Scrapping the Ships: Metal Cutting Procedures & Equipment
written by: Raunekk
• edited by: KennethSleight
• updated: 6/16/2009
In the previous article we saw the different methods that are used for ship breaking. Cutting the main metal structure of the hull requires expertise knowledge and extensive physical labor.In this article we will learn how the metal cutting is done and what are the main tools that are used.
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Ship Breaking - Not an Easy Job
In the previous articles we discussed what is ship breaking and what are methods that are used for the same and hazards in ship breaking. Sometimes it’s hard to believe how those thick metal structures from the ship’s body are cut and broken to small pieces. It is tough and requires specialized knowledge and experience to scrap huge tons of metal.
Metal cutting doesn’t mean that the ship is docked and the workers start cutting the body from any which way they like. Irregular and unplanned cutting may lead to life taking accidents. For example, if any unsupported part of the ship is cut without taking necessary steps, the whole structure may fall on the worker or damage the equipment and machineries lying around.
Moreover, metal cutting is not at all a “one man’s show". Thorough study of the whole structure needs to be done with the help of design diagrams in order to cut the metal efficiently and with utmost safety. In this article we will discuss the various resources that are used to carry out this tedious and dangerous process.
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As discussed in the earlier articles, scrapping of the ship starts by first cutting the superstructure and the upper decks and then the main and lower decks. Metal cutting, being a process carried out to recover materials that can be sold; the cutting is done with utmost care and precision. The process doesn’t include just steel but several grades and types of scrap metals. The large parts of metals are cut and shifted to a different area with the help of cranes to further cut them into smaller shapes and sizes as required by the buyer. Metal cutting is typically done by a variety of torches and mechanical cutter, but apart from that there are many other ways of doing it, e.g. for very thick metals detonation charges are used.
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An oxygen fuel torch is the most favorable instrument used for cutting metals. A variety of fuels such as propane, butane, natural gas, acetylene etc are used along with liquefied or compressed oxygen. The liquefied oxygen acts as an oxidizer and the fuels act as “cutting gas" to cut the iron along the cutting line. These torches work at a flame temperature of 3500 – 4000 degrees Fahrenheit with a flame velocity of 290- 425 feet/second.
The type of torch that is to be used for a particular work depends on the following factors:
The type of fuel and oxidizer to be used.
Supply pressure of fuel and oxidizer
The thickness of the metal to be cut
Environment where the work has to be done
All these torches have a cutting speed in the range of 17-26 inches/ minute depending on the thickness of the metal and the type of fuel and oxidizer to be used.
Apart from oxygen fuel torches, electric arc and plasma arc torches are also used. These torches generate very high temperatures and can melt almost any metal. Most of these torches are manual torches with cutting speed slower than the oxygen fuel torches. They cut at a rate no more than 10 inches/minute. The molten metal at the time of cutting is blown away with the help of pressurized compressed air.
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Shears are kind of large scissors with long blades, which are used to cut the large pieces of metals into smaller ones. Large industrial shears can quickly reduce large metal pieces into small dimensions with speed higher than that of normal torches. It also reduces the labor and does the work much faster.
Shears are generally used for cutting large metal pieces into small parts for putting them in re-melting furnace. Shears are generally of two types – mobile and stationary.
Selecting a shear depends on the following factors:
Dimension of the metal to be cut
Thickness of metal
Toughness of metal
Required cutting rate
Dimensions of the product as required by the buyer.
If cutting of a nonferrous metal is required, electric saws with circular or reciprocating blades are used.
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The cutting process generates lot of scrap metal. The scrap consists of several types and grades of metals known as scrap species. This scrap metal also has a very good selling market value. The scrap can be divided into types:
The ferrous scrap comes from various places of ship such as – shell plating, deck plating, frames, bulkhead, foundations, girders, doors and hatches, piping, hull attachments etc.
The nonferrous scrap with copper yielding scrap as the main type comes from various machinery, copper plates, broken valves and other copper alloys. Other nonferrous scraps are bronze and brass, which have a very high re-selling value.