Section 14(2) in The Static And Mobile Pressure Vessels (Unfired) Rules, 1981
(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 shall be covered with a metal jacket of a thickness not less than 3 mm. nominal 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 sq. cm. per hour per degree centigrade temperature differential) does not exceed the limit prescribed by the Chief Controller.
