So finaly it was a mistake... Thank you

there is no mistake, i've checked more than twice. the same configuration works with gaz from refinery and from light petrol craked into gaz. but if someone want to try, my save is here. i've try to connect the pipes above the spidertron. directly when you load the game. I've tried with mods activat...

I found this issue when i wanted to mix the output of heavy petrol cracked into light petrol, with the output of the refinery light petrol. the game says :" different liquid can't be mixed. But in my opinion they both are light petrol so they can mix!