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.
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.
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