Section 18(2)(xiv) in The Static and Mobile Pressure Vessels (Unfired) Rules, 2016
(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.