You should also see what your client's spec states.in some specs even when for example the material of transition/obround nozzle of header of air cooler is 316L PFHT is required for that nozzle.
Max temperature limit of SA 240 316L as per ASME II D is lower than the design temperature condition specified in table uha-44 for the austenitic chromium nickel.
MAWP is defined at corroded condition(see ug-98 b) and ug-99 b uses MAWP.as I said in previous post you can utilize ug 99 b for uncorroded or corroded condition.
There is no difference between the amount of test pressure between corroded and uncorroded condition per ug-99 b since the MAWP will not change.but the amount of test stress to be applied on vessel at corroded condition will be higher.you can use ug 99 b in pv elite for corroded or uncorroded...
about your body flange... You may know you can reach MDMT lower than -38 deg C you mentioned for curve B material without impact test.of course it depends on design condition.see ucs 66 b1(c),ucs 66 b1 (b) and ucs 68(c).
When you consider ug-99 b, yes it uses mawp, but the hydro test stress will be obtained at uncorroded condition. Hydro test at corroded condition is something different. the decrease in thickness and increase in diameter because of corrosion will result in increase in test stress at corroded...
And increasing reinforcement thickness will obviate the warning. Using self reinforced nozzle will be a better option especially when shell thickness is more than 50 mm, design temperature is upper than 345 degree celsius and in low temperature service.
Even for SSC resistance which could not be very hard to obtain, as rolled normalizing has not been permitted for low alloy steel by NACE MR0175/ISO 15156-2.
