(7)In-service test and examinations - (a) Every pressure vessel or plant in service shall be thoroughly examined by a competent person-(i)externally, once in every period of six months;(ii)internally, once in every period of twelve months :Provided that if by reason of the construction of a pressure vessel or plant, a thorough internal examination is not possible, this examination may by replaced by a hydrostatic test which shall be carried out once in every period of two years:Provided further that for a pressure vessel or plant in continuous process which cannot be frequently opened, the period of internal examination may be extended to four years; and(iii)hydrostatically tested once in every period of four years :Provided, that in respect of a pressure vessel or plant with thin walls such as sizing cylinder made of copper or any other non-ferrous metal, periodic hydrostatic test may be dispensed with subject to the condition that the requirements laid down in sub-rule (8) are fulfilled:Provided further that when it is impracticable to carry out thorough external examination of any pressure vessel or plant every six months as required in sub-clause (i) or if owing to its construction and use a pressure vessel or plant cannot be hydrostatically tested as required in subclauses (ii) and (iii) a thorough external examination of the pressure vessel or plant shall be carried out at least once in every period of two years and at least once in every period of four years a thorough systematic nondestructive test like ultrasonic test for metal thickness or other defects of all parts of failure of which might lead to eventual rupture of the pressure vessel or plant shall be carried out.(b)The pressure for the hydrostatic test to be carried out for the purpose of this sub-rule shall be 1.25 times the design pressure for 1.5 times the maximum permissible working pressure, whichever is less.
