Dear all;

As I know, B31.3 is for new plants. Then what we can do in stress analysis for runnig plants during revamping?

Same B31.3

It is upto the client to decide the code to be used.

API 579 has guideline on fittness for service.

Regards

Thanks anindya, I will try to find this API. I want to mention that the plant is more that 20 years and many operation cycles already done on plant piping with high temperature >500 degC. What is the advise?

I think stress range reduction factor should be considered as 0.9 or 0.8 based on how many more years plant is suppose to be further in operation.
When initial plant is design for 20 years stress range reduction factor is 1.0 considering roughly one cycle a day.