What Is The Purpose Of A Sight Glass?

The purpose of a sight glass is to allow plant managers and engineers to look inside of their systems without disrupting the environment within. Often referred to as viewing glasses, they offer you a way to see the levels of liquids or gases that are traveling through a critical point of your design, monitor pressure changes, and virtually anything else that sight glass windows could allow you to do. With their widespread use, sight glasses have become critical to operations such as steam pipelines, chemical processing, pharmaceutical processing, and food processing.

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Often simple in design, rudimentary sight glasses of the past consisted of a glass tube that was linked to an important part of your system, like a tank or a gas inlet. Modern industrial sight glasses are made from a glass disk that's securely held by bolts and gaskets between two metal frames, with the goal of maintaining the standards of sanitary fittings. The glass that's used is typically soda lime glass or borosilicate glass, while stainless steel is the go-to choice for the metal due to its superior strength and abrasion resistance, even with 304 steel.

It's necessary for a sight glass to be able to withstand whatever materials and fluids it will come into contact with so that the glass does not tarnish or degrade, impairing your ability to use it for system monitoring. Assuming it's kept in good condition, using a sight glass is as user-friendly as it is essential. When looking into a sight glass, the level of fluid present against the glass represents the level of fluid that you have at that point of your system. Bubbles against the glass show you that you have a liquid/vapor mix. If you’re looking at clear glass while your system is running, then that means that you have a full line of liquid (or complete lack there of). Patterned backplates or striped inserts make it easier to see the fluid levels and check them against desired outcomes. 

This is especially important for applications like steam boilers. With a steam boiler, you need to keep boiler water levels within a certain range to maintain operations and prevent system failure. Luckily, since the pressure of the water below and the steam above is equal, you can use a sight glass to see any change in the boiler's water level. A steam boiler's sight glass gauge usually has two different index marks: a low-water threshold and a high-water threshold. Go too far above the top mark and you risk overfilling the boiler, which can prevent steam from rising up into your system. Go too far below the lower threshold without shutting your boiler down and you risk damaging your equipment from the heat, which could also lead to a dangerous fire or explosion.