I would like to ask, why some vessel is lined and some is unlined?
Usually we are using 316SS as the material or DUPLEX. Any body got comparison of these two? and what makes them to use exotic material like Hastelloy C?
Dcasto's reply pretty much sums it up. I'd just mention that cladding or lining a vessel requires additional fabrication. Vessel size can play into the decision. For small vessels like vapor-liquid separators, it is often more economical to fabricate from solid alloy rather than clad or line a carbon-steel shell.