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Chrome, molyb- denum and nitrogen have a beneficial effect on the pitting and crevice corrosion resistance. Researchers suggest the use of the materials PREN, Pitting Resistance Equivalent Number, to evaluate the corrosion resistance. PREN = Cr(%) + 3.3 x Mo(%) + 16 x N(%) A technical paper presented at the NACE (National Association of Corrosion Engineers) convention in 1988 by T.J. Glover indicates that a PREN of 38 is sufficient to guarantee corrosion resistance of a stainless steel to seawater ex- posure. Another parameter used for evaluating the corrosion resistance of materials is the Crevice Factor. The formula developed from actual crevice corrosion testing performed in the labo- ratory, reads as shown below. CF = Cr(%) + 3 × Mo(%) + 15 × N(%) Experimental data shows that if an alloy has a minimum crevice factor of 35, the material will not crevice corrode in an aggressive acid chloride environment test. The Critical Crevice Temperature (CCT) of a material is yet another parameter used to indicate its corrosion resistance. It is the temperature of an acid chloride solution at which corrosion is first observed. The higher the CCT, the greater the corrosion resistance the alloy will exhibit. 4.2. Stress corrosion In addition to ferrite content, temperature and oxygen content, the stress corrosion resistance of the austenitic alloys is also a function of the chloride concentration and the molybdenum and nickel contents. In respect to nickel content, resistance to SCC is minimum at some intermediate value, with it being greater at both lower and higher nickel contents.
That intermediate value is the nominal content for the 300 series austenitics. Molybdenum, however, has a dramatic posi- tive effect on the SCC resistance. Duplex alloys exhibit higher SCC resistance due to the beneficial effects of ferrite and the low nickel content. Published results show that for chloride con- raining media, the high Molybdenum austenitic alloys are, at least, as good as the duplex alloys, whereas the 300 series grades have poor resist- ance. 4.3. Summary Alloy 885 has consistently been produced with a PREN from 39 to 40, exceeding the minimum specified by Glover. Alloy 885 is processed to conform to a CF of 35 or greater. The amount of ferrite phase is optimized. Maximum benefit is provided (strength) without increasing susceptibility of forming undesirable sigma phase, hard and cause of embrittlement. A low carbon content (below 0.03%) reduces the susceptibility of the alloy to sensitization. The addition of nitrogen further impedes carbide precipitation. Material can be welded with no post weld heat treatment. The alloy has a structure of approximately 85% austenite and 15% ferrite for ease of pro- cessing. This material shows, therefore, excellent per- formance in cold and warm seawater environ- ments due to its high localized corrosion resist- ance. The stress corrosion cracking resistance is also high. It is easy to process and weld. 5. Operating performance We have analysed some performance para- meters, more specifically, pump efficiency, turbine efficiency and specific consumption, i.e. kWh/m 3 of product. For the size of trains under consideration (2,500 mVd and above), for a feed water to permeate conversion around 45%, those para- meters, today, are within the values shown in Table 2. Table 2 High-pressure train - performance level Train size, Pump Turbine KWh/m 3 of mVd efficiency, % efficiency, % product 2,500 80-82 84-86 3.56-3.37 5,000 81-83 85-87 3.47-3.28 7,000 83-85 86-88 3.27-3.10 10,000 84-86 87-88 3.17-3.02 Flowserve has approximately 220 trains of these sizes in operation or under construction. That is a total installed production capacity of about 950,000mVd. Operating hours accumulated year to date are close to 7.5 million. 6. Conclusions There is a clear trend for high-pressure trains of larger size. For a given plant size it reduces the number of trains with the corresponding savings in installation costs. Furthermore, fewer, larger trains generally mean less capital than more, smaller trains. Larger units have, inherent- ly, higher efficiency. There is a preference amongst plant designers and operators for trains 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.