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. 高压泵反渗透设备列车英文文献和中文翻译(2):http://www.751com.cn/fanyi/lunwen_31599.html