The internal visual inspection of heater tubes is limited to heaters with fittings of the removable U-bend or plug type. On tubes up to about 30 ft (9 m) in length, it is possible to view the entire interior reasonably well if a light is inserted at the end opposite the one at which the tube is being examined and the examination is made from both ends of the tube.
The inside surface of a tube can be examined with optical instruments. Considerable time is required to inspect the full length of tube. Consequently, optical instruments are generally used for the more thorough inspection of questionable areas revealed by visual inspection or to assess internal fouling/deposits Most optical equipment today allows videotaping of the images. The videotape can serve as a record of the internal inspection and allows better comparison of conditions in the future if needed.
The internal visual inspection of tubes can be made to locate and determine the extent of the following types of deterioration commonly experienced in heater tubes:
a. Selective, spot-type, or pit-type corrosion.
b. Thinning of tube ends.
c. Cutting or other cleaning damage.
d. Loosening of the tube roll and flare.
f. Fouling/coke deposits
This type of corrosion is one of the most difficult to detect. Visual inspection can be hindered if the internal surfaces of the tubes are not free from coke and any other foreign matter. Mechanical cleaning will not always reveal spot- or pit-type corrosion. If this type of corrosion is apparent or suspected, the inside surfaces of the tube at the tube ends might be cleaned using an acetylene torch to burn coke or grit blasting material out of the pits. Grit blasting would be preferred and least likely to damage the tubes as could happen with an acetylene torch.