Section 6. All steam boilers and their appurtenances except those specified in the following section shall be thoroughly inspected externally and internally at least once a year. Upon written application made to it by the owner or user of a pressure vessel or boiler, the board may, when the public interest and convenience require, extend the time for the making of such inspection for a period not to exceed six months as the board may determine. A district engineering inspector of the division of inspection of the department of public safety shall perform the first inspection of a pressure vessel required by this chapter.