Throughout the Middle East and Arabian Gulf, multi-stage flash evaporation desalination is very much the preferred technology for the production of potable water. Several factors contribute to this, the key ones being the abundance of waste heat and the corrosive nature of the region’s seawater. Bryan Orchard looks at how one company, Japan’s Ebara, has addressed the issue of seawater intake pumping.
Multi-stage flash (MSF) evaporation dominates the desalination industry in the Middle East and Arabian Gulf, where almost 74% of all plants use this technology. Although a high energy user compared with seawater reverse osmosis (SWRO), the availability of waste heat, typically from adjacent power stations and oil and gas processing plants, gives MSF a high gain output ratio, this being the ratio that keeps operating and production costs to an economic level. The other compelling argument in favour of MSF is that the process does not require high levels of pretreatment as it is unaffected by seawater quality.
In fact, it is only the seawater intake pumps that really have to stand up to the corrosive and abrasive properties of the water. In this particular area of pumping technology, Ebara is one of the names that stands out.
Ebara’s pedigree in seawater desalination pumping stretches back to the mid-1960s when it built its first VL pumps for the Bureau of Reclamation in the USA . Since then it has built pumps for projects in many parts of the world, in the process accumulating a substantial volume of knowledge relating to operating conditions. Based on this knowledge and experience, Ebara has developed its VYB and VYM vertical mixed flow pumps, offering flow capabilities of up to 43 000 m³/h at 47.5 m and 7050 kW.
A VYM pump is a direct motor driven, vertical shaft mixed flow pump and is suitable for a small installation space for seawater intake. Its submerged impeller does not require a pump priming device or water-filling check device. The VYB is a barrelled vertical shaft mixed flow pump suited to the requirements of strict suction performance (low NPSH), and has thus been adopted for brine circulation, brine blow down and distillate pumps. Its impeller can be placed at any depth desired to avoid pump operation with cavitation, and this allows the pump driver to remain above ground.
The MSF process
To appreciate the contribution that Ebara’s seawater intake pumps make to MSF evaporation, it is necessary to understand the process. MSF requires enormous volumes of water in order to maintain continuous production, which means that the seawater feed pumps must provide the highest levels of reliability and resistance to corrosion. The MSF process involves the use of distillation through a number of multi-stage chambers, with each successive stage operating at a lower pressure.
The feed water is first heated under high pressure, typically to about 112°C, and fed into the first flash chamber where the pressure is released causing the water to boil rapidly, resulting in sudden evaporation or ‘flashing’. This ‘flashing’ part of the feed continues in each successive stage because the pressure at each stage is lower than the previous stage. The vapour created by the flashing is converted into fresh water by being condensed on heat exchanger tubes that run through each stage. The tubes are cooled by the incoming cooler feed water. There is an element of feed-water treatment, this involving de-aeration and polyelectrolyte dosing for scale and foam control, plus the addition of sodium bisulphate to scavenge oxygen and residual chlorine.
Middle Eastern demands
It is in the United Arab Emirates that Ebara has had its most recent success, supplying seven single-stage 1650-mm-bore VYM pumps with fully enclosed impellers, each having a capacity of 30 000 m³/h, to Fisia Italimpianti of Genoa, Italy, for the Shuweihat desalination treatment plant in Abu Dhabi . According to Roberto Borsani, the company’s desalination department manager, the Ebara pumps were selected primarily on their ability to withstand the highly corrosive nature of the seawater in the area and because of Ebara’s credentials and experience with this type of pump in MSF. That the pumps have performed as required has been a great satisfaction to Fisia Italimpianti, Borsani says.
The material specified for the seawater feed pumps requires special attention and Ebara has invested considerable resources into researching the materials and coatings for its pumps in order to combat the effects of corrosion. Where pumps are intended for use in highly corrosive sea areas, such as the Middle East, high-grade stainless steels are required. However, there are various stainless steels, and their corrosion properties were not quantitatively known in actual Middle Eastern seawaters.
In collaboration with Saudi Arabia ‘s Saline Water Conversion Corp (SWCC), Ebara has carried out immersion tests on various stainless steels to clarify their crevice corrosion behaviour in the region’s seas. As a result, it was discovered that the crevice corrosion occurrence rate in Middle Eastern seas is remarkably higher than that in other areas. The crevice corrosion growth rate is higher in the Arabian Gulf than in the Red Sea and the rate is lower in chlorinated seawater than in the raw seawater.
Based on these results, using super duplex stainless steels, in particular super duplex grades with a pitting resistance equivalent of 40 or more, allows for a reduction in the crevice corrosion occurrence rate and the corrosion growth rate. When super duplex stainless steel pumps are manufactured, it is critical to use appropriate technologies to attain and maintain the corrosion resistance of the heat-affected zones (HAZ) and the strongly cold-worked zones. Ebara has notable expertise both in working and soldering super duplex stainless steels and in corrosion rating technology. It is preferable to take into consideration the purpose, required lifetime and cost when selecting materials and to adopt appropriate countermeasures against crevice corrosion as required.
Dealing with aggressive seawater
Because the seabed in this part of the world is relatively shallow, the seawater is of a higher temperature than in other regions and carries high levels of turbidity. The seawater also has an unusually high level of salinity, far higher than in the nearby Mediterranean and almost twice as high as that found in the Atlantic. Were Sea Water Reverse Osmosis (SWRO) to be employed, a major investment in pretreatment technology would be required to withstand the aggressive nature of the water.
“Desalination plants are very demanding on pumps,” says Fisia’s Borsani. “The pumps are working a very severe duty, so we need to work with pump manufacturers that demonstrate very high levels of reliability in terms of performance and trouble-free operation. The process pumps in an MSF plant all work under vacuum conditions, so the issues of cavitation and NPSH are very stringent, requiring the pump to be designed and manufactured very carefully.”
Advanced modelling & design
The advancement of computational fluid dynamics (CFD) technology with cavitation modelling has made it possible to predict numerically pump head reduction under low NPSH conditions. Combining such advanced CFD technology with its unique inverse design method and numerical optimization methodology, Ebara is now able to improve the design for even better suction performance.
Another important issue is cavitation erosion. Cavitation erosion has traditionally been empirically ev-aluated based on the cavity length on impeller blades; however, such predictions show significant scatter.
To meet the even higher future demands of customers, Ebara is leading the research on the development of a more-accurate method to predict cavitation erosion utilizing the combination of numerical simulation and sophisticated experiments.
Future growth prospects
Fisia reports that there is a fast-growing requirement for potable water in the Arabian Gulf, with the drive behind this expansion identified as the planned investment in business and tourism. In the first half of 2007, Fisia received contracts for two very large desalination projects worth US$2 billion and $0.5 billion in the Gulf. Borsani says that the industry is also growing throughout parts of Southern Europe and North Africa. This is good news for pump manufacturers such as Ebara who have an extensive global operating base in MSF evaporation.