Each boiler and unfired pressure vessel used or proposed to be used within this state, except boilers or unfired pressure vessels exempt in RCW 70.79.080 and 70.79.090, shall be thoroughly inspected as to their construction, installation, condition and operation, as follows:

(1) Power boilers shall be inspected annually both internally and externally while not under pressure, except that the board may provide for longer periods between inspections where the contents, history, or operation of the power boiler or the material of which it is constructed warrant special consideration. Power boilers shall also be inspected annually externally while under pressure if possible;

(2) Low pressure heating boilers shall be inspected both internally and externally biennially where construction will permit, except that the board may, in its discretion, provide for longer periods between internal inspections;

(3) Unfired pressure vessels subject to internal corrosion shall be inspected both internally and externally biennially where construction will permit, except that the board may, in its discretion, provide for longer periods between internal inspections;

(4) Unfired pressure vessels not subject to internal corrosion shall be inspected externally at intervals set by the board, but internal inspections shall not be required of unfired pressure vessels, the contents of which are known to be noncorrosive to the material of which the shell, head, or fittings are constructed, either from the chemical composition of the contents or from evidence that the contents are adequately treated with a corrosion inhibitor, provided that such vessels are constructed in accordance with the rules and regulations of the board or in accordance with standards substantially equivalent to the rules and regulations of the board, in effect at the time of manufacture.