Glass

Table of Contents

What is Glass?

There are two types of glass which are most commonly used in engineering applications due to their mechanical and chemical properties. Tempered glass is glass that has been heated to very high temperatures and then cooled quickly in a process called "quenching". This gives tempered glass much better mechanical properties such that tempered glass is around 4 times stronger than regular glass [2]. Tempered glass has excellent impact resistance and can also maintain its mechanical properties at high temperatures, up to around 250°C [1]. Tempered glass will not shatter into large shards but instead break into tiny pieces which do not fly around everywhere and cause damage. Borosilicate glass is another type of glass which can be treated with heat to improve its strength. Borosilicate glass is more common due to better chemical and mechanical properties compared to tempered glass, but cannot be tempered due to its lower coefficient of expansion.

Applications of Tempered Glass in Piping

Tempered glass is used sparingly in piping applications. It is sometimes used as a sight glass, but borosilicate is generally preferred. Tempered glass is common outside of piping, like in windows and shower doors, where transparency and structural strength are required. 

Applications of Borosilicate Glass in Piping

Borosilicate glass is commonly used as a sight glass in other types of pipes. A sight glass is a place in a pipe where part of the regular piping material is removed and replaced with a material to look through. This can be used to monitor what is happening inside the pipe. Heat strengthened borosilicate glass is most commonly used for this purpose. It is transparent, very strong, heat-resistant and resistant to chemical attack. Heat strengthened borosilicate has an incredible tensile strength of 64 GPa and can withstand temperatures up to 500°C. Borosilicate is also commonly used in lab glassware due to its high chemical resistance [3].

Figure 1: Borosilicate sight glass for a pipe [6]	Figure 2: Borosilicate lab glassware [7]

Advantages and Disadvantages of Tempered Glass 

One of the advantages of tempered glass compared to annealed glass, is that it breaks into smaller fragments which do not cause as much damage when hitting someone or something. Tempered glass is able to withstand very high pressures and maintain its physical and chemical properties. Tempered glass is much stronger mechanically than annealed glass but not quite as strong as borosilicate. After tempering glass, you cannot re-work or re-cut any of the glass, which means it must be correctly shaped and sized before and during the tempering process. If there are any defects or damage on the edges of the glass, then it will fail and break completely if exposed to any stress [8].

Advantages and Disadvantages of Borosilicate Glass

Borosilicate has very low thermal expansion which allows it to keep its shape in high temperatures. It also retains very high strength at increased temperatures, which makes it an ideal choice for being cool and then quickly heated multiple times. The mechanical strength is much higher in borosilicate glass compared to normal or even tempered glass. It can have up to 64 GPa tensile strength. Borosilicate is also corrosion resistant which makes it perfect to use in chemical lab settings, or chemical transportation. The downside of all these benefits is its higher cost compared to regular glass. Though borosilicate is much stronger than regular glass with much lower thermal expansion, it still has the potential to crack if placed under extreme stress or very fast temperature changes [9].