Sight glasses are used in industries ranging from refineries to craft breweries. Perhaps because of their widely-varied applications, sight glasses are called by different names. Broadly speaking, a sight glass is any kind of window on a process vessel or pipeline. However, there are other terms with more specific definitions. This blog post will explain them.
This is a glass disc that mounts between gaskets and then is secured by a retaining ring. This is the older style, traditional sight glass. Because the glass is exposed to mounting forces, it could have imperceptible cracks that could lead to failure. After the glass disc is removed from service, such as for cleaning, it should be replaced.
I think of a sight glass window (or “sight window”) as glass fused to a metal frame to create a single-piece component. This is the way to make the glass stronger, because the metal frame places the glass under compression. (Like cement, glass is weak under tension, but amazingly strong under compression.) The metal frame also takes the stress of mounting, making the glass immune to the forces of uneven bolting and gasket residue. Therefore, sight windows are re-usable.
This is a term that distinguishes a sight glass used for viewing from a sight glass used for lighting. Often two sight glasses are installed on a tank: one on the top is dedicated to a light (called a “light port”) and one on the side is dedicated to viewing (a “view port”). As a cost-savings measure, some sight glasses are combined with a light at the same location. I have heard people call these “combos,” although I don’t know if that’s an official term.
It is not uncommon to encounter the term “sight port” used as a synonym for “view port” and even as a general term interchangeable with “sight glass.”
Pipelines are different from tanks and vessels in that the sight glass is used to observe flow. Commonly the sight flow indicator is an assembly consisting of two opposed sight glasses mounted to a housing. This arrangement allows ambient light to illuminate the flow from behind. There also versions using a tubular glass that admits more light and permits viewing from any angle. Another name for sight flow indicator is “visual flow indicator.” That’s my personal preference; the two names mean exactly the same thing.
Often tanks will have a vertical sight glass assembly used to observe the level of fluid inside the tank. Sometimes the glass has ridges in it that make it easier to see the line of fluid.
Boilers used in commercial and residential heating often have a tubular glass that makes it easy to observe water level. I’ve even seen this style of glass on an old-fashioned locomotive.
Lots of sight glasses, often made of plastic, are used in boats, lawn mowers, home brewing kits, etc. These are low-pressure, non-critical, low-cost components that are very different from the highly-engineered sight glasses used in industry.
That’s not to say that industrial sight glasses are not made of plastic. In many applications, such as the food and beverage industry, plastic sight glasses are widely used. The same definitions apply as above. Note that plastic sight glasses are never used with harsh media, high-pressure, or high-temperature processes.
Get more information about sight glasses and sight flow indicators from our guide: “Step-by-Step Guide to Sight Glass Selection.”