@THH
The question on the return pipe is whether the pipe was full or not with water. Was there a U in the pipe, for example. That is the question. The "trap" Barry was referring to was in the context of rust from the "big Frankies."
And the DN40 question relates to the exit pipe from the 1 MW plant not the return pipe.
Unfortunately, I have yet to see answers to these central issues. But I'm just barely scratching the surface of this doc dump, and I do need to get some other things done today. I'm afraid this is going to take several days to decipher.