As posted by ash9144, the type and quantity of scale and design of your boiler will dictate your approach to cleaning the tubes.
We have a HGRS boiler (115,000 lbs/hr) @ 650 psig, 750F, in a thermal reduction (NOx)unit that requires periodic cleaning of the tubes due to feathers from refractory dust. These tubes are sand blasted using Al2O3 abrasive and a very good operator. One thing is we have hooked a vacuum system to help collect the residue. We tried water blasting, 15,000 psi, but it took too long and it took too long to dry the refractory. We still water blast the preheater section. This boiler has a Atemperator.
All our Therminol Vaporizers of D-Tube construction have been sand blasted several times in their 50 yr, operation.
We have partially abrasive blasted our power boiler due to having to burn high Vanadium fuel oil for short periods.
No matter which approach you undertake make sure you have experienced people to do the work.