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.
I need a sight glass. What type of glass do you recommend?
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.
No Metaglas window has ever been known to fail in service, even those subjected to significant overpressure and/or temperature surges. That high strength in extreme conditions has made Metaglas is the top selling fused sight glass, proven in thousands of installations around the world. Unlike some other sight glasses, it meets stringent DIN 7079 and DIN 7080 quality standards, and has been tested and proven to meet the USP Type 1 standard. To get all the details, watch the video at: http://www.sightglassblog.com/blog/08.html.
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.
Increasingly stringent specifications are creating greater demand for improved surface finish on most metal components that are part of process equipment. In particular, the pharmaceutical and biotechnology industries require surfaces in contact with the process media to have finishes that meet the ASME-BPE-2009 Standard. This standard provides specifications for the design, manufacture and acceptance of vessels, piping and related components for application in equipment used by the bioprocessing, pharmaceutical, and personal care product industries. This standard includes aspects related to sterility and cleanability, materials, dimensions and tolerances, surface finish, material joining, and seals.
To learn about techniques commonly used to reduce the surface roughness of metals used in fluid vessels, piping, and related components, request your free copy of the L.J. Star white paper, Surface Finish: An Important Characteristic in Fluid System Components.
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.
An industrial sight glass failure has the potential to be extremely dangerous. Even minor cracks, scratches or abrasions of the glass can be a source of weakness within the glass, which will most likely lead to failure. A catastrophic failure can cause severe operator injury or death, as well as costly downtime. Typically, the failure of an industrial sight glass on a piece of equipment or within a piping system will halt the whole process until the equipment can be repaired or replaced. Moreover, this failure may lead to scrapping the process media. In the case of a pharmaceutical process, the product loss could cost a manufacturer millions of dollars. Could you afford that kind of loss? (inside voice answring “NO”) We thought so, too.
That said, we would like to invite you to learn more about choosing the right industrial site glass for a particular environment. You can do so by downloading our free white paper, 6 Tips for Critical Sight Glass Applications.
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.
Our “6 Tips for Critical Sight Glass Applications” white paper provides expert advice on how to select a sight glass designed to withstand these challenges:
- Temperature: Depending upon the temperature range, certain glass types will perform better than others. At less than 300°F, standard Soda Lime glass may be used. For temperatures up to 500°F, borosilicate glass may be used. At higher temperatures such as in high temperature steam applications, we recommend quartz or sapphire glass.
- Thermal Shock: Some glass types are particularly vulnerable to cracking as a result of rapid temperature change, due to their low toughness, low thermal conductivity, and high thermal expansion coefficients.
- Corrosion: Materials in process media like hydrofluoric acid, hot phosphoric acid and hot alkalis can etch the glass, producing a cloudy view with weakened integrity that requires the sight glass to be replaced.
- Abrasion: Fluids that contain granular particles in suspension, or with particles carried in process gasses can abrade and erode a site glass, limiting visibility and affecting its strength.
- Pressure: The glass materials selected, the unsupported diameter, and the glass thickness all play a role in determining the pressure capabilities of a sight glass assembly. Fused sight glass windows offer high pressure ratings and high safety margins. The strongest fused sight glasses are made from duplex stainless steel and borosilicate glass; this combination creates the highest compression.
- Impact: Objects that strike the sight glass are seldom sufficient to cause immediate failure, but they can create scratches or gouges that provide a point for tensional force to concentrate. Scratched sight glasses should be replaced immediately.
For details on how to take on these challenges, download our free copy of the “6 Tips for Critical Sight Glass Applications” white paper.
“Fusion” is fine for fancy restaurants, but “compression” makes the real difference in sight glasses.
“Fusion” is sometimes used to describe cuisines that combine ingredients and cooking techniques from different cultures. However, fusion is also 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.
During manufacturing, the glass is melted within the metal ring as the ring expands. Then, the temperature is raised to the point where the glass and the metal ring fuse together. When the unit cools, the glass hardens before the metal ring shrinks back to its original size. This places the metal ring in tension and the glass in uniform radial compression. The most compression is produced by using an alloy of metal that shrinks a lot as it cools in combination with a type of glass that shrinks little when it cools: the greater the difference, the greater the compression. Fusion – getting the glass to stick to the metal – is easy to achieve but not as critical to sight glass reliability as compression. Compression strengthens the glass because it is stronger than tensile forces that could create internal torque on the glass. Therefore, it is the level of compression, rather than fusion, that predicts the reliability of a sight glass.
A recent study shows that the combination of glass and metal that provides the most compression also provides the highest strength. The study compared a sight glass made of soda-lime glass and a Duplex stainless steel frame with a borosilicate sight glass (also with a Duplex stainless steel frame) to analyze the amount of radial compression created by the difference between the glass and the metal in coefficient of thermal expansion as the sight glasses cool. The analysis showed the borosilicate glass produces significantly more compression than one made with soda lime glass. Also, there is a direct relationship between the amount of compression and the strength of the sight glass. The borosilicate sight glass has far greater pressure capability. Strength is also important for worker safety and because it reduces the need for sight glass maintenance and replacement.
The study also compared the compressive force of a sight glass made with proprietary soda-lime glass coupled with a Hastelloy C22 stainless steel ring (which is touted for its high glass-to-metal fusion) with that of a Duplex/borosilicate sight glass. The borosilicate glass/Duplex stainless steel construction provided four and a half times the pressure capability of the soda-lime/Hastelloy sight glass. The reliability of a sight glass has almost nothing to do with fusion; compression is what matters most because it secures the glass to the metal and creates a leak proof seal. Click here to see a full line of borosilicate sight glass products.
For all the details on this study, take a few minutes to read the white paper on compression vs. fusion.