There are so many differences which can be observed after the deep study of two code, however I would like to highlight some differences between the above two code.

B3 1.3 deals with process piping: piping typically found in petroleum, refineries, chemical, pharmaceutical, textile, paper, semiconductor, and cryogenic plants, and related processing plants and terminals.

The factor of safety considered is 3, so thickness is less and more risk is involved.
Normal designed life of the plant is approx. 20-25 years.

B3 1.1 deals with power piping: piping typically found in electric power generating stations, in industrial and institutional plants, geothermal heating systems, and central and district heating and cooling systems.

The factor of safety considered is 4, so thickness is more and less risk is involved. Normal designed life of the plant is approx. 40-45 Years. However there is also difference in the way the “stress intensification factor” is considered in both the code. In 31.3 stress intensification factor is distinguished in two part i.e In-plane stress and out plane stress, however in 31.1 both are same.

There are so many differences. However, the above are some important one. Please correct me if I am wrong.