Section 18(2) in The Static and Mobile Pressure Vessels (Unfired) Rules, 2016
(2)Pressure relief-(i)every vessel shall be provided with two or more pressure relieving devices in accordance with the provisions of the Design Code referred to in rule 13;(ia)for cryogenic pressure vessels, the outer vessel shall be provided with a vacuum valve and safety relief device (disc) to release internal pressure. The discharge area of such device shall be at least 0.34 sq.mm/litre of water capacity of the inner vessel but not exceeding 5000 mm2. The relief device shall function at a pressure not exceeding the internal design pressure of the outer vessel;(ii)the relief valves shall be spring loaded and shall be set-to-discharge and reach full flow conditions as required by the Design code referred to in rule 13;(iii)weight loaded relief valves shall not be permitted;(iv)the relief valves be so designed that they cannot be inadvertently loaded beyond the set pressure;(v)the design of the valves shall be such that the breakage of any part will not obstruct free discharge of the liquid under pressure;(vi)safety relief valves on any vessel shall be set to start-to-discharge at a pressure not in excess of 110 percent of the design pressure or maximum working pressure of the vessel and shall have a total relieving capacity sufficient to prevent the maximum pressure in the vessel of more than 120 percent of the design or maximum working pressure;(via)in case of cryogenic pressure vessels, the safety relief devices shall be set to discharge in such a manner that at least one such device shall be set at a pressure not higher than the maximum allowable working pressure and the other device may be set at a pressure not higher than 110 percent of the maximum allowable working pressure."(vii)each safety relief valve shall be plainly and permanently marked with the pressure in Kg/Cm2 at which it is set to discharge, with the actual rate of discharge of the device in cubic metres per minute of the gas at 150 C and at atmospheric pressure, and with manufacturer's name. The rated discharge capacity of the device shall be determined at a pressure of 120 percent of the design pressure or maximum working pressure of the vessel;(viii)connections of safety relief devices shall be of sufficient size to provide the required rate of discharge through the safety relief valves;(ix)safety relief valves shall be so arranged that the possibility of tampering is minimised and if the pressure setting or adjustment is external, the safety relief valve shall be provided with suitable means of sealing adjustment;(x)each safety relief valve for static vessel shall be provided with angle type approved safety valve with shut-off valve between it and the vessel. The arrangement of the shut-off valve installed between the safety relief valve and the vessel shall be so designed as to afford full required capacity flow through at least one of the safety relief valves. In case of cryogenic vessel the safety relief valve may be provided with multi-way flow diverter valve;(xi)safety relief valves shall have direct communication with the vapour space of the vessel;(xii)for static vessels for storage of flammable, toxic or corrosive gases, relief valves shall be fitted with extended vent pipes adequately supported and having outlets at least 2 metres above the top of the vessel and at least 3.5 metres above the ground level and the vent pipes shall be fitted with loose-fitting rain caps;(xiii)relief valves shall be tested by a Competent Person for correct operation at least once in a year and a record of such test shall be maintained. The test certificate shall be issued in the prescribed proforma;(xiv)Safety relief valves of road tankers carrying flammable, toxic or corrosive gases shall not have isolation valve between the vapour space and the relief valve. Such relief valves shall be provided in a recessed cup formation with suitable rain caps. The relief valves of such tankers shall be replaced with new valves every ten years or whenever found defective during operation or periodic testing, whichever is earlier.
