ABSTRACT
Sight flow indicators are used when a visual inspection of the process is necessary. The flapper design is used in transparent or slighdy opaque solutions and in gas services. The glass is normally a borosilicate which can be rated to 450 F (230 C) for sight glass applications and has good resistance to mechanical and thermal shock. The glasses can also be made of silica glass or quartz, thus enabling temperature ratings in excess of 1000 F (535 C). Tempered glass is not recommended for fluorine, hydrofluoric acid, or phosphoric anhydride service, because the erosion causes uneven stresses and eventual failure. In high-temperature services the gradient across each glass is reduced. The outer glass protects the inner glass from thermal shock caused by splashes of cold water. Special designs are available that can be used in sanitary services such as food processing.
