minimum thickness for apply PWHT

hai all,

i have a doubt with PWHT process. we are using asme ix and asme B31.1 for contruction code.

i already made prewps for this,

material P11 to P11 ( p no 4 and group no 1) and 6'' sch120 wall thickness 14.27mm.

my question is. what the minimum thickness for applying PWHT?

i already referred to the code QW 407.1 PWHT but not understand it.

kindly please advise