Not much info provided...sounds like a good question to ask your engineering dept.
Regarding the weld procedures:
SA-335 Grade P11 is a ASME P4 mat'l and the stainless's are P8 mat'ls.
Changing from the P4 to a P8 mat'l will obviously require a different WPS/PQR, ..do you intend to substitute and join the stainless pipe directly to the P4 mat'l as well?
not much information provided,
which fluid is passing through SA-335GR P11 pipe, i.e fluid class. What temperature exactly ? What is the NDT class for that pipe class, Working and test pressure for that pipe classes.
these questions are vital, without knowing that nobody can give an answer.
as far as i know sa312 tp 316 working temperature is up to 300 degC. if your working temperature is close to 300 degC or over, is not possible to use 316
may be you can think to use martensitic and ferritic stainless steels, such as "420",which has high heat and wear resistance up to 600degC.
for high temperature usage incaloy or inconel type materials are used.
but in anycase SA335grP11 will be cheaper for you.
Regards,
T
My response would be "No - not unless your design engineer has evaluated the change and says it is OK."
Stainless steels expand at a different rate than P11 does as it heats up to the operating temperature. This means both the diameter and pipe length changes at different rates. A dissimilar girth weld between P11 and 304 or 316 will have additional thermal expansion stress across the weld. The changes in length at operating temperature may increase bending loads at elbows that may either be too high (resulting in failure) or require changes to the pipe support load settings. And the allowable stress values for 304 and 316 are much lower than P11 for a given temperature, so the minimum pipe wall thickness required will be higher.
I recommend you not make such a change without consulting a design engineer.