Preparation of Tubes for Inspection
The tubes must be externally and internally cleaned to permit inspection. The external cleaning is usually accomplished by wiping with steel wool, sandblasting, or by brushing. The inspector may scrape, hammer, or chip loose the fire scale in isolated individual spots as required for gaging and visual inspection.
To permit internal inspection, tubes are usually cleaned by mechanical reaming, washing, or by dry firing (steam-air decoking). If tube cores are used, they must be pulled out, cleaned, and inspected. (In locations where severe coking has resulted in high maintenance costs, cores may be installed in the tubes to increase velocity, reduce coke formation, and to improve heat transfer.) Removal of internal coke is very important when gaging by mechanical or electronic means is to be done.
On tube coils which have return bends instead of headers, the internal surfaces will not be available for inspection unless the return bends are cut off. Normal cleaning is by steam air decoking. Inspection of these coils is done by ultrasonics for wall thickness and radiography for the presence of coke. See Section 960 for a discussion of tube decoking.