(7)In service test and examinations. Every pressure vessel or plant in service shall be thoroughly examined by a competent person(a)externally, once in every period of six months;(b)internally, once in every period of twelve months;If by reason of the construction of a pressure vessel or plant, a thorough internal examination is not possible this examination may be replaced by a hydrostatic test which shall be carried out once in every period of two years:Provided 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;(c)hydrostatically tested once in every period of four years: and(d)The hydrostatic pressure to be carried out for the purpose of this rule shall be 1.25 times the design pressure or 1.5 times the maximum permissible working pressure whichever is less:Provided that in respect of a pressure vessel or plant with thin walls, such as ceasing cylinder made of copper or any other nonferrous 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 clause (a) of this sub-rule, or if owing to its construction and use a pressure vessel or plant cannot be hydrostatically tested as required in clauses (b) and (c) of this sub-rule, 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 non-destructive test like ultrasonic test for metal thickness or other defects of all parts, the failure of which might lead to eventual rupture of the pressure vessel or plant shall be carried out.
