The heat transfer rate is much different for wetted and unwetted surface area. During a fire, unwetted surfaces only transfer a minimal amount of heat to the vessel contents. Consequently, the unwetted vessel wall isn't sufficiently cooled. That's why PSVs can't adequately protect a vapor-filled vessel from fire. Without any liquid on the inside, the vessel wall temperature continues to rise.
To calculate the heat transferred to the vessel contents, you need to know whether the surface is wetted or unwetted. If you only have a minimal amount of wetted surface and a high amount of unwetted surface, then you should look at other methods of protecting the vessel. You still have to install a PSV for code compliance, but other protective measures such as fire resistant insulation, water spray, and automatic depressurization should also be considered.
