Fused sight glass is sometimes used to promote some brands of sight glasses based on the belief that greater fusion of glass to metal during the manufacturing process will produce a stronger finished sight glass for use in chemical and pharmaceutical processes.
We have recently learned that inferior gauge glass, with a historically common industry name, from an overseas supplier has entered the North American market and does not meet accepted standards of safety and performance.
While inherently resistant to corrosion, sight glasses can begin to corrode under the right circumstances, particularly if the wrong material is spec’d for the application. If a sight glass happens to corrode, it becomes increasingly vulnerable to sight glass failure. When glass fails, it is often sudden and catastrophic, damaging your system, putting workers at risk, and causing unplanned downtime. That’s why it’s important to understand the dangers of sight glass corrosion and failure so that you can adequately prepare for them… and avoid a costly disaster.
A sight glass seems pretty straightforward: a fitting with a window installed on the top or side of a vessel that you can look through and see what’s going on. Simple, right?
Not so fast. There is a wrong way to do anything, and choosing the wrong type of glass for a sight glass can have bad
“Learn the Secret of Sight Glass Strength” is a recent video from L.J. Star that outlines the precision process of fusing borosilicate glass to a stainless steel ring, then cooling it carefully to create a METAGLAS® sight glass window. Once the process is complete, the ring compresses the glass continuously, which gives it its exceptional tensile strength, preventing dangerous ruptures or fragmentation for greater reliability and safety. By combining the transparency of glass with the strength of steel, METAGLAS virtually eliminates the possibility of a sudden, unexpected rupture, and any crack that might develop, at worst, may slightly affect visibility. In addition to the unsurpassed pressure and impact resistance, the metal frame provides rigidity to prevent breakage if bolts are tightened unevenly during installation. METAGLAS products have been tested and approved by safety testing departments of several major chemical and other companies under a variety of conditions, most exceeding those normally encountered in practice.
For a process engineer, being smooth means more than knowing which wine to order in a fancy restaurant. In plant operations, the surface finish of a process vessel, piping, and related components can have profound effects on how well a fluid system performs. Surface roughness can affect fluid flow resistance (friction), adsorption/ desorption, bacteria growth, the build-up of chemicals from a process fluid, corrosion formation, pressure drop, etc. Ultimately, the smoothness of a surface finish can affect service life and maintenance costs. In a sight glass, for example, the surface roughness of both the glass and the metal mounting ring are critical for achieving a good seal in the installation.
An industrial sight glass is a critical part of virtually any type of process equipment because it allows for visual inspection of industrial and pharmaceutical processes. Luminaires are often used in conjunction with an industrial sight glass to illuminate tank and pipe interiors.
In a system made primarily of metal, like a pressure vessel, the weak spots are generally sealing joints and glass. However, an industrial sight glass is a highly engineered product. Although different brands of industrial sight glass might look alike, differences in their specs have tremendous impact on worker safety, sterile processes, and maintenance costs. In industrial applications, sight glasses are often subject to extremes of pressure, temperature, thermal shock, caustics, abrasion, or impact. Choosing the right industrial sight glass depends on taking all of these considerations into account.
Is your sight glass tough enough to handle “The Big Six” challenges?
Sight glass failures can be extremely dangerous. A sight glass that fails catastrophically has the potential for severe operator injury or death, as well as costly downtime and product losses. Even minor cracks, scratches or abrasions can create weakness within the sight glass, which will most likely lead to failure. In normal use, sight glasses are regularly subjected to forces involving extremes of pressure, temperature, thermal shock, caustics, abrasion, or impact.