External Inspection of Tubes
Tubes should be carefully inspected with adequate light for bulges which may be the result of hot spots or flame impingement. The use of snap gages, preset to the established safe bulge limits, is a rapid means of locating bulges on a go/no-go basis. Bulges are sometimes located by shadows formed by shining a flashlight longitudinally down a tube. When bulges are located, they should be measured accurately with a dial gage, a pair of OD calipers, or by strapping.
Defects, where found, may be additionally inspected by the use of dye-penetrants or by acid etching. It is good practice to grind out cracks and repair with weld buildup as necessary because they tend to propagate with continued operation and may result in tube failure.
Evidence of hot spots or overheating may be noted by discoloration of the tube accompanied by heavy scaling on the fired side. Overheated or burned tubes may have excessive sagging or bulging. Where excessive scaling is noted, the scale should be removed and the tube ultrasonic (UT) gaged in the most severely affected areas.
Carbon steel tubes may be hammer-tested for a quick thickness check. Hammertesting is not recommended for most alloy tubes. External ultrasonic thickness measurements may be taken on either carbon steel or alloy tubes to establish accurate wall thicknesses. When ultrasonic testing is used, measurements must be taken at many locations on each tube to provide adequate evaluation.
The wall thickness of studded tubes has been satisfactorily established by ultrasonically gaging through the stud and tube from the stud face and subtracting the length of the stud. Radiography has also been used for this purpose.
Skin point protector tubes attached to furnace tubes should be inspected for cracks and defects in the welds. On replacements, be sure that the failed protector tubes are properly removed and that the area is ground smooth. The area must be free from cracks. Replacement skin points should not be installed in the same spot. Move the new one approximately 6 inches or more on the length of the tube.
The tubes should be checked for excessive sagging. Accepted practice is to replace a tube when it has sagged over six diameters; however, each case should be considered independently.
Inspect the exterior surfaces of the tubes behind the headers for cracks or leaks. Look for evidence of over-rolling or bulging immediately behind the header.
Make sure that the coil is properly supported and has room to expand.