Mystery process - GIF

[BuffaloJohn]

Looks like they are cleaning boiler tubes

Yup, cleaning the tubes. The brush is rotary and powered by the water in the blue hose.

The tubes carry the hot gases from the combustion chamber through the water in the boiler to the other end of the tubes where it is eventually exhausted. It might go up the stack or it might go through another operation where more heat or energy is extracted to do more work (pre-heating, supercharging, etc.).