stoker-man wrote:What is a good product and procedure to remove hard water scale from the 520 boiler?
The first question that comes to mind is why or how did is scale up in the first place?
The only way to get significate scale in a boiler is to have considerable amount of make up water enter the system. The make up would be needed only from a leak in the system that was left unchecked for considerable time.
If you made up that much water to create scale, that water also had oxygen in it. That oxygen no doubt attacked the metal of the boiler. You may remove the scale and find out you need tube/plate repairs.
I'm with Sting. If the rest of it is in good condition, salvage the stoker, base, jacket, etc. and buy a new pressure vessel.