15-01-2013, 04:47 PM
UNDER WATER WELDING
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INTRODUCTION:
• What is Welding ?
Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done bymelting the workpieces and adding a filler material to form a pool of molten material (the weld pool) that cools to become a strong joint, with pressuresometimes used in conjunction with heat, or by itself, to produce the weld. This is in contrast with soldering and brazing, which involve melting a lower-melting-point material between the workpieces to form a bond between them, without melting the workpieces.
Many different energy sources can be used for welding, including a gas flame, an electric arc, a laser, an electron beam, friction, and ultrasound. While often an industrial process, welding may be performed in many different environments, including open air, under water and in outer space. Welding is a potentially hazardous undertaking and precautions are required to avoid burns, electric shock, vision damage, inhalation of poisonous gases and fumes, and exposure to intense ultraviolet radiation.
Until the end of the 19th century, the only welding process was forge welding, which blacksmiths had used for centuries to join iron and steel by heating and hammering. Arc welding and oxyfuel welding were among the first processes to develop late in the century, and electric resistance welding followed soon after. Welding technology advanced quickly during the early 20th century as World War I and World War II drove the demand for reliable and inexpensive joining methods. Following the wars, several modern welding techniques were developed, including manual methods like shielded metal arc welding, now one of the most popular welding methods, as well as semi-automatic and automatic processes such as gas metal arc welding, submerged arc welding, flux-cored arc welding and electroslag welding. Developments continued with the invention of laser beam welding, electron beam welding,electromagnetic pulse welding and friction stir welding in the latter half of the century. Today, the science continues to advance. Robot welding is commonplace in industrial settings, and researchers continue to develop new welding methods and gain greater understanding of weld quality.
• UNDER WATER WELDING:
Underwater welding is a type of welding which takes place underwater. A number of differentwelding techniques can be used underwater, with arc welding being among the most common. There are numerous applications for underwater welding skills, including repairing ships, working on oil platforms, and maintaining underwater pipelines. People with skills and experience in this field can find employment all over the world.
In underwater welding, the environment around the welder is wet. He or she wears a dive suit, and uses welding equipment which has been customized for wet environments. This equipment is designed to be as safe as possible for the welder, reducing the risk of electric shock and the development of dangerous situations. Someone who practices underwaterwelding must be both a skilled welder and a skilled diver, with the ability to safely and effectively prepare a scene for welding and to confirm that the welds are of high quality.
HISTORY:
With all the power and precision machinery involved in production welding, you might think of welding as a relatively new process. In reality, welding has been around for thousands of years. Early examples of welding have been found in locations ranging from Ireland to India, with some dating back to the Bronze Age. Naturally, these civilizations lacked the vast array of tools and machinery that welders have access to now. How did they manage to weld?
The process they used is known as forge welding. To start the process, blacksmiths would heat the metal until it was bright red in color (but still not at its melting point). The blacksmiths would then place the two pieces, slightly overlapping, on an anvil and pound them together. Forge welding has multiple limitations. Only relatively s¬oft metals can be forge welded, and the process is very labor intensive. In places without electricity, however, the process is still used.
Hyperbaric Welding (dry welding)
Hyperbaric welding is carried out in chamber sealed around the structure o be welded. The chamber is filled with a gas (commonly helium containing 0.5 bar of oxygen) at the prevailing pressure. The habitat is sealed onto the pipeline and filled with a breathable mixture of helium and oxygen, at or slightly above the ambient pressure at which the welding is to take place. This method produces high-quality weld joints that meet X-ray and code requirements. The gas tungsten arc welding process is employed for this process. The area under the floor of the Habitat is open to water. Thus the welding is done in the dry but at the hydrostatic pressure of the sea water surrounding the Habitat
CRYOGENIC FUELS/PROPELLANT:
In a cryogenic propellant, the fuel and the oxidizer are in the form of very cold, liquefied gases. These liquefied gases are referred to as super cooled as they stay in liquid form even though they are at a temperature lower than the freezing point. Thus we can say that super cooled gases used as liquid fuels are called cryogenic fuels.
These propellants are gases at normal atmospheric conditions but to store these propellants aboard a rocket is a very difficult task as they have very low densities. Hence extremely huge tanks will be required to store the propellants. Thus by cooling and compressing them into liquids, we can vastly increase their density and make it possible to store them in large quantities in smaller tanks.
As we now know the properties of cryogenic fuels, we can proceed further and see how they are employed to impart thrust to the rockets. Normally the propellant combination used is that of liquid oxygen and liquid hydrogen.
WORKING:
Cryogenic Engines are rocket motors designed for liquid fuels that have to be held at very low "cryogenic" temperatures to be liquid, they would otherwise be gas at normal temperatures. Typically Hydrogen and Oxygen are used which need to be held below 20°K (-423°F) and 90°K (-297°F) to remain liquid.
The engine components are also cooled so the fuel doesn't boil to a gas in the lines that feed the engine. The thrust comes from the rapid expansion from liquid to gas with the gas emerging from the motor at very high speed. The energy needed to heat the fuels comes from burning them, once they are gasses. Cryogenic engines are the highest performing rocket motors. One disadvantage is that the fuel tanks tend to be bulky and require heavy insulation to store the propellant. Their high fuel efficiency, however, outweighs this disadvantage.
The Space Shuttle's main engines used for liftoff are cryogenic engines. The Shuttle's smaller thrusters for orbital manuvering use non-cyogenic hypergolic fuels, which are compact and are stored at warm temperatures. Currently, only the United
States, Russia, China, France, Japan and India have mastered cryogenic rocket technology. The cryogenic engine gets its name from the extremely cold temperature at which liquid nitrogen is stored. Air moving around the vehicle is used to heat liquid nitrogen to boil, Once it boils, it turns to gas in the same way that heates water to forms steam in a steam engine. A rocket like the Ariane 5 uses oxygen and hydrogen, both stored as a cryogenic liquid, to produce its power. The liquid nitrogen, stored at -320 degrees Fahrenheit, is vaporized by the heat exchanger. Nitrogen gas formed in the heat exchanger expands to about 700 times the volume of its liquid form. This highly pressurised gas is then fed to the expander, where the force of the nitrogen gas is converted into mechanical power.
CONCLUSION:
Cryogenic engine has brought some revolutionary changes in the field of rocket propulsion and space research. Apart from this it also finds its applications in various other fields. The discovery of this engine has brought about a solution to some major problems that were faced by the scientists in the field of space exploration.
Further researches on this engine can make it the ultimate engine in many fields , mostly rocket propulsion.