API 521 Question- Insulation Credit for Environmental Factor

[n9ne0hse7en]

Hi, new member here just getting into my first analysis of PSV sizing using the environmental factor for API 521 fire case sizing. Per API 521 4.4.13.2.7, it appears that insulation properties (e.g. thermal conductivity) should be calculated at the AVERAGE insulation temperature during a fire case. However, my interpretation of 4.4.13.2.7.4 is that the outside temperature of the insulation should be taken as the equilibrium temperature of the banding, 1660 F. This would mean that the average temperature, assuming a minimum process boiling temperature of 0 F, would be at a minimum 830 F. For a higher-boiling point fluid, such as at 600 F, the average temperature of insulation would be in excess of 1000 F.

This seems to conflict with Table 6 where thermal conductivity values for most materials (aside from cementitious) are not provided above 1000 F. In addition, vendor brochures will advertise "API 521 insulation" but not provide thermal conductivity values in excess of 800 F. Am I using the wrong equation for average temperature, or how else to explain this discrepancy? Many thanks!