Sight Glass Safety A Major Process Plant Concern
For industrial processing facilities, sight glass failures are a major concern
. Failures are expensive because they interrupt production and waste valuable product fluids. Cleanup and repair is also costly. Moreover, a sight glass failure can present a significant health and safety hazard. In three such incidents known to the author, sight glass failures resulted in more than $67 million in damages, $1.5 million lost due to business interruption, and the loss of 28 lives.
Usage Basics. Proper design, installation and maintenance of sight glasses are the keys to their safe and effective use. Conventional sight glasses may serve well in less demanding process applications, but not where the risk of failure has unacceptable consequences. The better alternative in those circumstances may be sight glasses with the new mechanically prestressed windows, which makes the glass as strong as steel. While this involves certain costs, purchasing conventional sight glasses or ignoring good installation and maintenance practices creates a high level of risk.
A review of sight glass characteristics is the first step in determining how well they are likely to survive in process monitoring applications. An understanding of these characteristics can then guide your decision on when and how to use newer, more reliable technology.
Glass Characteristics. There are many different types of glass, including soda-lime formulations and borosilicate types. The latter is preferred for highly reliable sight glass windows, especially in chemical, biotech and pharmaceutical processing. Borosilicate glass is widely used because of its mechanical strength and chemical resistance. It is capable of withstanding temperatures up to 500F, has low thermal expansion, and exhibits broad-spectrum corrosion resistance while retaining excellent transparency.
In some respects, the mechanical strength of borosilicate glass is greater than steel. For example, the tensile strength of virgin borosilicate glass is one million psi, compared to 60,000psi for steel. At 200,000psi, the compression strength of borosilicate is also excellent.
Still, taking aside the type of glass used, a sight glass almost always fails in tension rather than compression. This is similar to the case for concrete, because glass is not ductile and cannot stretch like metal. Therefore, tiny imperfections in a sight glass window can create stress concentrations, which are potential failure points. Just the touch of a finger on the window can reduce the tensile strength of a virgin glass element by three orders of magnitudefrom one million to 1000psi.
Another general characteristic of sight glass elements is their failure mode when placed under mechanical stress. When these elements fail, the failure is usually sudden and catastrophic. Since glass cracks at the rate of about five miles per second, the result can be glass shards ejected at speeds up to 24 times the speed of sound.
Conditions Contributing to Failure. Failure analysis reveals several factors that can contribute to the demise of a sight glass. Improper design can be the starting point. For instance, if a cover or hold down flange is too thin, it will flex during bolt tightening on installation. This transfers a bending load to the glass that can cause a subtle crack under the flange. The result is a hidden hazard before the process system ever goes into operation. Even if a flange has the appropriate thickness, installing the sight glass can cause cracks if the flange doesnt have enough bolts around its perimeter. Such cracks occur due to point-loads from uneven glass compression.
Improper handling during production can also be a source of trouble. The detection of manufacturing flaws in sight glass components can be exceptionally difficult. because most are not visible to the naked eye. The manufacturer must have a strict and effective QA/QC system, and use it rigorously.
Although design and manufacturing flaws are important, most sight glasses fail due to improper installation. Mechanical stress is a frequent cause, arising from the over-tightening or uneven torquing of bolts that generate bending loads on the glass. When an existing sight glass window is replaced, trapped debris may become a problem if old gaskets have baked onto the flanges. While this may seem trivial, it is actually very dangerous. Even small contaminant particles or build-up might be enough to scratch, pit or bend the new glass during installation.
Maintenance induced failures are far too common, but easily avoided. Typically, these failures are due to misuse or mishandling after installation. One example is a maintenance person laying a wrench on the lip of a sight port installed in the dished head of a vessel. A slight tap of the wrench on the glass can create a stress problem. Another common problem can be inspectors that rub the glass with a gloved hand to remove surface debris. Although well meaning, they are leaving tiny scratches that progressively weaken the glass. Another habit of maintenance workers is to tap the sight glass, such as they might do on the face of a sticky temperature gauge. Again, this has the potential for weakening the glass.
In fact, any mechanical impact or abrasion on a sight glass window can eventually result in failure, because it produces stress concentrations that reduce the mechanical strength of the glass. By themselves, these small scratches and impacts may not cause failure, but they can result in subtle cracks. Then if a bending load is present, it will cause the cracks to open.
Finally, a common maintenance practice is trying to reuse the old glass removed from a window during repair operations. Simply put, this should never happen. According to Corning Glass Works, the most prominent maker of industrial glass, its essential that conventional glass discs and gaskets removed from a sight glass assembly never be reused
Operating Problems. After installation, sight glass failures are typically caused by one or more operating conditions, i.e., thermal shock, overpressure, or glass corrosion/erosion. Thermal shock occurs due to a variety of situations. Most commonly, during system start-up under low ambient temperatures, a fast rise in fluid temperature puts excessive stress on the glass. When the inner surface of the glass expands much faster than the outer surface, it can crack. A similar problem can occur during external wash-down, if a cold liquid is sprayed on a hot sight glass. This is the reverse of first situation, with the outer surface of the glass being quickly cooled to a temperature significantly lower that of the inner surface.
Over-pressurization of sight glasses also is a fairly common cause of failure. Even pressure relief valves may not relieve a pressure spike quickly or thoroughly enough to prevent damage to the glass.
Corrosion or erosion over time can also degrade the sight glass. Most process fluids will corrode/erode sight glass to some extent. Erosive degradation can be accelerated by the continuous flow of the process fluid against the glass, and thereby weaken it considerably. The only recourse is periodic inspection and replacement of the glass when excessive etching due to erosion or corrosion becomes apparent.
A Safer Solution. The strength and chemical resistance of prestressed borosilicate glass is why it is the preferred material in most types of process service. It is increasingly being used as an alternative to standard glass. Because of inherent design features, this relatively new technology avoids most failure modes associated with standard sight glass windows.
The new designs consist of a stainless steel ring encircling a mechanically prestressed borosilicate glass disc. The reason for its high reliability is the prestress introduced during manufacturing. This causes the steel ring to apply a uniform radial compression to the glass. Afterward, the ring assembly is heated, causing it to expand and the glass to melt within the metal ring. The temperature is high enough that the glass and the metal ring fuse together. Then the assembly is cooled and the glass solidifies within the ring before the metal can shrink back to its original size.
The cooling process places the metal ring in tension and the glass in uniform radial compression. The compressive force is so great that were the metal ring to be cut, compressive forces would be released and the ring would pull away from the glass. While some might interpret that as an indication the glass was not properly fused to the ring, it actually means the inherent compressive force was stronger than the adhesion between the glass and metal.
The performance and reliability of a sight glass are improved by radial compression, which offsets the effects of tensile stress. There are few, if any, sight glass operating conditions where external tensile stress will overcome the compressive stress. In a sight glass assembly where the window is under a strong compressive force, the glass, in effect, becomes elastic, allowing it to accommodate external tensile stress.
Furthermore, the metal ring in a prestressed assembly resists failure because mounting and sealing pressures are applied to it rather than the glass. This virtually eliminates most dangers imposed by any bending loads on the glass. Still, exceptionally strong bending forces can extend beyond the metal ring into the glass. Even so, the glass remains elastic and able to tolerate bending as long as the compressive force applied by the steel ring exceeds the tensile stress on the glass.
In an extreme case, bending force could theoretically exceed compressive force. Even if that happens, the mechanically prestressed window will not shatter. Instead, because of the glass elasticity, the sight glass assembly will fail safe. When the glass surface stretches beyond its elastic capability, small cracks usually develop near the surface. While the cracks can interfere with visibility, they do not compromise window integrity. Below the surface, the glass is still under compressive stress and can withstand the pressures. This same mechanism relieves the stress of over-pressurization. Finally, the steel ring acts as a heat-sink and helps the glass resist thermally induced bending.
Summary. The use of conventional sight glasses in many process monitoring applications will outweigh the risks they impose, provided the assembly is properly designed, installed and maintained. However, to increase safety and reduce maintenance costs, mechanically prestressed windows should be considered for applications where failure could result in injury, excessive downtime, or loss/contamination of costly process fluids.
References. A free 20-page Sight glass Application Handbook is available from L. J. Star at
by:David Star
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