I don't have a copy of B31.3 with me to check, but I suggest you look at it very closely.
About 10 years ago, I was writing welder qualification and hydro testing procedures for HDPE to B31, and was suprised to find that the testing parameters are much more stringent than for steel.
I always figured it was becase HDPE, in industrial settings, is often used to carry caustic materials, but that's just an opinion.
Tim