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CHAPTER 1
INTRODUCTION
1.1 Introduction
A pressure vessel is a closed container designed to hold gases or liquids at a pressure different from the ambient pressure. The pressure differential is potentially
dangerous and many fatal accidents have occurred in the history of their development and operation. Consequently, their design, manufacture, and operation
are regulated by engineering authorities backed up by laws.
The need to protect the public became apparent shortly after the steam engine was conceived in the late 18th century. In the early 1800s, there were literally thousands
of boiler explosions in the United States and Europe, each of which resulted in some deaths and a few injuries. The consequences of these failures were not of a
catastrophic level that brought a lot of attention to them. It was not until the failures became more catastrophic that attention was brought to bear on the explosions.
Canonico, D. A. 2000.
For both economic and safety reasons, the pressure vessel steel with sufficient strength and toughness is required in commercial industry. In particular, the WM and
HAZ must have sufficient toughness. Effects of mechanical loading, inclusion size, chemical composition and cooling rate on the toughness in pressure vessel steel
welds have been extensively investigated for the last two decades. Low fracture toughness has been correlated with the crack propagation behavior of the weld.
Cracks have been found in various regions of the weld with different orientation in
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the weld zone, such as centerline cracks, transverse cracks and micro-cracks in the underlying WM or HAZ.
1.2 Problem Statement
Failures of welded structures can and do occasionally occur, sometimes with serious human, environmental and economic consequences. Study shows approximate
failure rates for various types of welded structure. For example the explosion boiler at USA in year 1900 recorded the failure rate is approximately 400 per year and for
year 1970 is approximately 200 per year. For onshore gas pipeline at Western Europe traced the failure rate is 0.6 per 1000 km per year while for petroleum products
pipeline at USA give the failure rate 0.55 per 1000 km per year.
It shows amongst other things how the use of experience-based engineering codes and standards can reduce failure rate whereas the ASME Boiler Code Committee
was established in 1911, when boiler explosions in the USA were occurring at the rate of virtually one per day. Although such occurrences are much less common a
century on, the continued prevention of failure requires careful attention to design, materials, construction, inspection and maintenance.
A useful way of categorizing failures in welded structures is to distinguish between instant failure modes and time-dependent failure processes. In all cases, the failure
occurs when the driving force for failure for example applied stress that exceeds the materials resistance such as fracture toughness. Consequently, instant failure modes
are quite likely to occur early in the life cycle of the structure, perhaps due to errors in design, construction, materials or inspection. Smith, T. A. and Warwick, R. G.
1983.
The temperature and maximum thickness of plates is limited only by the capacity of the composition to meet the specified mechanical property requirements. However
the crack of the material due to life cycle for the required value in the investigation.
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Discontinuities may be classified as defects depending on acceptance criteria in a particular specification or code. Discontinuities are rejectable only if they exceed
specification requirements in terms of type, size, distribution, or location. Discontinuities may be found in the weld metal WM, heat-affected zones HAZ,
and base metal BM of weldments.
These may eventually lead to final failure by one of the instant failure modes described above. Welded joints are particularly susceptible to fatigue, typically
initiating from discontinuities at the weld toe. The region affected is called the heat affected zone that lies outside the fusion zone in pure metals and outside the partially
melted zone in alloys similar to the area in the undisturbed tank metal next to the actual weld material. Messler, R. W. 1999a. This area is less ductile than either the
weld or the steel plate due to the effect of the heat of the welding process. Literature show that HAZ is frequent where damage start to occur. This zone is most vulnerable
to damage as cracks are likely to start here. Thus, the zone is uncovered for exposure to influence the serious damage.
1.3 Objectives of project
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