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Chemical carriers: an important market for increasing application of duplex stainless steel

DataTime:[8/19/2020]

Chemical carriers are specialized in transporting liquid chemicals, which may be toxic, harmful, flammable and explosive, or highly reactive and corrosive. There are strict regulations for the design, construction and operation of chemical carriers, and more and more molybdenum containing stainless steel cargo tanks are used. They travel long distances around the world, carrying liquid chemicals safely around the world. The chemical carrier industry has become an important market for duplex stainless steel and molybdenum applications.

What is a chemical tanker?

The so-called chemical carrier should be defined as a liquid cargo ship, which is constructed or refitted to carry all kinds of toxic, flammable, easy to play or corrosive chemical substances. Most of the liquid chemical carriers have double bottom and double side sides, and the cargo hold is equipped with special cargo pump and piping system. Stainless steel or anti-corrosion coating is used for inner wall and piping system of cargo hold.

When referring to the term "oil tanker", people often think of those super oil tankers carrying bulk cargo and oil products on the sea. The carrying capacity of these huge objects can reach more than 400000 DWT. The tonnage of chemical tankers is small, generally between 5000 and 40000 tons, but they are equally important in the world economy. Since chemical cargo may be extremely corrosive, many chemical carriers use molybdenum containing stainless steel cargo hold, which not only has long service life and high safety, but also is suitable for a variety of goods. Duplex stainless steel is increasingly becoming the necessary material for harsh transportation environment.

Rules and classification of chemical carriers

All ships are subject to the strict supervision of safety navigation, air pollution and solid-liquid discharge regulations. Vessels sailing in international waters must comply with the international code for the construction and equipment of ships carrying dangerous chemicals in bulk (IBC Code) issued by the International Maritime Organization (IMO). The rules stipulate the design criteria for the risk level of the goods transported.

Because chemicals cause a variety of safety and pollution risks, the construction and operation requirements of chemical carriers are much more stringent than those of oil tankers and ordinary commodity carriers. The design and manufacture of cargo hold is also more complicated. IBC rules put forward requirements for cargo handling and monitoring, warehouse coating, cleaning, ventilation, steam detection and fire fighting. Special regulations on warehouse heating, cargo compatibility, corrosion protection and cargo density have also been promulgated. Chemical carriers are undoubtedly the most technologically advanced of all large cargo ships.

Chemical carriers are classified according to size, cargo type and usage. The bulk chemical carrier has a large hull with many separate cargo tanks for the transportation of high-grade chemicals. Stainless steel cargo tanks are widely used. The volume of the product oil carrier is also large, but it usually transports goods with low corrosivity, and the cargo hold is generally made of coated steel. Special carriers are small and medium-sized transport vessels, which transport a limited number of chemicals in special trade. According to the types of goods, coated steel or stainless steel cargo tanks are used.

The deck of a chemical tanker is covered with pipes and valves

The deck of a chemical tanker is covered with pipes and valves © Nordic tankers

Hull

There are rows of pipes on the deck of the chemical carrier, and the loading and unloading of liquid chemicals are completed through these pipes and valves. For example, the ODFJELL se bow star, a large-scale chemical carrier built in the new shipyard of Szczecin in Poland in 2004, has 34 square and rectangular 2205 stainless steel cargo tanks under the deck, and 6 cylindrical tanks are installed on the deck. The hull is 183 meters long and 32 meters wide, with a deadweight of 39832 tons. The ship was built using 3000 tons of duplex stainless steel (about 90 tons of molybdenum) and a storage capacity of 52106 cubic meters.

Location of stainless steel cargo hold of ODFJELL se bow star

Location of stainless steel cargo hold of ODFJELL se bow star © Odfjell

Cargo hold

Design and build

The inner cabin contains hatch, escalator, heating device, piping and discharge device. In order to improve the rigidity of cargo hold, the corrugated plate with thickness of 20-25 mm is generally used. Compared with the flat panel, this method improves the structural stiffness and reduces the weight of the hull. The width and depth of profiled corrugated plate are 1 meter. Precision welding technology must be used to ensure the integrity of the weld. In order to ensure the surface quality and corrosion resistance of stainless steel, careful cleaning must be carried out after processing. About 1500 tons of stainless steel is required for a normal size carrier.

Chemical tanker stainless steel cargo hold interior

Chemical tanker stainless steel cargo hold interior © Stolt Tankers

Material Science

Early chemical ships used 304L stainless steel. Nowadays, stainless steel with molybdenum content of 2-3.5% has become the standard material. 2205 duplex stainless steel is widely used, and 316 austenitic stainless steel and its derivative grades are sometimes used.

2205 duplex stainless steel is popular because its strength and corrosion resistance are better than 316 stainless steel. High strength can reduce the use of steel, reduce the weight of the hull. The addition of 3% Mo enhances the corrosion resistance and can transport all kinds of corrosive liquid chemicals. The thermal expansion coefficient of 2205 dual phase steel is closer to that of carbon steel than 304 and 316, and it is more compatible with the surrounding carbon steel members in terms of thermal expansion and cold contraction.

Clear

Cargo hold cleaning is a key step in the operation of chemical tanker, which is directly related to the quality of goods and operation cost. Many shippers see it as the most important part of operating costs because it is the most controllable cost. For a certain cargo hold size ship, strict design standards and operation supervision make the investment close to most of the operating costs, and effective cleaning methods can bring certain competitive advantages.

The material used in the hold and its coating (if any) determine the method of cleaning. Unlike the epoxy coating on carbon steel, stainless steel does not absorb liquid and has no pores of zinc silicate coating. Due to their corrosion resistance, they are suitable for the use of a variety of cleaning methods and detergents, providing significant advantages in terms of operating costs for shippers.

The chemical vessel bow sagaml is loading and unloading at the wharf

The chemical vessel bow sagaml is loading and unloading at the wharf © Odfjell

Chemical carrier market

The average life span of a chemical carrier is 23 years. According to a survey of 138 chemical carrier operators in 2012, 400 of the 1800 ships operated at that time used stainless steel cargo tanks. According to the data, the annual demand for chemical tankers is about 75-80, of which 15-20 use stainless steel cargo tanks. According to the recently released his Markit statistics, a total of 144 stainless steel chemical vessels were under construction and ordered during 2016-2017.

According to historical data, the market growth rate of new chemical tanker is 1.3-1.7 times of global GDP. The demand for chemicals and the globalization of their production and marketing are conducive to the continuous expansion of the chemical fleet, especially stainless steel chemical vessels. Moreover, the hull is getting bigger and more complex. With the upgrading of ships and increasing market demand, cargo hold materials are constantly upgraded. The demand for duplex stainless steel in the chemical tanker market reaches tens of thousands of tons every year. This means that the demand for molybdenum will continue to maintain a strong growth momentum.

Source: China DSS Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)

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