Section 21(10) in The Static And Mobile Pressure Vessels (Unfired) Rules, 1981
(10)[ Aboveground vessel for storage of corrosive, flammable or toxic gas in liquefied state shall be provided with enclosure wall all around the ground. The minimum distance between vessel and enclosure wall shall be the diameter of the vessel or five metres, whichever is less. The ground shall be graded to form a slope away from pumps, compressors or other equipments. The height of the enclosure wall shall be thirty centimetres on the upper side and gradually increasing to maximum sixty centimetres on the lower side, at the end of which a shallow sump for collection of the spilled liquid, if any, shall be provided. The minimum separation distance between the vessel and the sump shall be,-(a)diameter of the vessel, in case of vessels with water capacity not exceeding forty thousand litres,(b)fifteen metres, if the water capacity of the vessels exceeds forty thousand litres.]
