Urias,
Under FRA regulations, enough tubes need to removed at the time of the mandatory FRA 1472 inspection to inspect the interior of the boiler shell for soundness. As a matter of practice, this involves removing
all of the tubes so that the shell can be needle-scaled to remove deposits and visually checked, especially along seams. Then the exterior wrapper is removed, along with the insulation (and generally most of the appliances), a chalk grid is inscribed on the outside of the boiler shell, and an ultrasound thickness test is conducted at each point of intersection on the grid, and the results are recorded. If the metal composition of the shell is known, the thickness can be used to calculate the safe operating pressure of the boiler. Getting the scale (which can throw ultrasound measurements off) off is much more difficult due to space constraints between the firebox and the outer wrapper, so thickness readings there must be closely scrutinized. While the tubes are out, the front and rear tube sheets can be inspected, as well as the staybolts and the dry pipe, for wastage. Thin sections of plate and wasted staybolts can be replaced--to a point. One always has to ask if the cost justifies the labor or materials, or if a replacement boiler is in order.
Boiler tubes and flues are generally not reused out of both safety concerns (too many lawyers) and cost considerations. Because the rolled ends of the tubes are burned off when they are removed, they no longer are of sufficient length to be reused. They can be "safe-ended" by carefully welding on short extensions, but the time involved, particularly in carefully examining all the welds, mitigates in favor of the old tubes with new ones.
Staybolts are removed and replaced if and when they show signs of breakage or wastage (thinning). This does not mean that all staybolts always have to be replaced at every 1472-day inspection.
I'm sure any errors in my comments will be pointed out by Mike Ramsey!
Hope that helps
