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Glass Selection for Process Industrial Applications
By: Harvey Travers, Engineering Consultant
There are two types of glass commonly used in industrial applications: soda lime glass and borosilicate glass. The two have distinctly different chemical and physical characteristics. Soda lime glass is the most common, representing some 90% of all glass produced and used in the U.S. The majority of it is used for common glass products such as window panes, household glassware and glass-wool insulation. In industrial applications, soda lime glass is useful in non-critical processes and liquid storage. Borosilicate glass is more costly to produce and much more limited in application, being used for a few consumer products and extensively for critical industrial applications in such areas as pharmaceuticals, chemical processing and food processing.
Soda Lime Glass
Soda lime glass has been with us for millennia, having been discovered by the ancient Egyptians and, in various formulations, used extensively through the centuries for a wide variety of needs. It is inexpensive to produce since common sand, silica, is the primary basic ingredient. Other ingredients in modern glass formulation include soda, lime, magnesia and alumina. It is easily formed, reasonably transparent despite a bluish tint and heat resistant within limits. Tempering, also known as heat treating, significantly improves its surface hardness.
In industry, it finds applications wherever requirements do not involve temperature exposure over 150° C (302°F), significant resistance to corrosion or erosion, and high resistance to impact or stress. It is offered under a variety of trade names, some perhaps implying a boron content, but, in general, any industrial glass product is most likely soda lime glass unless it is specifically identified as borosilicate glass by name.
Borosilicate Glass
Borosilicate glass is a relative newcomer to the field, having been developed in the 1893 when a German scientist discovered that adding boron salt to the formula greatly improves the resulting glass’ resistance to temperature shock, chemical corrosion and physical erosion. It is commonly recognized under the Corning Inc. trade name “Pyrex” and known especially for its high temperature capabilities and its resistance to thermal shock in bakeware and coffee makers, among other kitchen implements.
In industrial applications its use is much broader. Virtually all laboratory glassware is now borosilicate, as is much process industry tubing, sight ports, glass linings and other components where the heat resistance or purity of the product is important.
It is often used as the transparent element of a prestressed glass viewport in which a borosilicate disc is fused inside a steel ring. As the two elements cool they become bonded together by virtue of the differing coefficients of thermal expansion, that of the glass being less than the steel. The result is a permanently fused viewport in which the glass is held under very large radial stress making it much more resistant to pressure.
Transparency is also a plus for Borosilicate Glass with 90% or more transmission from the low UV range through visible light and well into the IR range.
Which Glass is Right?
In most instances the temperature to which a glass element is exposed is the primary selection key to be considered. Standard DIN 7080 rates tempered borosilicate glass to 280°C (536°F), while annealed borosilicate is rated to 232°C (450°F) which is sufficient for most process industry applications. For even more demanding applications such as furnaces and ovens the requirement would be quartz which is rated at 1000°C (1832°F)!
Soda lime glass is more limited. DIN 8902 rates tempered soda lime glass for applications to 150°C (302°F).
The next major characteristic in glass selection is resistance to chemical corrosion. Contrary to what one might expect from casual observation, glass is not chemically inert. It reacts to a variety of chemicals, including water. In a series of tests reported in a report, “VGB Kraftworksstechnik” in 1979, Dr. Peters of (??????) reported that borosilicate glass is 10 times more resistant to chemical degradation than soda lime glass. The difference is significant at slightly acidic conditions (pH 6) and even more marked when the value was highly alkaline (pH 10).
Soda lime glass is not suitable for parenteral (injectable) pharmaceuticals because the glass could leach into the process material. In contrast, the USP standard allows the use of borosilicate glass in parenteral applications, and gives borosilicate glass a Type I classification, its highest grade.
Another determining factor, sometimes the determining factor, is the matter of strength, most often measured as the ability of a glass element to resist pressure without failing, and the resistance to thermal shock. This is all the more important since when glass fails it most often fails catastrophically, shattering as does a water glass when dropped on a tile floor.
Because borosilicate glass has a lower coefficient of expansion than soda lime glass, its use in sight glass construction creates higher levels of compression inside the metal ring. A recent paper by Karl Schuler concludes that 2.8 times higher compression raises the maximum operating pressure of a sight glass 4.5 times. Thus borosilicate sight glasses are stronger by orders of magnitude than sight glasses made with ordinary soda lime glass. Strength translates into more reliable, safer operation, as well as resistance to scratching during handling.
When selecting a sight glass for an industrial process application, the type of glass is an important consideration. When heat and pressure are factors, when resistance to chemical corrosion is needed, and when worker safety is a concern, sight glasses made with borosilicate glass are usually the better choice.
By: Harvey Travers, Engineering Consultant
There are two types of glass commonly used in industrial applications: soda lime glass and borosilicate glass. The two have distinctly different chemical and physical characteristics. Soda lime glass is the most common, representing some 90% of all glass produced and used in the U.S. The majority of it is used for common glass products such as window panes, household glassware and glass-wool insulation. In industrial applications, soda lime glass is useful in non-critical processes and liquid storage. Borosilicate glass is more costly to produce and much more limited in application, being used for a few consumer products and extensively for critical industrial applications in such areas as pharmaceuticals, chemical processing and food processing.
Soda Lime Glass
Soda lime glass has been with us for millennia, having been discovered by the ancient Egyptians and, in various formulations, used extensively through the centuries for a wide variety of needs. It is inexpensive to produce since common sand, silica, is the primary basic ingredient. Other ingredients in modern glass formulation include soda, lime, magnesia and alumina. It is easily formed, reasonably transparent despite a bluish tint and heat resistant within limits. Tempering, also known as heat treating, significantly improves its surface hardness.
In industry, it finds applications wherever requirements do not involve temperature exposure over 150° C (302°F), significant resistance to corrosion or erosion, and high resistance to impact or stress. It is offered under a variety of trade names, some perhaps implying a boron content, but, in general, any industrial glass product is most likely soda lime glass unless it is specifically identified as borosilicate glass by name.
Borosilicate Glass
Borosilicate glass is a relative newcomer to the field, having been developed in the 1893 when a German scientist discovered that adding boron salt to the formula greatly improves the resulting glass’ resistance to temperature shock, chemical corrosion and physical erosion. It is commonly recognized under the Corning Inc. trade name “Pyrex” and known especially for its high temperature capabilities and its resistance to thermal shock in bakeware and coffee makers, among other kitchen implements.
In industrial applications its use is much broader. Virtually all laboratory glassware is now borosilicate, as is much process industry tubing, sight ports, glass linings and other components where the heat resistance or purity of the product is important.
It is often used as the transparent element of a prestressed glass viewport in which a borosilicate disc is fused inside a steel ring. As the two elements cool they become bonded together by virtue of the differing coefficients of thermal expansion, that of the glass being less than the steel. The result is a permanently fused viewport in which the glass is held under very large radial stress making it much more resistant to pressure.
Transparency is also a plus for Borosilicate Glass with 90% or more transmission from the low UV range through visible light and well into the IR range.
Which Glass is Right?
In most instances the temperature to which a glass element is exposed is the primary selection key to be considered. Standard DIN 7080 rates tempered borosilicate glass to 280°C (536°F), while annealed borosilicate is rated to 232°C (450°F) which is sufficient for most process industry applications. For even more demanding applications such as furnaces and ovens the requirement would be quartz which is rated at 1000°C (1832°F)!
Soda lime glass is more limited. DIN 8902 rates tempered soda lime glass for applications to 150°C (302°F).
The next major characteristic in glass selection is resistance to chemical corrosion. Contrary to what one might expect from casual observation, glass is not chemically inert. It reacts to a variety of chemicals, including water. In a series of tests reported in a report, “VGB Kraftworksstechnik” in 1979, Dr. Peters of (??????) reported that borosilicate glass is 10 times more resistant to chemical degradation than soda lime glass. The difference is significant at slightly acidic conditions (pH 6) and even more marked when the value was highly alkaline (pH 10).
Soda lime glass is not suitable for parenteral (injectable) pharmaceuticals because the glass could leach into the process material. In contrast, the USP standard allows the use of borosilicate glass in parenteral applications, and gives borosilicate glass a Type I classification, its highest grade.
Another determining factor, sometimes the determining factor, is the matter of strength, most often measured as the ability of a glass element to resist pressure without failing, and the resistance to thermal shock. This is all the more important since when glass fails it most often fails catastrophically, shattering as does a water glass when dropped on a tile floor.
Because borosilicate glass has a lower coefficient of expansion than soda lime glass, its use in sight glass construction creates higher levels of compression inside the metal ring. A recent paper by Karl Schuler concludes that 2.8 times higher compression raises the maximum operating pressure of a sight glass 4.5 times. Thus borosilicate sight glasses are stronger by orders of magnitude than sight glasses made with ordinary soda lime glass. Strength translates into more reliable, safer operation, as well as resistance to scratching during handling.
When selecting a sight glass for an industrial process application, the type of glass is an important consideration. When heat and pressure are factors, when resistance to chemical corrosion is needed, and when worker safety is a concern, sight glasses made with borosilicate glass are usually the better choice.
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