Vessels are often full of process fluids that carry heat away from the vessel walls, whereas the skirts will heat up and fail in very short order.

Nathan Brink

See API RP 2218 Fireproofing Practices in Petroleum & Petrochemical Processing Plants, Third Edition.

Regards
r6155

IMO, it's the question of the extent of the fireproofing scope, effectiveness of the fire protection, and associated cost issue.

Paulettea

Some Horton Spheres (used for LPG / Propylene storage) do have fire-proofing everywhere. Just because of safety issues, though.

DHURJATI SEN

My understanding is that supports are the one closest to the main source of heat input (i.e. pool fires). Also, if the equipment is insulated and within a fire scenario envelope, a non-combustible insulation should be used and the vessel itself would be less exposed to the heat source.

That is in addition to NBrink's comment that the bottom part of the vessel typically have liquids which can absorb the heat.

Vertical vessel skirts and horizontal saddles are fireproofed so that in the case of a fire the vessels do not collapse, rupture and add more fuel to the fire (escalation).
I have seen vessels offshore fully fireproofed and not just the supports. These were fireproofed with Chartek and it was applied due to the possibility of a jet fire impinging on the vessels. Typically offshore vessels can be blowndown (remove the gas inventory) in say 15 mins. The Chartek is there to protect the vessel whilst blowndown takes place. Onshore I have only typically seen tall vertical vessels (columns) with their skirts fireproofed on the outside - sometimes on the inside dependent on vessel size and CLient requirements.