ASME SECTION I – BOILER REQUIREMENTS
PG60.1 : All boilers having a fixed water level (steam and water interface) shall have at least one gauge glass (a transparent device that permits visual determination of the water level).
PG60.1.1: Boilers having a maximum allowable working pressure exceeding 400psi (3Mpa) shall have two gauge glasses. Instead of one of the two required gauge glasses, two independent remote water level indicators (two discrete systems that continuously measure, transmit and display water level) may be provided.
PG220.127.116.11: When the water level in at least one gauge glass is not readily visible to the operator in the area where control actions are initiated, either a fibre optic cable (with no electrical modification of the optical signal) or mirrors shall be provided to transfer the optical image of the water level to the control area. Alternatively, any combination of two of the following shall be provided:
(a) an independent remote water level indicator
(b) an independent continuous transmission and display of an image of the water level in a gauge glass
PG18.104.22.168 : When two independent remote water level indicators are in reliable operation (continuously indicating water level), the one required gauge glass may be shut-off but shall be maintained in the serviceable condition.