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Abstract There is clear trend for high-pressure trains of a larger size. For a given plant size it reduces the number of trains with the corresponding savings in installation costs. Furthermore, fewer, larger Irains generally mean less capital than more, smaller trains. Larger units have, inherently, higher efficiency. There is a preference amongst plant designers and operators for la'ains capable of operating reliably without pressure lubrication. Today this limits the train size to, approximately, 10,000m3/d, considering a 40-45% conversion rate. Reliability, availability, dependability are paramount. Metallurgy with proven track record, adequate material combination, reasonable cost and ease of processing is the key to the success of the installation. Keywords." High-pressure pump; Alloy 885; Materials; RO train I. Description In an analogy between a reverse osmosis plant and the human body we could say that the membranes are the lungs of the system and that the feed pumps are the heart. The main feed pumps, also known as the high- pressure trains, are, undoubtedly, one of the critical components of the plant. As such, every effort should be made to ensure this equipment is most reliable, dependable and efficient. 34154
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This brief paper focuses on trains of mid to large size, i.e. trains for production of 2,500 mVd and larger. The high-pressure train on today's plants typically consists of a high-pressure pump, driven by an asynchronous electric motor, and an energy recovery turbine (ERT). The pump increases the feed water pressure to that required by the Presented at the European Conference on Desalination and the Environment: Water Shortage, Lemesos, Cyprus, 28-31 May 2001. membranes; the brine, rejected at high pressure, is past through the ERT, which recovers that energy. The electric motor provides the net power. The most common configuration is that in which the electric motor, featuring a double extended shaft; is placed between the pump and the ERT, three separate elements -- pump, motor and turbine, for much improved reliability, avail- ability and maintainability. 2. Equipment configuration On seawater plants, the high-pressure pump is of the centrifugal type, horizontal configuration and with multiple stages. The pump casing could either be axially, also referred to as horizontally, split or radially split of the segmental ring type. It is not the intention of this paper to compare both designs,
however, an analysis of the in- stallations yields the following results (Table 1). Table 1 High-pressure pump - construction Train size, Segmental, Axially Number of m3/d radially (horizontally) trains split, % split, % considered 2,500-4,000 67 33 90 4,000-5,500 32 68 75 Greater than 7 93 130 5,500 In the smaller segment, 2,500-4,000, the trend is to install segmental radially split pumps. The same holds true for the middle segment, 4,000-5,500, but at a slower rate. It is the opposite, however, in the large segment, beyond 5,500. The trend is for axially split machines, Reliability, availability and efficiency are the most important factors on these large installations, generally under BOO/ BOT financing schemes. Equipment first cost is secondary. As for energy recovery, today's plants feature turbines of the Pelton type. Initially, reverse running pumps and, in some cases, Francis turbines were used. However, as they became more reliable, the more efficient Pelton turbine became the element of choice. The Pelton turbine allows a more simple plant design and simplifies the operation. Other energy recovery devices are being con- sidered but, to date, some are restricted to small plants or retrofits and others need to gather expe- rience and operating hours on larger units. 3. Materials of construction. Alloy 885 Metallurgy is one factor that makes special the specification and construction of a seawater reverse osmosis high-pressure pump. Typical seawater where reverse osmosis plants are installed has salt contents ranging from 20,000 to 40,000 ppm and temperatures from 15 to 35°C. Dissolved oxygen levels are moderate. Fluid velocities within the pump are rather high; values of 40m/s are representative. Fortunately, inorganic suspended solids levels are very low. Simplistic material recommendations are, therefore, impractical and the designer has to consider, among other things, localized corrosion (pitting and crevice corrosion) and stress corrosion cracking (SCC). Castability and weld- ability are also to be taken into account. The families of materials under consideration are: 300 series austenitic stainless steels, duplex stainless steels, super austenitic stainless steels and specialized alloys. 3.1. 300 series austenitic stainless steels Their susceptibility to localized corrosion (pitting and crevice) and SCC make them a marginal material for this service. Improved service life has been experienced with steels containing molybdenum in colder seawater. The chrome oxide layer is very tenacious and, there- fore, the corrosion rate is not velocity sensitive. They are readily available and easily weld repaired (the low carbon grades) in the field. 3.2. Duplex stainless steels Materials with a ferrite and austenitic micro- structure, hence the name "duplex"; ferrite is the matrix phase. Higher levels of chrome, molyb- denum and nitrogen make them more resistant to localized corrosion than the 300 series austenitics. On the negative side, these materials still present some degree of difficulty in pro- cessing and welding; post weld heat treatment may be required. Ferrite content must be controlled to avoid its transformation to sigma, a hard brittle phase. 3. 3. Super austenitic stainless steels These materials keep the basic austenitic structure with higher contents of chrome and molybdenum with nitrogen. Nickel must also be increased to offset the ferrite forming effect of Cr and Mo. Drawbacks: their substantially higher cost, processing difficulties and weldability problems. 3.4. Specialized alloys Alloy 885, a patented material, is an alloy developed with a corrosion resistance equal to or better than most duplex alloys, approaching that of the super austenitic stainless steels and, at the same time, possessing the ease of casting and welding of the 300 series austenitics. 4. Alloy 885. A stainless steel casting alloy for pumps in seawater applications 4.1. Localized corrosion There has been a considerable amount of data published regarding the metallurgical variables that affect the localized corrosion behavior (pitting and crevice corrosion).