Despite the advances in measuring flow and level electronically, a need for visual confirmation often exists in chemical processing. There is no simpler and more-reliable means to verify measurements, witness process steps or observe fluid characteristics than with a sight glass. That’s why many process vessels are ordered with sight glasses and others are retrofitted with them. However, the potential failure of sight glass windows is a real concern. In many chemical processing applications, poorly specified or installed sight glasses may pose the greatest risk of leaks and accidents in the system.
Properly specified and installed sight glasses can be incredibly pressure-, temperature- and corrosion resistant. However, when glass fails, it often fails suddenly and catastrophically. Glass can crack at five miles per second, ejecting shards at speeds approaching Mach 25. Investigators of a failure at a nuclear power plant found glass embedded in a steel railing six feet away. In three incidents, sight glass failures cost more than $67 million in damages, $1.5 million in downtime and multiple lives. Proper sight glass selection, installation and maintenance are critical to avoiding these risks.
Seven factors contribute to sight glass failures:
1. Improper design. Sight glasses are highly engineered assemblies of flanges, bolts, gaskets and glass. Each component in the assembly is critical to its performance. The type of metal, the kind of glass and the gasket material all must meet design criteria. A flange that is too thin may bend during bolt tightening, transferring a bending load into the glass. Even flanges of proper thickness can cause cracks if they have too few bolts, by creating point loads from uneven glass compression. Improper design of the sight glass for the application can be the first step toward a failure.
2. Faulty installation. Over-tightening or uneven torqueing of bolts can generate bending loads on the glass (Figure 1). When replacing glass, trapped debris (often residue from baked-on gaskets) can cause point loads or scratch glass during installation. Any scratches to the glass or damage to gaskets prior to installation can lead to leakage or failure.
3. Mishandling during operation. Maintenance personnel may use a sight glass as a convenient place to rest a wrench. A tiny impact or scratch, even from a gloved hand, can weaken the glass. Just tapping the glass or using harsh cleaning agents can damage the glass. Once installed, the less contact with the glass, the better.
4. Glass disc re-use. According to a technical glass handbook published by Corning Inc., glass removed from service should not be reused. Stressing and then destressing the glass can cause tiny faults. Visual inspection won’t always detect these tiny cracks, so it’s best to replace glass removed from an assembly. The exception is the type of sight glass that is fused to a steel frame; here, the frame takes all the stress of installation.
5. Thermal shock. This may occur during a system startup in which temperature rises rapidly. If the inner surface of the glass expands faster than the outer, the glass can crack. Alternatively, during external washdown, cold water on a hot sight glass can cause the outer surface to contract faster than the inner. Borosilicate glass (e.g., Pyrex) resists damage from thermal stress better than common soda lime glass. It also can handle higher service temperatures: tempered borosilicate glass is rated to 280°C (536°F) compared to the 150°C (302°F) rating of tempered soda lime glass.
6. Over-pressurization. Even though safety devices will relieve a dangerous pressure rise, they may not act fast enough to prevent damage to the glass. Sight glasses are designed with a safety factor but design pressures never should be exceeded during normal operation.
7. Degradation. Glass can suffer erosion over time from contact with coarse process media. In addition, glass itself can react with process fluid, so chemical erosion also is common. Degradation, theoretically, may weaken the glass to the point of catastrophic failure. However, the gradual degradation more likely will lead to a slow-motion failure, which is the loss of visibility. Sight glasses can acquire a cloudy appearance, especially if the process media is caustic, necessitating frequent sight glass replacement. Specifiers should consider the better resistance to chemical and physical erosion of borosilicate glass compared to soda lime glass (Figure 2).