Water gauge glasses

1. Each steam boiler must have one or more water gauge glasses attached to the water column or boiler by means of valved fittings not less than one-half inch [12.70 millimeter] pipe size, with the lower fitting provided with a drain valve of the straight type with opening not less than one-quarter inch [6.35 millimeters] diameter to facilitate cleaning. Gauge glass replacement must be possible under pressure. Water glass fittings may be attached directly to a boiler. 2. The lowest visible part of the water gauge glass must be at least one inch [25.4 millimeters] above the lowest permissible water level recommended by the boiler manufacturer. With the boiler operating at this lowest permissible water level, there must be no danger of overheating any part of the boiler. Transparent material other than glass may be used for the water gauge provided that the material will remain transparent and has proved suitable for the pressure, temperature, and corrosive conditions expected in service.