of the burner front, and they are called parallel cracks. Parallel cracks usually appear at or slightly behind the leading edge of the bladed cone. They are not dangerous until they actually loosen pieces of  the  burner  front.  Improper  installation  and boiler operation are usually the cause of parallel cracking. A slanting crack in the narrow section between the burners sometimes joins a radial crack. When this  occurs,  pieces  of  plastic  tend  to  break  off. This type of damage can usually be repaired by a  plastic  patch. If  during  your  inspection  you  find  that  a castable  burner  front  is  breaking  up  after  very little service, it is likely that too much water was used in mixing the material during installation. Sometimes  the  material  is  already  partially  set before  installation;  a  common  cause  of  this trouble  is  that  the  castable  material,  while  in storage,  reacted  with  moisture  in  the  air  and started to set. When castable material sets before it  is  used,  it  can  never  reach  full  strength. Castable  material  is  also  subject  to  spalling after   several   hours   of   service.   The   peeling material, usually in 1/8-inch strips, should not be removed  unless  it  is  in  the  burner  cone  and  is interfering  with  combustion. If a castable front is chalky or crumbly, find out how deep the condition goes. If no more than the  surface  can  be  rubbed  off,  the  burner  front is  not  seriously  damaged.  Do  not  remove  the crumbly  material.  The  condition  is  serious  only if the burner cone is affected or if the casing shows signs  of  overheating. Burner   tile   should   be   inspected   for   loose segments  and  broken  pieces  that  might  cause improper  cone  angles.  The  broken  or  damaged segments  can  be  repaired  by  patching  with  plastic fireclay refractory. In some cases a new segment of  tile  can  be  installed. When   you   inspect   boiler   refractories,   it is  a  good  idea  to  keep  in  mind  the  possibility that  damage  may  occur  because  of  operational problems. Although boilers must occasionally be operated under very severe and damaging condi- tions, a lot of damage to refractories (and, in fact, to  other  boiler  parts  as  well)  is  caused  by  poor operating  procedures  that  are  really  not  necessary under  the  circumstances.  It  may  be  helpful  to show operating personnel any refractory damage that  appears  to  be  directly  related  to  poor operation  of  the  boiler. 12-16 Tube  Inspection When inspecting the exterior of boiler tubes, look  for  signs  of  warping,  bulging,  sagging, cracking,   pitting,   scaling,   acid   corrosion,   and other  damage.  All  tube  sheets  should  be  inspected for  signs  of  leakage,  especially  the  superheater tube sheet. Inspection   of   boilers   sometimes   shows   an unexpected  condition  in  which  adjacent  boiler tubes are warped in such a way that they touch each other. When this condition exists, the tubes are said to be married. Tube marriages can result either  from  overheating  of  the  tubes  or  from stresses developed in the tubes during installation. For the latter reason, newly erected boilers and boilers that have been retubed should always be inspected  for  tube  alignment  after  the  initial period  of  steaming. When  inspection  reveals  one  or  more  tube marriages, the decision as to whether or not the married  tubes  should  be  renewed  should  be based  on  the  following  considerations: 1. If the tube marriage occurs in screen tubes 1 1/2 inches or larger, or if the furnace side wall or  rear  wall  tubes  are  bowed,  tube  replacement is usually required. 2.  If  1-inch  or  1  1/4-inch  tubes  in  the  main bank of generating tubes are married, replacement is usually not required if the tube joints are tight under hydrostatic test. 3. Inspect the external surfaces of the tubes. If  they  show  blistering  or  other  signs  of  over- heating,  the  tubes  should  be  renewed. 4.  Inspect  the  watersides.  Where  tube  mar- riages  exists,  a  poor  waterside  condition  may indicate hard scale or oil within the affected tubes. If hard scale or oil does exist, the married tubes should  be  replaced,  and  all  appropriate  steps should be taken to remove the scale or oil from the  rest  of  the  boiler.  If  the  condition  of  the tubes  is  uncertain,  or  if  a  large  number  of  tube marriages  have  occurred,  remove  one  or  more sample  tubes,  split  them,  and  examine  them carefully. 5.  Tube  marriages  may  cause  gas  laning, and  gas  laning,  in  turn,  may  cause  local  over- heating  of  the  inner  casing,  the  bottom  part of  the  economizer,  and  other  parts.  Inspect the   boiler   carefully   for   signs   of   local   over- heating  that  might  have  been  caused  by  gas laning   resulting   from   the   tube   marriages.   If the  local  overheating  from  this  cause  is  found, renew the married tubes.