Those calculations are likely correct (18 thou per deg F), but on a 50foot run, the expansion can occur at the ends. If there was 200 feet of PVC conduit, the expansion that occurs in the center of the run (at approx 100ft) has nowhere to expand easily enough, and thus, buckle the center of the conduits overall entire run. It is an interesting figure you've commented on....I'll remember that, 18thou per deg F.