Section 15(2) in The Static and Mobile Pressure Vessels (Unfired) Rules, 2016
(2)Insulated vessels,-(i)the shell of the vessel and its manhole nozzle shall be insulated with a material approved by the Chief Controller. The entire insulation may be covered with a metal jacket and flashed around all openings so as to be weather-tight;(ii)the insulation shall be of sufficient thickness so that the thermal conductance at 150C (expressed in calories or square centimeter per hour per degree centigrade temperature differential) shall not exceed the limit prescribed by the Chief Controller.
