For chemical, biotech, and pharmaceutical processing applications, it is important to consider the effect of chemicals on the sight glass over time. Glass is not chemically inert, and will react even with water. Glass can suffer chemical degradation from caustic or acidic substances and suffer pitting from mild abrasion. These conditions may dangerously weaken glass, obscure its transparency, and possibly create crevices where bacteria can hide or leach into the process media. Borosilicate glass is significantly more resistant to chemicals than soda lime glass, as shown in the following chart.
In investigating for chemically induced disintegration of Soda Lime and Borosilicate Glass, it was found that Borosilicate was much more resistant. This occurred not only at slightly acidic conditions (pH 6), but also when the pH was raised to a value of 10 (alkaline). As shown on the chart, the degradation of Soda Lime glass is 10 times greater than that of Borosilicate glass. This considerable divergence in resistance properties begins at 134°C, the initial temperature in the study.
Translated from “VGB KRAFTTWORKSTECHNIK,” Dr. A. Peters, Feb. 1979.
Independent Statements Regarding Use of Borosilcate Glass
There is a wealth of citations in literature that strongly support the use of borosilicate glass in sight glasses. Following are a few:
“Borosilicate glasses have the widest use for chemical equipment. Because of low thermal expansion, these withstand higher operating temperatures and show greater resistance to thermal shock than soda-lime glasses. They also have excellent chemical durability.”
Glass Engineering Handbook
McGraw-Hill Book Company, 1984
“Soda-Lime glass (Herculite®) is relatively inexpensive but has the poorest thermal-shock and thermal-stress resistance.”
Selecting Sight Flow Indicators
Chemical Engineering, July 4, 1977
“Resistance to thermal shock generally decreases as the coefficient of thermal expansion increases.”
Kopp Glass Inc.
Color Filter Glasses, February 1991
“Lime glasses are low in cost, easily hot worked and are usually specified for service where high heat resistance and chemical stability are not required.”
Properties of Selected Commercial Glasses
Corning Glass Works
“Soda-Lime glass is the most common (90% of glass made), and least expensive form of glass. It usually contains 60–75% silica, 12–18% soda, 5–12% lime. Resistance to high temperatures and sudden changes of temperature are not good and resistance to corrosive chemicals is only fair.”
Corning Museum of Glass
“Because of its high thermal expansion (three-fourths of steel), lime glass in its annealed state does not have the thermal shock resistance required for chemical process equipment.”
Installation and Maintenance of Glass Equipment
Chemical Engineering, April 26, 1965
“Where fracture due to thermal stressing cannot be handled with safety toughened glass, borosilicate glass rather than soda-lime glass might be considered.”
CBD.60 Characteristics of Window Glass
Canadian Building Digest
“Soda-Lime glasses are not very chemically durable and are subject to fracturing from quick thermal changes.”
Glass Alchemy, Ltd. Glossary