Advanced  Reactive™  Desiccant  Dehumidifiers  for  Long  Term  Layup  and  Corrosion  Control  

 

Todd  Bradley,  Application  Specialist,  Controlled  Dehumidification  IMS.  

Controlling  corrosion  in  metals  is  best  accomplished  by  removing  atmospheric  water  vapor  from  ambient  air.    The  goal  is  to  condition  the  ambient  (air)  moisture  vapor  pressure  (dew  point)  to  always  be  below  the  dew  point  temperature  of  the  metal  surfaces.    To  prevent  moisture  from  collecting  on  the  metal  surfaces  the  ambient  vapor  pressure  must  be  maintained  to  a  lower  vapor  pressure  than  that  of  the  surfaces.  

Carbon  steel  and  other  metals  can  oxidize  when  ambient  dew  point  levels  exceed  the  surface  dew  point  temperature.  Microscopic  moisture  particles  in  the  moisture  laden  boundary  layer  will  collect  on  surfaces  as  the  metal  cools.    As  more  water  molecules  collect,  visible  moisture  appears.  This  moisture  can  create  rapid  oxidation  or  flash  rust.    Other  materials  present  from  the  combustion  processes  on  the  gas  side  of  boilers  and  auxiliaries  can  also  interact  with  moisture  to  form  acids  which  can  further  accelerate  the  degradation  of  some  metals.  

 Typical  large  power  plants  involve  tons  of  solid  mass  that  react  slowly  (lag)  to  ambient  atmospheric  changes  in  temperature.    Ambient  weather  changes  can  happen  rapidly:    the  typical  daily  swings  in  ambient  temperatures  can  be  30⁰F  or  more.    Ambient  humidity  swings  can  also  be  extreme.  The  peak  of  the  ambient  temperature  generally  happens  in  the  afternoon  when  net  solar  radiation  gains  are  the  largest.  The  large  mass  of  metal  surface  reacts  slowly  to  air  temperature  changes  because  of  the  arrangement  of  the  structure  and  specific  heat  of  the  materials.  The  night  convective  cooling  effect  can  remove  energy  from  the  materials  and  the  daytime  convective  heating  effect  can  warm  the  materials.  Radiant  energy  during  the  daytime  can  also  warm  the  exposed  surfaces.  

   

 Chart  1:  ASHRAE  Design  

   

 

 

 

 

 

 

 

This  net  energy  balance  can  provides  a  cool  surface  that  can  act  as  a  condenser.  When  the  vapor  pressure  temperature  is  higher  than  the  surface  vapor  pressure  temperature  of  the  metal,  corrosion  and  pitting  can  result.        

Chart  2  above  shows  ASHRAE  design  conditions  for  Houston’s  Hobby  Airport.  The  peak  ambient  dew  point  for  a  dehumidification  application  shows  78.7  ⁰F:  any  surface  that  is  below  that  temperature  will  collect  moisture  as  it  acts  as  a  condenser.  Controlling  the  air  dew  point  below  that  level  is  critical  to  avoiding  moisture  related  corrosion.  

 

Elevated  humidity  levels  on  the  combustion  side  of  the  systems  can  also  react  with  residual  process  deposits  to  form  acids.  The  industry  to  date  has  used  relative  humidity  %  RH  has  a  control  point  to  reduce  the  effects  of  atmospheric  humidity.    It  is  more  accurate  to  protect  the  surfaces  by  allowing  the  controlled  ambient  dew  point  to  always  be  below  the  coldest  surface  temperature.  A  differential  dew  point  between  the  ambient  air  and  the  metal  surfaces  will  reduce  or  eliminate  this  corrosive  effect.  

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“The most important factor in atmospheric corrosion, overriding pollution or lack of it, is

moisture, either in the form of rain, dew, condensation, or high relative humidity (RH). In

the absence of moisture, most contaminants would have little or no corrosive effect.”

James Mathews EPRI  

Chart  2:  Radiant  Solar  Insolation  

 

The  challenge  of  controlling  the  internal  moisture  levels  is  the  size  and  complexity  of  typical  generating  plants.  Generating  plants  can  expose  the  structures  to  variable  temperatures;  from  a  sub-­‐basement  feed  water  system  to  external  flue  gas  treatment  devices.  The  structure  can  have  a  variety  of  structural  temperatures,  thereby  making  it  difficult  to  have  one  dew  point  set  point  that  would  provide  protection  for  all  components.  The  areas  located  in  cool  areas  could  see  average  summer  steel  temperatures  of  50°  to  60°  F  Degrees  while  components  located  in  higher  areas  or  external  to  the  plant,  might  be  exceed  75°  F.  During  cold  weather  months  components  connected  or  exposed  to  the  outdoors  could  see  average  steel  temperatures  of  0°  to  40°  F.    The  specific  vapor  pressure  of  air  at  the  recommended  40%  RH  at  normal  heating  temperatures  is  very  different.  (See  Chart  4).  The  chart  shows  a  steel  temperature  of  40°  F  with  an  interior  air  condition  of  75°  to  80°  F  at  40%  RH.    Both  of  those  air  readings  show  a  dew  point  condition  higher  than  the  steel,  there  will  be  condensation  in  this  example  

 Chart  3:  Comparison  of  Metal  Temperature  to  Ambient  Temperature  and  Humidity  

 

Average  steel  temperatures  will  vary  with  atmospheric  and  environmental  forces  over  the  layup  period  resulting  in  an  elusive  thermal  equilibrium.    Dew  point  control  and  measurement  can  provide  a  more  accurate  predictor  for  corrosion  prevention.  Structuring  a  dew  point  monitoring  system  that  will  provide  a  set  point  based  on  the  coldest  surface  temperatures  will  provide  consistent  and  predictable  corrosion  prevention.  

Generating  plants  can  be  protected  by  proper  conditioning  of  both  the  combustion/gas  side  and  the  steam  side  of  the  system.  By  reducing  the  dew  point  of  the  air  in  contact  with  steel  surfaces  with  an  advanced  reactive  desiccant  system,  the  dew  point  can  be  adequately  controlled  to  eliminate  the  moisture  reaction  on  the  coolest  surface  of  the  pressure  generating  system.    Typically  the  wet  side,  turbines,  super-­‐heaters  and  the  boiler  wet  side  can  be  controlled  by  one  system(s)  and  the  gas  side  including  the  boiler,  economizers  and  baghouse  can  be  conditioned  with  a  separate  dehumidification  system(s).    

Sizing  the  system  is  done  by  calculating  the  cubic  volumes  of  the  spaces  and  piping  to  be  conditioned.  Additional  capacity  must  also  be  provided  to  account  for  leakage  rates  to  ambient.  Advanced  Reactive  Desiccant  systems  can  be  sized  to  provide  long  term  protection  for  components  through  proper  sizing  and  application.  

 References:  Pidwirny, M. (2006). "Daily and Annual Cycles of Temperature". Fundamentals of Physical Geography, 2nd Edition. Date Viewed. http://www.physicalgeography.net/fundamentals/7l.html Numerical computation of time lags and decrement factors for different building materials H. Asan_ Kingston  Technical  Software,  Corrosion  Engineering  principles  and  practice.  Layup  Practices  for  Fossil  Plants.  James  Mathews  February  13,  2013  Power  Magazine http://corrosion-doctors.org/Corrosion-Atmospheric/Corrosion-resistance.htm

 Typical  advanced  reactive  

dehumidifier  with  temporary  ducting  

 Permanent  Damage  to  Tubes  and  

Boiler  drum:  Courtesy  EPRI