Compression Trumps Fusion in Sight Glass

“Fusion” is fine for fancy restaurants, but “compression” makes the real difference in sight glasses.

     Low-pressure bolt-on sight glass“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, 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.


Let There be (More) Light!

You may remember the old story about a man who is looking for his lost keys beneath a streetlamp. A passerby asks the man if he is sure about where he dropped them, and the man says, “Actually, I think I dropped them in the grass over there.” The passerby asks, “Then why aren’t you looking where you dropped them?” The first man answers, “Because the light is better here.”

On a street corner or in a processing facility, sufficient light makes all the difference in spotting what you need to find. The interiors of vessels and pipelines are typically far too dark to allow operators to observe important stages of their process readily through a sight glass. Flashlights usually don’t cast enough light and often produce glare, complicating the inspection process. Luminaires mounted onto sight glasses offer a more effective solution. Many processes take place in hazardous environments where explosive conditions may be present. For these applications, explosion proof lighting combines safe interior illumination and glare-free viewing.

There is a wide variety of explosion-proof sight glass lights to choose from. When choosing a light for a specific sight glass application, consider factors like type of technology, size, weight, voltage, wattage, materials of construction, mounting configuration, and light pattern. Energy consumption, location requirements, heat output, ambient conditions, and vibration must also be taken into account.

The “Understanding and Specifying Sight Glass Lighting” handbook addresses these considerations to help specifiers choose the most appropriate lighting solution for the application. Download your free copy today: