Your objects have self intersections and that doesn’t work well with boolean union without some extra care. If you don’t join the mirrored pipes but keep the S shapes separate, there are no self intersections.
By using the loop components from Anemone, you can create a boolean union of all parts, one by one.
I changed your clusters and added the loop in your file.
seamless pipe panel_mrtn.gh (300.2 KB)