Griswold  Water  Systems:  White  Paper  Tower  Metal  Selection  Guide  Based  on  Water  Chemistry  

 Two   characteristics   of   makeup   water   intended   for   use   in   a   cooling   tower   can   provide  guidance  in  selecting  the  type  of  metal  specified  for  cooling  tower  basins  and  other  water-­‐immersed  areas  of  cooling  towers.  The  choice  of  metals  is  typically  between:    

• Galvanized  steel  • 300  series  austenitic  stainless  steel  

 Good   water   treatment   practices   can   successfully   achieve   good   corrosion   properties   on  either  metal,  but  knowing   the  water  chemistry  of   the   local  makeup  water   to  be  used  can  lead  to  a  material  selection  that  will  enable  lower  operational  costs  and  increased  customer  satisfaction.   The   two   characteristics   of   local   makeup   water   important   to   know   prior   to  specification  are:    

• pH  • Chloride  level  

 These   two  values  must   be   ascertained  by   laboratory   testing   or   field-­‐testing.  GWS  has   an  extensive   library   of   water   analyses,   which   can   often   provide   reasonably   accurate  assumptions  on  source  water  quality  but  a  current  local  water  analysis  is  the  best  way  to  provide  accurate  water  chemistry  information.    Water  utility  publications  cannot  be  relied  on   because   the   ranges   of   values   reported   are   too   broad   to   be   helpful.   pH   and   Chloride  levels   can   vary   from   town-­‐to-­‐town,   from   neighborhood-­‐to-­‐neighborhood,   and   between  city-­‐verses-­‐well  water  sources.     Seasonal  variations  also  occur  and  are  due   to  changes   in  the  source  of  the  water  supply.      Makeup  Water  having  High  pH  If   makeup   has   a   pH   of   8.0   or   above,   there   is   extra   complexity   in   conditioning   (i.e.,  passivating)   galvanized   steel   against   the   formation   of   “white   rust”.   White   rust   is   a  detrimental  condition  which  destroys  the  protective  properties  of  a  zinc  coating  and  results  in  shortened  equipment  life.    To  condition  galvanized  steel  against  white  rust,  conductivity  actuated  control  of  blowdown  discharge  is  usually  used  to  maintain  makeup  water  below  8.0   during   the   first   six   weeks   of   operation   such   that   the   pH   of   recirculating   condenser  water  remains  below  pH  =  8.3.    This  is  possible  in  many  waters  since  incoming  pH  is  often  less  than  8.0.  If  this  is  accomplished,  galvanized  steel  will  become  permanently  passivated;  from  then  on  the  pH  can  vary  up  or  down  without  concern  for  white  rust  formation.    If  the  makeup  water  has  a  pH  of  8.0  or  above,  however,  chemicals  must  be  used  to  either  lower  the  pH  for  the  first  six  weeks  of  wet  operation,  or  make  other  chemical  changes  in  the  water   for   even   longer   than   six  weeks,   in   order   to   achieve   passivation.   This   process  will  require  extra  cost  and  labor-­‐intensive  attention  including  chemical  addition  equipment  and  frequent  service  visits.    

In  cases  of  makeup  having  a  pH  of  8.0  or  above,  stainless  steel  may  be  a  preferable  choice  for  immersion   areas   of   a   cooling   tower   in   order   to   entirely   avoid   the   problem   of   passivating  galvanized  steel.    Makeup  Water  having  High  Chlorides  If   makeup   has   a   chloride   content   of   100   mg/liter   (often   reported   as   PPM   [parts   per  million])  or  above  all  the  metals  in  a  cooling  systems  can  be  subjected  to  “chloride  pitting”  after  the  chloride  content  is  concentrated  by  evaporation  to  higher  levels.  Chloride  pitting  is   a   damaging   form  of   localized   corrosion   that   can   cause   quick  penetration   of  metal   and  significantly  shorten  equipment  life.    After  evaporation  concentrates  the  chlorides,  the  sensitivity  of  metals  in  a  cooling  system  can  be  summarized  as  follows:    

• Unprotected   iron   and   steel,   galvanized   steel,   and   copper  will   tolerate   800  PPM  of  chlorides   (as   concentrated   by   evaporation)   without   significant   risk   of   pitting  corrosion.  

• 301  and  304  stainless  steels  will  tolerate  400  PPM  of  chlorides  (as  concentrated  by  evaporation)   without   significant   risk   of   pitting   corrosion.   316   stainless   steel   is  occasionally  specified  because  it  can  tolerate  chlorides  well  above  800  PPM.  

 It   can   be   seen   that   the   stainless   steels   are   more   sensitive   to   chloride   levels   than   other  metals.  If  makeup  water  is  100  PPM  or  more,  evaporation  concentration  should  be  limited  to  4  times  resulting  in  400  PPM.  This  limitation  can  limit  water  conservation  and  increase  the  water  purchase  cost  and  sewage  discharge  cost  of  cooling  tower  operation.    In  cases  of  makeup  having  chloride  content  of  100  PPM  or  higher  galvanized  steel  may  be  a  preferable   choice   for   immersion   areas   of   a   cooling   tower   to   facilitate   good   water  conservation  without  creating  a  risk  of  pitting  corrosion.    Makeup  Water  having  both  High  pH  and  High  Chlorides  If  makeup  has  both  high  pH  and  high  chlorides,  tradeoffs  are  required  and  priorities  must  be  ordered:    

• If  guarantee  of  a   trouble   free   startup   is  most   important,   stainless   steel  might  be   the  preferred  choice.  

• If  continued  water  conservation  and  reduced  cost  for  future  years  is  most  important,  and   close   attention   and   some   extra   expense   can   be   provided   for   a   six  week   startup  period,  galvanized  steel  might  be  the  preferred  choice.