Section 61(1)(d) in The Punjab Factory Rules, 1952
(d)thoroughly examined by a competent person -(i)externally, once in every period of six months, to ensure general condition of the vessel and the working of its fittings;(ii)internally, once in every period of twelve months, to ensure condition of the walls, seams and dies both inside and outside of the vessels, soundness of the Parts of the vessel, and the effects of corrosion. If by reason of construction of the vessel thorough internal examination is not possible, this examination may be replaced by a hydraulic test which shall be carried out once in every two years; provided that for the vessels in continuous processes which cannot be frequently opened, the period of internal examination may be extended to four years; and(iii)hydraulically tested at intervals of not more than four years; provided that in respect of pressure vessels with thin walls such as sizing cylinders made of copper or any other non-ferrous metal periodic hydraulic test may be dispensed with on the condition that the requirements laid down in clause (2) are fulfilled: