Go with the Flow
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THE MAGAZINE DEVOTED TO NICKEL AND ITS APPLICATIONS |
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WORLDWIDE DESALINATION capacity of 31 million cubic meters per day is projected to grow to 62 million by
2015. |
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THE COST OF DOUBLING worldwide desalination capacity by 2015 is estimated to be US$95 billion. |
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THERE ARE ABOUT 7,000 DESALINATION plants in the Middle East with a daily production capacity of
more than 100 cubic metres of fresh water. |
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HIGH PRESSURE PIPES in desalination plants are typically S32750 stainless steel, containing 7%
nickel. |
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HIGH PRESSURE PUMPS are also made of nickel-containing stainless steel. |
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DEMAND FOR high-nickel stainless steels in reverse osmosis plants such as this is almost
certain to increase. |
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THESE TUBES are made of S34565 stainless steel, containing 17% nickel. |
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日本語 |
A glimpse into the future of fresh water
By Dr. Gerry Crawford
Nickel Magazine, November 2005 -- The complex industrial process of turning salty water into fresh, drinkable water is used today in at least 120 countries.
As human populations increase in arid regions and in island communities with inadequate and dwindling sources of fresh water, the need to change sea and brackish water into potable water intensifies.
At the 2005 IDA World Congress on Desalination and Water Reuse, H. E. Abdullah Al Hussayen, Minister of
Water and Electricity, Kingdom of Saudi Arabia opened the conference with a keynote address that stressed the
importance of desalination and water management in meeting the growing demand for clean fresh water.
Only 0.01% of the earth’s water is renewable and available for safe drinking water. The worldwide health
costs related to the consumption of unsafe drinking water is estimated to be US$300 billion per year. Since
1955 population growth, pollution, and climate shifts have reduced the amount of available fresh water per
person by more than 50%.
Because of these trends, the desalination industry is currently experiencing rapid growth. The current worldwide desalination capacity of 31 million cubic meters per day is projected to grow to 62 million by 2015 and the cost of this increased capacity is estimated to be US$95 billion.
There are currently about 13,600 land-based plants, capable of a daily production of 100 cubic metres or more fresh water. Half of these plants are in the Middle East, about 20% are in the Americas, 13% are in Europe, and 12% are in Asia.
There are five commercial processes for turning salt water into fresh water, the two most common being, in roughly equal measure, multi-stage flash distillation (MSF) and reverse osmosis (RO). Both processes use nickel-containing materials because of their high strength and resistance to corrosion.
Since billions of dollars are being invested each year to increase desalination capacity worldwide, demand for high-nickel stainless steels in this essential sector is sure to increase.
The nickel-containing duplex, super-duplex and super-austenitic stainless steels are becoming the materials of choice because of their superior corrosion resistance and greater strength. The standard S31603 and S31703 stainless steels and the cupronickel alloys, which remain workhorses for desalination (at least at lower temperatures and chloride concentrations), simply cannot always provide the reliability required.
Although higher-nickel stainless alloys have greater unit costs, their strength allows them to be used in thinner gauges, thereby cutting both weight and cost to the point where the alloys are competitive with existing alternatives. This becomes more important as the design life of newer, larger plants increases (it is now upwards of 40 years on average).
At the NACE International conference in April 2005, Wilhelm Schleich, of KM Europa Metal AG, reviewed the properties of copper-nickel alloy C70600 and outlined its application in seawater service. Over several decades, many thousands of tonnes of C70600 have been used as seawater piping material, Schleich said. Its most notable property is its resistance to bio-fouling and to crevice and stress-corrosion cracking. Schleich acknowledged that although C70600 remains the alloy of choice for seawater piping, it is still susceptible to erosion and galvanic corrosion, but he stressed that this susceptibility can be eliminated at the design stage.
In 2004, Outokumpu (formerly Avesta Polarit AB) published two information sheets on nickel alloys for desalination. For MSF plants, the recommendations included:
- condenser tubes (commonly of titanium or copper-nickel alloys) – S31254, S32750 and S32654;
- brine heater tubes and tube sheets – S32654;
- flash chamber shells – S32205 (eliminates painting, decreases maintenance) and S31254, S32654 (sealing surfaces, flange joints); and
- ‘downstream’ stages – S32205.
- In seawater RO plants, the recommendations were:
- high-pressure pumps – S31254, S32750 (for high chloride) and S32205 (for low chloride);
- high-pressure piping – S32750.
A useful performance rating for various stainless steels, in ascending order of preference, was provided by Jan Olsson, et. al., of Avesta Polarit AB in two papers in 2003: S31603, S31703, S32205, N08904, S31254, S32750.
The authors reached several conclusions:
- S31603 and S31703 really should not be used unless the feed is deaerated;
- S32205 and N08904 are preferable to S31603 and S31703 but may also be subject to corrosion;
- super-duplex and super-austenitic 6% Mo alloys are needed for high-pressure piping;
- welded pipes can be less expensive than seamless piping; and
- super duplex S32750 can reduce gauge, weight and cost.
A second paper by Jan Olsson, et. al., on MSF chambers of solid duplex stainless steel provided a review of corrosion problems experienced with evaporator materials in MSF plants, starting with mild steels and progressing to various epoxy coatings, metallic linings, and stainless steel-clad plates. A preferred alternative is an evaporator shell of solid duplex, austenitic-ferritic stainless steel S32205.
The high strength of S32205 allows thinner, lighter evaporator vessels -- about 30% thinner and lighter than S31603, and at competitive cost -- for a 16,000-cubic-metre-per-day MSF plant in Libya. It was the first commercial use of S32205 for evaporator vessels since the initial proposal to use solid duplex stainless steel for this application was made by Groth and Olsson at a conference in Yokahama, Japan, in 1993.
At the 2005 IDA World Congress On desalination and Water Reuse, Outokumpu announced the selection of "Dual Duplex" construction at the Taweelah B extension and Jebel Ali units. The evaporators at these plants will be constructed with of S32205 duplex stainless steel on bottom portion of the evaporators that is exposed to deaerated seawater and S32101 duplex stainless steel on the top portion of the evaporators, which is exposed to the less aggressive condensate.
A super-austenitic 6% molybdenum alloy, N08367, which has 24% nickel and 20% chromium, has been used extensively in seawater-handling applications. However, in 2003, Grubb and Gerlock presented a paper at NACE describing its use in seawater desalination applications in both RO and MSF plants. It was chosen as a competitive alternative to more traditional copper alloys that offer certain advantages, including high thermal conductivity, but suffer from flow-induced, erosion corrosion, particularly in the presence of pollutants, including sulphides and amines.
N08367 is more costly than copper-nickel alloys yet is competitive with them because it can be used in thinner sections with less weight and cost. In recent years, this alloy has been specified for several plants, including critical pretreatment piping for the largest RO plant in the Western Hemisphere, in Trinidad, in 2002. The design capacity of the plant is 109,000 cubic metres per day.
In 2003, Francis and Byrne pointed out that the wrought alloy S32760, which has 7% nickel, 25% chromoum and 3.5% molybdenum, has been used in seawater systems since the mid-1980s in a wide range of applications. In firewater and seawater cooling systems, however, there have been numerous failures, most of which are associated with corrosion at welds. These problems have been overcome by the development of "approved and qualified welding procedures" to avoid sigma phase formation at welds, the authors say in their NACE paper (sigma phase formation causes a large, local reduction in corrosion resistance). As a result, S32760 is now widely accepted for piping in RO plants and for other components, including seawater intakes for MSF plants
An article in this magazine in 2001 highlighted the application of thin-walled, laser-welded S34565 austenitic stainless steel for tubing in the evaporator chamber of the heat recovery section of MSF plants as an economic alternative to 90/10 copper-nickel alloy. S34565 contains, nominally, 17% nickel, 24% chromium and 4% molybdenum.
The article was based on a 2001 NACE paper by Felton, Oldfield, Peet, et. al., who pointed out that galvanic corrosion occurred at some welds between tubes and tube plates and that further development work was required to identify suitable tube plate materials. In 2002, the same authors, in another NACE paper, concluded that the most appropriate tube plate material, welded to tubes made of S34565 alloy, was also S34565.
This update is not a comprehensive review of nickel-containing alloy developments for desalination in recent years. Rather, it identifies some of the more significant options and confirms nickel-containing alloys as both essential and in growing demand for the complex industrial process of making potable water from seawater.
Dr. Gerry Crawford is a Toronto-based consultant to the Nickel Institute. Dr. Jim Fritz, a
Pittsburgh-based consultant to Nickel Institute, also made a valuable contribution to this
article.
PHOTOS: Nickel Institute
Following, is a list of NACE technical papers (including reference numbers) that were used to compile
the above article. All of these papers are available from NACE
International. |
