Slagging  and  spalling  are  two  of  the  main causes of refractory deterioration. Slag is formed when ash and other unburnable materials react with the brickwork. Although the ash content of fuel oil is low, there is always enough present to damage  the  refractories.  The  most  damaging  slag- forming  materials  are  vanadium  salts  and  sodium chloride. If the slag that forms on the brickwork would remain  in  place,  it  would  not  cause  any  particular trouble;  however,  the  slag  does  not  remain  in place. Instead, it peels off or melts and runs off, taking  some  refractory  with  it  and  exposing  a fresh  layer  of  refractory  to  further  slag  attack. When  deterioration  of  the  brickwork  has  pro- gressed until only a 3-inch thickness of firebrick remains,  the  wall  should  be  replaced.  When sufficient  slag  has  accumulated  on  the  deck  to cause striking with resultant deposits of carbon, the  slag  should  be  removed.  If  less  than  1  1/2 inches  of  firebrick  remain  after  the  slag  is removed,  the  entire  deck  must  be  replaced. Another type of slag that results from using fuel  oil  that  is  contaminated  is  usually  more damaging than peeling slag. This type of slag is very  glassy  in  appearance,  and  when  this  slag melts,  it  usually  covers  the  entire  wall  or  deck. Firebrick   shrinkage   is   another   cause   of furnace  deterioration.  True  shrinkage  (permanent shrinkage) is quite rare in firebrick approved for naval  use.  However,  this  defect  can  occur  even in  approved  firebrick.  In  any  case,  it  is  impor- tant to recognize the appearance of true firebrick shrinkage  because  of  the  extremely  dangerous condition it could create if it should occur. When the firebrick shrinks, the hot-face dimensions of each brick become measurably smaller than the cold-face  dimensions.  This  condition  leaves  an open space around each brick, and the entire wall or  floor  becomes  loose.  A  wall  or  floor  having this appearance is DANGEROUS and should be completely  renewed  as  soon  as  possible. Also,  during  your  inspection,  look  for  signs of  unequal  stresses  that  are  caused  by  rapid raising of the furnace temperature while raising steam  too  rapidly.  Emergencies  may  arise  that require  the  rapid  raising  or  lowering  of  furnace temperatures, but it is important to remember that the  refractories  cannot  stand  this  treatment  often. As a rule, you will find that raising the furnace temperature  too  rapidly  causes  the  firebrick  to break   at   the   anchor   bolts,   and   lowering   the temperature too rapidly causes deep fractures in the  firebrick. Also,  look  for  signs  of  mechanical  strain caused by poor operation of the boiler. Continued panting  or  vibration  of  the  boiler  can  cause  a weakened section of the wall to be dislocated so that  the  bricks  fall  out  onto  the  furnace  floor. Improper oil-air ratio is the most common cause of boiler panting and vibration. Proper operation of  the  boiler,  with  particular  attention  to  the correct  use  of  the  burners  and  forced  draft blowers,  generally  prevents  panting  and  vibration of  the  boiler. Inspection  should  also  be  made  of  the  lower side  of  the  floor  pan.  Any  overheating  indicates a  loss  of  insulation  and  excessive  heat  penetra- tion.  Under  normal  conditions,  the  brickwork  in a boiler should last for a number of years without complete  renewal. Expansion  joints  should  be  inspected  often  for signs of incomplete closure. It is important to keep the  joints  free  of  grog,  mortar,  and  refractory particles so that the joints can close properly when the  boiler  is  fired.  You  can  tell  if  an  expansion joint  is  closing  completely  when  it  is  heated  by inspecting it when it is cold. If the inside of the expansion joint is light in color when the furnace is  cold,  the  expansion  joint  is  closing  properly. If  an  expansion  joint  does  not  close  properly  when heated,  the  inside  is  dark  and  discolored. The same method can be used to tell if cracks in refractory materials are closing properly when the furnace is fired. If the cracks are dark, show- ing that they do not close, they should be repaired. Since  the  first  firing  of  a  plastic  or  castable burner  front  does  more  damage  than  any  other single firing, the first inspection after installation is a very important one. The unfired burner front may   appear   to   be   in   perfect   condition   while actually  containing  defects  of  material  or  work- manship that will show up immediately in the first firing. After  the  boiler  has  steamed  for  several  hours, slabs  of  plastic  about  1/2-  to  1-inch  thick  may separate from the burner’s front surface and fall off.  This  is  because  the  surface  layer  is  more densely   rammed   during   installation   than   the remainder  of  the  material. Radial  cracks  in  the  burner  fronts  may  be found  on  the  first  inspection.  These  cracks  are  not harmful.  They  are  caused  by  stresses  resulting from the normal expansion and contraction of the refractory  as  it  is  heated  and  cooled.  After  the radial cracks occur, the stresses are relieved and there should be no further cracking of this type. The cracks that eventually result in extensive damage  run  approximately  parallel  to  the  surface 12-15