Dear members,

In CAESAR II "Application Guide" pp.9-34(Tutorial B),
Some comments on local vessel stresses are described as follow;

"As a very rough guide for evaluating local vessel stresses, one
can check the code defined stress on the pipe connected to the vessel.
If those stresses are below 6000 psi, the vessel stresses should be OK."

My question is
What is the engineering basis of 6000 psi in above sentence?

Is there any code or paper that evaluates the "6000 psi"?

Thank you.