Because glass is so sensitive to imperfections, there are several factors that will contribute to sight glass failures.
First is improper design. A cover flange, or hold-down flange, that is too thin will create a bending load on the glass as the flange bends during bolt tightening. This scenario can crack the glass, which creates a hazard even before the system is in operation. This situation may easily go undetected prior to start-up since the cover flange can hide the cracks. Even too few bolts will create point loads from uneven compression on the glass, which again may result in cracking.
Another design flaw may be a cushion gasket that is too soft or too thick. Under pressure, this will not create good flat support, similar to lying in a feather bed, and will create a bending load on the glass that will result in a tensile stress on the surface of the glass. Again, an undetectable hazardous situation created even before start-up.
A third and more obvious design flaw is inadequate glass thickness. This can lead to failure as the internal pressure exceeds the maximum pressure capability of the glass. This can occur as a result of the use of an incorrect coefficient, improper formula or a miscalculated unsupported glass diameter.
Another reason for conventional glass failure is improper installation. Examples are over-tightening and uneven torquing which produce bending loads on the glass and can potentially result in cracking. One more common cause of failure due to improper installation is trapped debris. Even the smallest dirt particles or product spill build-up can be enough to scratch, pit or bend the glass. Often, gaskets become baked onto the flanges and portions are left behind when removed. Again, this creates a very dangerous situation.
A very difficult flaw to discover is created during the manufacturing of the sight glass components. The lower and upper flanges, and even the glass, may not have flat and parallel surfaces as a result of poor or low tolerance manufacturing. These are difficult to determine since most of these defects are not visible to the naked eye. A flaw in any of these components will again result in a bending load on the glass.
One of the most common causes of glass failure is incorrect use and mishandling, such as using the sight port as a handy place to rest a wrench. Any surface defects, even those that cannot be seen by the naked eye, create a source for breaks to originate, also known as stress concentrations. Even though a bending load is required to open these cracks, they significantly reduce the pressure capabilities of the glass.
Impact from an object will also weaken the glass considerably. Such an impact can create a pitting, or depression, in the surface, which produces a stress concentration point, resulting in a hazardous situation in pressure applications. This is comparable to making a slight cut or tear on a piece of paper, and then pulling on it. In fact, the same thing is done when glass is cut. A glasscutter is used to score the glass, creating a small depression in the surface, and then a bending moment breaks the glass along the scratched surface. This is similar to a sight glass with a surface scratch and pressure behind it creating a bending moment.
Another misuse of glass often occurs during maintenance and inspection. Conventional glass discs and gaskets should not be removed from the sight glass assembly and re-used. Corning Glass Works states this in its Sight Glass Use and Care Manual because compressing the glass between two flanges will create scratches and pitting in the glass.
The last possible cause of failure may result from unknown variables during processing. There are many situations that may not be realized, understood, or expected after start-up. One such example is thermal shock, or a quick or drastic change in temperature. This can occur in various ways. First, and most common, is when a sight glass is subjected to cold ambient temperatures and the start-up causes a quick rise in temperature. If the bottom surface of the glass expands too quickly relative to the top, the surfaces will move against each other until they crack. Another thermal shock scenario happens during external wash down cleaning; a cold liquid directed on the top surface of the glass quickly cools down a hot sight glass. Again, a quick change in temperature on one surface and the glass can fail. Heat generated from a high wattage luminaire can also cause thermal shock.
A less obvious type of thermal shock is called shadowing. The 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. This “shadowed” area remains closer to ambient temperature while the exposed viewing area sees temperatures near that of the product.
This extreme temperature difference within the glass will produce opposing forces that may exceed its limitations. Again, the result is failure.
A second service condition that creates a hazardous situation is over-pressurization. A miscalculation or an unexpected increase in system pressure could exceed the design pressure of the sight glass. Often, there are systems in place to relieve this sudden increase in pressure; however, these systems may not eliminate the initial burst of pressure, which is exerted throughout the vessel or pipeline, which includes the glass.
The last service condition that can create a sight glass failure is the degradation of the glass over time. Chemicals, and even water, will corrode glass. There are several chemical resistance charts that illustrate the loss of weight of glass when exposed to various chemicals. Another source of degradation is the continuous friction from an abrasive, and even a non-abrasive, product against the glass, which we call erosion. In both cases, this occurrence will etch the glass and weaken it considerably, to where normal operating conditions may result in catastrophic failure.