This is just a small addition to Vannella's comment on solar heat gain.

I have seen for outdoor and overground pipes, carrying fuids even at ambient temperature, the design temperature of pipe is considered as 85ºC or 90ºC. The idea is, the pipe may be empty and will absorb solar heat until an equilibrium temperature is reached when incoming solar heat will equal heat lost from the surface of pipe by convection and radiation.

Thank you,