qualification as a bidder. Similarly,
international customers were not comfortable with Indian suppliers as
far as supply of critical equipment was concerned. Some of the Indian
heavy engineering industries took this up as a challenge to
demonstrate that they were as good if not better then foreign
fabricators.
Developments
In Materials And Weldability
There is
continuous development of materials for all the industries to improve
process efficiency, reduce the weight of equipment, improve plant
life and reduce plant maintenance/ shut down. Designers are coming up
with a newer variety of materials thereby posing challenges in front
of manufacturing industry to come up with suitable technology for
processing the same.
Creep
Resistant Cr-Mo Materials- Conventionally,
creep resistant 2.25 Cr-1Mo material is very widely used in Refinery
& Fertilizer applications up to 4500
C. Increase in temperature and pressure conditions and also
susceptibility to hydrogen attack in such environment called for
improved materials. Thus in the late 90's steelmakers came out with
never variety of 2.25 Cr-1Mo material, known as vanadium modified
2.25 Cr-1Mo material. Use of these high strength materials helps in
substantial reduction in vessel weight due to thickness reduction.
Typically, changing the material from conventional 2.25 Cr-1Mo steel
to 2.25 Cr-1Mo-0.25V steel will result in nearly 30% reduction in
weight in a typical 1000MT reactor. This is a huge saving and as a
result, all designers are changing over this new generation material
to take advantage of this benefit.
Development
In Welding Technology & Automation
Welding is one of the important operations in
fabrications. Recent developments in design and operation have put a
lot of challenges in front of welding engineers which has led to many
innovations such as the introduction of new processes/ variants of
processes, new techniques, mechanization and several others.
Quality and on-time delivery of equipment are the two
most important requirements in today's globalized world. Therefore,
fabricators are working towards more and more mechanization of
welding operations. Some examples of mechanization of welding carried
out by Indian Heavy engineering industries are:
Narrow Gap Saw: Most of the reactors and
vessels manufactured nowadays are heavy wall thickness (>100mm).
While welding of high thickness welds in such equipment, adoption of
Narrow Gap SAW technique provides great advantages in terms of
reduction in welding consumables and cycle time. In NG SAW, the
sidewalls are nearly vertical (with 0.50 angle) and the
top opening of the groove is as low as only 28~30mm irrespective of
thickness. It is very important to get the welding operation 'first
time right' since it is extremely difficult to carry out post weld
repairs. Use of contractor non-contact type seam tracking devices and
turning rollers with drift control is mandatory for successful
welding of such joint. This technique has been successfully applied
in welding high thickness Carbon, Cr-Mo and Stainless Steels. Narrow
Gap Tandem SAWis one of the process variations of SAW, wherein two
(or more) wires are fed from separate welding heads and power sources
into the same weld puddle. Use of two wire Tandem SAW increases the
productivity by about 90% and is regularly used by fabricators.
Capability to weld up to 800 mm thick joints have been demonstrated
by Indian fabricators.
Weld
Overlay by ESW/SAW: for
equipment operating with fluid which is corrosive, normally, inside
surface of C-Mn or low alloy steel is cladded/ weld overlaid with
corrosion resistant material. A typical reactor requires nearly 25MT
of weld overlay (assuming 4.5mm thick weld overlay) to cover the
entire inside surface of shell courses and heads. The requires
development of high deposition welding techniques like Electro Slag
Welding (ESW) or Submerged Arc Welding(SAW) using strip electrode.
Welding is carried out by using strips of up to 120mm wide and 0.5mm
thick, which results in deposition of 42 Kg/arc-hr. ESW overlay of
stainless steel and nickel alloys are regularly carried out by Indian
fabricators.
Weld
Overlay of Nozzle Pipe/ Fittings by Mechanized Processes:
all nozzle attachments in a clad/ overlayed reactor call for weld
overlay on the inside surface as well as on the faces. Special
welding torches to carry out weld overlay by mechanized FCAW, GTAW or
Thin-wire SAW (1.2mm/1.6mm dia) inside nozzle pipes, forgings, and
900
elbows.
Weld overlay has been carried out successfully on nozzles with a very
small bore (as low as 25mm) and extra length (as high as 400mm) wear
resistant overlay operations have also been carried out on OD of bars
by Plasma Transferred Arc Welding(PTAW) Process.
Development
In Quality Control & Assurance of Welded Constructions
Each weld joint of a vessel calls for stringent
inspection and testing requirements as per the requirement of
manufacturing code, customer specifications, and other applicable
standards. The test generally includes Non-Destructive Testing (NDT)
like Radiography (RT) Ultrasonic Test (UT), Magnetic Particle Test
(MPT), & Penetrant Test (PT) in addition to the thorough visual
examination. Out of these tests, RT & UT are given maximum
importance. Due to the higher wall thickness of the vessels, RT is
being preferably done using a high power Linear Accelerator (LINAC).
On the other hand, Micro focal anode X-ray is being used for
detection of a flaw in Tube to Tubesheet joints for critical nuclear
applications.The concept in NDT has shifted from 'only flaw
detection' to 'flaw detection, characterization and flaw sizing'.
There is a huge advancement in UT technology over the last few years.
His resolution UT including Time of Flight Diffraction (TOFD) has
become a mandatory requirement for all critical reactor weld joints.
Stringent requirements of nuclear and aerospace projects have taken
capabilities in carrying out various NDT to its zenith.
Significant changes have taken place over the years in
welding and allied areas in heavy industries in India. From making
simple equipment with basics materials to fabricating the most
complex ones involving stringent quality requirement, the Indian
heavy engineering industry has envolved a lot. The industry has
become mature and can compete globally for various orders, due to its
demonstration capabilities in welding and allied fields.
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