Credit factors for insulation in determining the minimum Pressure Relief valve sizes for fire exposure should be as per API RP 521 with following additional guidelines:

Credit can be taken for the insulation systems on steel vessels and vessels constructed of low melting point material, such as aluminum, when the insulation is held in place by systems that will withstand a fire e.g. a galvanized steel or stainless steel jacket. Cellular glass, mineral fiber (Class II), expanded Perlite, calcium silicate (Types I and II), lightweight or dense cementitious. Jacketing may not be required for cementitious.

For plastic foam, e.g., polyurethane and similar types of materials, which melt or decompose under fire exposure conditions if covered with materials such as above and protected with a galvanized or aluminum coated steel, or stainless steel, jacket.

Plastic foam must have a flame spread classification of 25 or less per ASTM E 84. The covering should prevent the plastic foam from decomposing or melting when the insulation system is exposed to a temperature of 1660 °F (900 °C) for two hours. Credit may also be taken for the insulation property of the covering material.

Plastic foam shall have a flame spread classification of 25 or less per ASTM E 84. The covering shall prevent the plastic foam from decomposing or melting when the insulation system is exposed to a temperature of 1660 °F (900 °C) for approximately two hours. Credit may also be taken for the insulation property of the covering material.