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While sightglasses may seem rather simple in design, they are actually highly engineered products. If not properly specified, sightglasses can easily become the weakest link in a chemical or pharmaceutical processing system. And by neglecting the importance of this component, end-users may find themselves coping with less than optimal observation capabilities; frequent maintenance and replacement; difficult illumination capability; and, in the worst case, sightglass failure, thus endangering workers and causing extensive destruction and downtime. On the other hand, a proper understanding of sightglass design options, materials of construction and maintenance procedures will ensure that sightglasses serve and survive through the life of the system.
Glass is strong and durable, actually stronger than steel in tensile stress and similar in compression if flawless and untouched. However, glass is exceptionally sensitive to minor imperfections occurring in the production process. Most system designers will never have the advantage of using flawless glass — however, sightglasses can be designed, installed and maintained in such a way that can avoid most of the causes of sightglass failure.
1. A sightglass cover flange or hold-down flange that is too thin in design or manufacture will bend during bolt tightening, creating a bending load on the glass. If the load is great enough, the glass will crack and create a hazard even before the system is in operation. Worse still, if the cover flange hides the cracks, these will go undetected prior to system start-up. Cracks in a sightglass also may occur if not enough bolts are used when installing a cover flange or hold-down flange. The uneven compression creates point loads that lead to stress fractures in the glass.
2. Another design flaw may be a cushion gasket that is too soft or too thick. Under pressure, the gasket will not provide good flat support, instead creating a bending load on the glass. Once again, there is the potential development of an undetectable hazardous situation even before system start-up.
3. Inadequate glass thickness, as a result of the use of an incorrect coefficient, improper formula or a miscalculated unsupported glass diameter, is a third and more obvious design flaw. Installing a sightglass with an incorrect dimension can lead to failure as the operating pressure exceeds the glass’s maximum pressure tolerance.
A recommended minimum thickness of a window required to withstand a specified pressure differential can be calculated by the following formula:
The chart on the following page shows the allowable stress (PSI) of commonly used glass.
Finally, the right quality glass must be specified. Sightglass service is not very tolerant of flaws in the production of the glass, and faults are difficult to determine since most are not visible to the naked eye. As a result of poor quality control, the lower and upper flanges, or even the surfaces of the glass itself, may not be flat and parallel. A flaw or in any of these components, or the use of the wrong type or quality of glass, may result in stress points or bending loads on the glass. Fortunately, there are widely accepted standards that can be depended upon to assure that the material used in a particular sightglass is appropriate for the intended service.
Specifications & Standards
|Uneven bolt tightening or debris can cause a bending moment that can break a sightglass.
DIN 7079 Standard for Fused-Glass Sightglasses in Metal Frames: The DIN 7079 standard for fused-glass sightglasses sets standards for fusion, thermal properties, strength and chemical resistance. This is a stringent standard, and very few sightglasses can meet its specifications.
DIN 7080 Standard for Borosilicate Glass Quality: The DIN 7080 standard means the glass has passed tests for material strength, shock endurance, chemical resistance and compression. There is no reason not to insist on this high quality glass.
Factory Mutual (FM): Factory Mutual has defined standards that sightglasses should meet in order to provide safe operation. Use of FM-approved products may in some cases reduce plant insurance costs. This standard assures that a sightglass meets high test standards for pressure, impact, and thermal shock.
|A scratch on a conventional sightglass creates a stress concentration
point that may break the glass under pressure. In contrast, glass
compressed inside a metal ring will not fail because part of the glass
remains in compression.
Installation & Maintenance
Perhaps the most common example of improper installation occurs when the mounting bolts are over-tightened or unevenly torqued, resulting in bending loads and glass fractures. Installers must also watch for any debris that might be trapped between the mounting components and the sightglass. Often occurring when a sightglass is being replaced and gaskets are baked onto the flanges, even the smallest dirt particles or product spill build-up in the spaces between the various components can be enough to scratch, pit or bend the glass.
An unfortunately common cause of failure of a sightglass is incorrect use and mishandling. Something as simple as using the sight port as a handy place to rest a wrench can be the prelude to failure. Any surface defects, even those that cannot be seen by the naked eye, create a fault that becomes a stress concentration — a place for breaks to originate. Even though a bending load is required to open these cracks, such fractures significantly reduce the pressure capabilities of the glass.
|Thermal shock can break a conventional sightglass.
One hard-and-fast general rule is this: Conventional glass discs and gaskets should never be re-used after they have been removed from a sightglass assembly. Corning Glass Works states this clearly in its “Sight Glass Use and Care” manual. No matter how intact these discs might seem to the naked eye, the act of compressing the glass between two flanges during installation and subsequent removal will create scratches and pitting in the glass.
Situations that may not be realized, understood or expected after start-up sometimes do happen in the real world of processing. But simply understanding what variables might possibly arise and how they might affect sightglass safety is a good defense against bad outcomes.
One such example is thermal shock, the result of a quick or drastic change in temperature. This can occur in various ways. First, and most common, is when a sightglass is subjected to cold ambient temperatures and the start-up causes a quick rise in temperature. If the inner surface of the sightglass expands too quickly relative to the outer surface, the thermal zones will attempt to move at different rates of speed, causing the glass to crack. Another thermal shock scenario happens during external washdown cleaning cycles, where a cold liquid happens to be directed onto the outer surface of a hot sightglass. Again, a quick change in temperature on one surface could cause the glass to fail. Heat generated from a nearby high wattage luminaire also can cause thermal shock.
A less obvious type of thermal shock, called shadowing, occurs when an area of glass that is protected and sealed by the gasket and flanges is not directly exposed to the heat or cold of the processing fluid. This “shadowed” area remains closer to ambient temperature while the exposed viewing area sees temperatures near that of the product. An extreme temperature difference within the sightglass may produce opposing forces that may exceed the design limits of the glass, resulting in failure.
Another service condition that creates a hazardous situation is over-pressurization. A miscalculation or an unexpected increase in system pressure could exceed the glass’s design pressure. Often, there are systems in place to relieve this sudden increase in pressure; however, these provisions may not respond fast enough to eliminate the initial burst of pressure exerted throughout the vessel or pipeline system, including the sightglass.
A more common service condition that can create a sightglass failure is the degradation of the glass over time. Glass will corrode or erode to some extent in service, depending on the quality of the glass and the conditions of application it services. Chemicals, even water, will cause corrosion, while friction from a product will erode the glass. In both cases the glass will become etched, degrading its transparency and possibly weakening it to the point of catastrophic failure in what would otherwise be normal operating conditions. Several chemical resistance charts are available that address the loss of weight of glass when exposed to various chemicals, and they should be consulted when adding a sightglass into any system. These guides are generally quite expensive, so users would be advised to consult their sightglass suppliers for resistance charts.
Andrew Obertanec is vice president of business development for L.J. Star, Inc. He has 30 years of experience in the area of process observation equipment. He is a member of ISPE and sits on subcommittees and task groups for the ASME-BPE Committee. Mr. Obertanec can be reached at email@example.com.
This article is a follow-up to an article titled “Are You Gambling with Your Sightglass,” which appeared in the July 2008 issue of Flow Control (pages 18-24). Both articles are based on content included in L.J. Star’s “Chemical and Pharmaceutical Application Handbook” publication.