PREPARING  TUBE  SHEETS The  tube  sheet  holes  must  be  prepared  before replacement tubes are inserted. The best way is to use a piece of hardwood turned to a diameter slightly less than the  diameter  of  the  hole  and  covered  with  a  medium fine-grit  emery  cloth.  Pass  the  wooden  piece  in  a circular motion, back and forth through the tube sheet or header holes to smooth the surface. Finish the job by using a fine emery cloth wrapped around your finger. Keep working until the hole is clean and smooth. When  preparing  tube  seats,  check  the  size  and trueness  of  the  tube  holes;  use  a  tube  nipple  of corresponding  size  as  a  template.  It  is  impossible  to make  tube  seats  tight  if  the  tube  holes  are  much enlarged or if they are too elliptical (out-of-round). To ensure the tightness of the tube seats, be sure that the maximum enlargement and the maximum ellipticity of the tube holes do not exceed the figure shown in table A, appendix II. REPAIRING  TUBE  SHEETS Out-of-round tube holes, small steam cuts, and other minor  defects  may,  in  some  cases,  be  corrected  by welding. NAVFAC approval is not required for this type of welding repair on drums and headers made of low carbon   steel,   carbon-molybdenum   steel,   or   steel containing less than 1 percent chromium if a qualified welder   uses   approved   welding   procedures   for   the welding,  filler  metal,  and  position  of  welding  under MIL-STD-248.  Always  check  the  blueprints  for  the material of the drums and headers before welding. PREPARING  TUBE  ENDS After  the  tubes  have  been  thoroughly  cleaned, prepare  the  tube  ends  inside  and  outside.  Clean  the ends with a wire brush and polish them with abrasive paper  and  a  liquid  cleaner  until  the  tube  ends  are completely  clean,  free  of  burrs  and  mill  scale,  and thoroughly polished. Clean and polish the tube ends for a distance equal to the thickness of the tube seat plus 2 or 3 inches. Round off the tube ends with a file, so no square or sharp edges remain. If the tubes are not rounded off at the ends, the tubes may split when they are belled. FITTING  TUBES When installing tubes, always fit the tubes into the steam drum before inserting the other end in the water drum  or  header.  Inserting  the  tubes  into  drums  and headers is not particularly difficult, since all tube holes are drilled normal to the tube sheet. If you are renewing a complete row of tubes, fit a tube  at  each  end  of  the  row  and  then  work  toward& middle. You may find slight differences in the lengths of tubes required, if the boiler has been in service for some years. These differences are more likely to show up at the ends of the rows than in the middle. When fitting tubes into drums or headers, be sure each tube extends far enough into the header or drum. Tubes  up  to  (but  not  including)  2  inches  in  outside diameter (OD) should project 3/16 to 5/16 inch into the drum or header. Tubes 2 inches OD and larger should project 5/16 to 7/16 inch into the drum or header. After  you  have  fitted  a  tube  and  allowed  for  the amount it must project into the steam drum and into the water drum or header, remove the tube and cut off the excess. You may be able to use one tube as a guide for cutting  off  the  excess  on  several  other  tubes;  if  you recall,  the  tubes  may  vary  slightly  in  length, particularly in older boilers. Do NOT use one tube to measure the rest of the tubes in a row if you believe there are big differences in tube lengths in the row. If your sample tube happened to be a little on the short side, you would end up with a whole row of tubes that were too short; therefore, they could not be used. Each tube must be carefully aligned with the other tubes. Use a plywood batten about 1/2 inch thick, 3 feet long, and 3 inches wide to align tubes in the generating bank. After positioning the tubes, check them with the batten. Then place small, wooden wedges to hold the tubes in place until they have been expanded into the tube  sheets.  Be  sure  to  remove  the  batten  and  the wedges  before  starting  work  on  the  next  row.  These wooden  pieces  cannot  be  left  in  the  boiler.  You  will have a real job on your hands if, after installing five or six more rows of tubes, you suddenly discover that you have overlooked the batten or one of the wedges. EXPANDING  TUBES The  basic  joint  in  boiler  construction  is  an expanded  joint  that  must  not  leak  nor  lack  holding power. Leakage, if permitted to go uncorrected, leads to   deficiency   of   holding   power   because   of deterioration  of  the  tube  seat.  Slight  leakage  itself should not be taken as cause for alarm, but rather as evidence  to  correct  the  fault  as  soon  as  possible. Deficiency of holding power causes the tube to pull out of its seat. In most cases, the tubes are installed within the furnace of the boiler, and any danger to personnel, if  the  tubes  pull  out  of  the  seat,  is  reduced  since  the 2-10