Night and day? Thermal expansion sizing is very small volumes (mL scale) for a liquid-full vessel, designed for fairly low grade energy input. The fire case is a much higher rate of energy input.
In my opinion, Liquid thermal expansion is a condition resulted from direct solar radiation contact while the segment is blocked. Usually the expected temp from solar heating is up to 70 oC only (so the P increase and relied flow will correspond to volume increase of your liquid from its temp to approximately 70 Deg C).
For fire case, your blocked segment is exposed to fire. Hence, the limit of heating is the flame emissive power being exposed to the segment (depends on the fuel), area being exposed to fire, etc.
