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.
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.
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 theContinue Reading