It was quite common in days gone by to reuse flues. After removal, they were inspected, and some railroads used a device known as a "flue rattler." This thing was a v-shaped table with a number of oval rollers on it. A number of flues would be placed on the rattler and the air or electric motor would be started. The flues would roll around against each other, and the vibration and bouncing would theoretically knock scale loose.
Once cleaned of scale,the flues could be cut off to the proper length for a smaller (shorter) boiler, or perhaps "safe ends" would be swedged on and welded. Thus flues could be reused several times, as long as they passed inspection.