Section 19(1)(a) in The Static and Mobile Pressure Vessels (Unfired) Rules, 2016
(a)Except specifically otherwise provided in this rule, all vessels shall be hydraulically tested by a competent person at a pressure marked on the vessel at intervals of not more than five years after the date of first test, provided that in the case of vessels, containing corrosive or toxic gases, the periodic test shall be done at an interval of two years. Internal inspection along with residual thickness measurement shall be carried out during the periodic testing.