I tested in a blank project with ducts made of Marble and pipes made of Grass (image attached).
As a test, copy some the offending Pipes and/or Ducts to a new project and attempt to render them. Also, could you attach a full-screen (to include application buttons at the bottom) of the 3D View you're rendering?
Corey D. ────────────────────────────────────────────────────────────────────── ⁞|⁞ Please use .Accept as Solution and Give Kudos as appropriate to further enhance these forums. Thank you!