NCEDA analysis of IDA World Congress technical sessions
Failure of components in desalination plants results in forced shutdown and loss in production, headaches for operators as they search for solutions and increased costs. One after one in this Session, the papers showed that proper material processing and selection are the key to minimize the chances of corrosion of material and provide longevity to the plant and its components. High corrosion resistance, durability and good mechanical properties must be balanced against cost and servicability to achieve the best material selection possible.
Corrosion behavior of some superaustenitic, superduplex and superferritic materials in seawater environment
Authors: Prof. Anees Uddin Malik, Engr. Saleh Ahmed Al Fozan, Engr. Mohammad Al-Hajiri
Dr Malik qualified his presentation by placing his focus on thermal seawater desalination plants where selection of materials from copper to iron, plain carbon steel and cast iron, is very important due to the high propensity for corrosion associated with continual contact with seawater. Austenitic stainless steels have been shown to be the most resistant to corrosion in marine environments. However, problems do arise. In particular, chloride ions have been found to play a major role in crevice corrosion in both austenitic and super-austenitic steels.
Following extensive testing, Dr Malik’s team found that corrosion rates depend on the balance in the steel manufacturing process between chromium, molybdenum and nickel. The corrosive behaviour of two types of super austenitic steel, seven of super duplex and two super ferritic alloys in seawater were compared and the suitability of their application to MED and MSF plants.
Perth seawater desalination plant materials selection
Authors: Mr Tako Heiner, Mr David Parravicini
Mr Heiner began by stating that the goal of his paper was to “raise awareness” and for others to learn from the experiences being detailed. While responsibility for Perth material selection tends to placed ultimately on the designer, the owner is concerned about costs, the operator about functionality, the constructor has a range of other priorities. With selection of materials, minimising crevice corrosion and being able to resolve it when it occurs without loss of production was a priority for the Perth SWRO.
The types of failure that have been observed in the five years of operation are
1. Seawater intake grille SAF2205 heads of bolts hollowed out
2. Non-return valves showed perforation through the body after three years’ service
3. Non-return spring collapse where the mechanism was found to be galvanic coupling
4. Rack inlet valve showed seal faces and stem crevice corrosion
The conclusion: no material is or is likely to be good in seawater because of the inherent variability of the medium in oxygen, salinity and other water quality criteria
Critical components of SWRO desalination plants
Authors: Mr Andreas Broeckmann, Mr Stephen J. Roddy, Mr Peter Eccleston
Presented by: Mr Andreas Broeckmann
A similar paper to that on the Perth SWRO plant but with a slightly different focus. This paper discussed the selection of essential process systems and elements for the Sydney Seawater Desalination plant rather than the materials component themselves.
At Sydney SRWO, the elements that were identified as likely to be the source of problems were seawater, 1st pass concentrate and the permeate. For the intakes, biological growth and dealing with chlorinated seawater were identified as factors in materials selection. For the pre- treatment systems, the issues were again chlorinated seawater, spray and splash, varying fluid levels and aeration. Mr Broeckmann commented that coatings for concrete structures and metallic equipment may not be as protective against corrosion as just stainless steel.
For the RO system, the concerns were rack design, the operational envelope, performance and energy consumption. For critical process components, the use of super duplex steel was deemed essential , despite cost, due to its corrosion resistance and design flexibility. The pumps need hydraulic testing to ensure optimal performance. Vibration and noise continue to be difficult to manage.
Other points made: casting quality is critical and is welding of a high standard. Tanks for chemicals and for storing treated water, in particular need quality welding followed by integrity tests and QA. The final point emphasised was the need to robust and careful management with attention to detail at all stages.
