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