Ballast water treatment systems

Stainless steel for ballast water treatment systems

Seawater must be treated before it can be carried in the ballast tanks that stabilize ships. Therefore, it needs to pass through a ballast water treatment system (BWTS) to remove and inactivate biological organisms such as bacteria, viruses and plankton. This step is vital to prevent the transfer of invasive marine species into new environments. As the BWTS operates in demanding corrosive conditions with high chloride contents and potentially other chemicals, the selection of a suitable material is essential for reliable operation of the system.

Long service life and minimal maintenance with stainless steel

Material selection plays an important role in determining the reliability and lifetime of a BWTS. Premature failure in the system causes unwanted downtime, expensive maintenance work and the need for replacement parts. BWTS components are currently manufactured from various materials, including different stainless steel grades, copper alloys, titanium and coated carbon steel. High performance stainless steels, such as Ultra SMO 254 and Forta SDX 2507, are required to prevent premature failures caused by localized corrosion.

Benefits of using stainless steel in ballast water treatment systems

Applications

  • UV Chambers
  • Filters
  • Chemical treatments
  • Pipework

Benefits

  • Excellent performance in seawater handling
  • Reduced dimensions, weight and cost
  • Long service life with minimal maintenance
  • High mechanical strength and wear resistance
  • A sustainable solution

Excellent performance in seawater handling

Seawater is a neutral chloride solution with a chloride level between 18,000 to 30,000 ppm. At the moderate temperatures in most seawater systems, crevice corrosion and pitting corrosion are the main corrosion risks. High performance stainless steels such as Ultra 254 SMO and Forta SDX 2507 offer excellent resistance to pitting and crevice corrosion in seawater applications, ensuring reliable operation and a long lifetime for the system.

Reduced dimensions, weight and cost

Unlike copper alloys, such as Cupronickel 90/10, stainless steel can resist very high water velocities. This has been recognized for example in seawater piping, where it is possible to significantly reduce pipe dimensions, weight and costs. Design exercises have shown that it is possible to save 50 % of piping weight and 35 % of piping cost by using Ultra 254 SMO compared to Cupronickel 90/10.

High mechanical strength and wear resistance

Stainless steel offers significantly higher strength levels compared to competing materials such as Cupronickel 90/10. Duplex stainless steels in particular, such as Forta SDX 2507, have extremely high strength combined with excellent corrosion resistance in seawater applications. The higher strength enables wall thickness reduction, which leads to cost and weight savings, as well as improved wear resistance.

Long service life with minimal maintenance

Due to the excellent performance of high performance stainless steels in seawater, costly replacements caused by corrosion can be avoided. This has a significant influence on the total life cycle costs of the system.

Sustainable choice of material

Stainless steel is, just like water, 100% recyclable. Outokumpu is the world leader for using recycled material in its production of stainless steel, with an average of over 85% recycled content. Outokumpu also leads the way in sustainability with the lowest carbon footprint in the global stainless steel industry.

Stainless steel for ballast water filters

In order to perform successful filtration and to minimize the need for expensive replacements, it is essential to select suitable material solutions for the filter. This is because the filter is exposed throughout the treatment process to flowing seawater.

In filters, both corrosion and wear resistance are required. Therefore, Forta SDX 2507 is an excellent choice as it is highly resistant to both wear and corrosion in seawater. 

In contrast, less corrosion resistant grades, such as Supra 316L/4404 and Forta DX 2205 are expected to suffer from corrosion that leads to premature replacement. 

Stainless steel for UV chambers

The UV Chamber is exposed to flowing seawater throughout the treatment process, which means that highly corrosion resistant grades are required. High performance stainless steel, such as Ultra 254 SMO and Forta SDX 2507, offer excellent performance in this application. 

Ballast water treatment system - Alfa Laval reference

In the same way as with filters, it is expected that less corrosion resistant grades, such as Supra 316L/4404 and Forta DX 2205, will be likely to suffer from corrosion leading to premature replacement.

Stainless steel for chemical treatments

Stainless steels might be used in systems, where chlorine or chlorine dioxide are produced on board and used as disinfectants provided that exposure times are sufficiently short and is followed by rinsing with fresh water. 

Chlorine and chlorine dioxide can be highly aggressive for stainless steels and this should be carefully evaluated before material selection. Factors to be considered are:

  • Concentration of chlorine compounds
  • Chloride concentration
  • Temperature of the solution
  • Water rinsing
  • Continuous or intermittent exposure

Outokumpu has broad experience and knowledge of using stainless steel grades in different corrosive environments consisting of chlorine compounds. We can perform laboratory-scale or field corrosion testing to support our customers in the selection of the ideal material for their specific application.