There are several derating factors involved depending on the code that you are obligated to use. Service derating, temperature derating, location derating etc... B31.3 Appx B does not give you any of them. They are HDS values only and I guess B31.3 is still in development stage for plastic piping.

The most developed one is Code Case N-755-1 for nuclear applications and we were told it is going to be used for the other industrial applications. I guess you can use B31.3 with the information taken from the Code Case N-755-1. But you still need to know almost everything about the materials you are using. Without knowing the material and their behaviour in operation you should not directly do any calculation, the responsibility is great; you may not be able to handle alone. Ask the provider of the materials to give you hand on the material types, and their allowable. None of them will give you any service factor. Your client or you are to decide.

If there is a recognised ISO or European standard for the material you are trying to investigate, and this standard investigation is an option in your analysis program, I would suggest you to use it.
Additionally please note that all the pipes with the materials you mention above, if they are containing pressure, they expand under internal pressure similar to thermal expansion, and they may be as significant as or even large than thermal expansion. Assume 50 metres of PE pipe with 50 deg C of differential wall temperature; it expands around 500 mm under thermal load only. So you need to prepare your piping for the thermal+pressure expansion before start doing anything else.

Good luck.

Ibrahim Demir