Aside from functionality, the most sought-after feature of most sight windows is their margins of safety. Virtually every commercial vendor of such products touts its safety record, and makes various claims regarding why their products are "safe". The most common of such generic claims about safety relates to numerical safety factors on pressure scale, and conformity to miscellaneous industry standards. And yet, sight window failures have been happening for centuries, and they continue.
Virtually all sight window failures arise from misidentification of risks associated with the application environment. In other words, the original specifications ignored a major risk factor that eventually caused the failure. Stated that way, it should be obvious that the first issue to be addressed before specifying a sight window is a holistic assessment of the risks associated with a given application. Indeed, the failure to carry out this preliminary assessment is the ultimate reason for eventual failures; catastrophic process parameter excursions that lead to sight window failures are exceedingly rare.
Aside from sight window geometry, there are three parameters that usually characterize a unit:
Mechanical - pressure rating and impact resistance.
Chemical - corrosion resistance with respect to wetting fluids.
Thermal - temperature related issues.
Each of these areas is associated with its own set of potential problems and failure modes, and a proper specification of sight windows needs to accommodate all of these holistically. Failures occur because one area is addressed to the exclusion of another. Also contributing to this overall problem is the fact that commercial vendors are in the business of selling products, and only advertise facts that show their own products in favorable terms. While this is not unlawful or even unethical, it leads to reason that vendor provided information would not discuss risk factors that show their products in unfavorable terms. Thus, it is important for buyers of sight windows to make sure that they take it upon themselves to develop a holistic picture of risks for their own applications, and not rely merely on vendor's discussions.
The following three-part discussion presents a summary of major issues that should be considered before finalizing a specification for sight window. Video is included with each section.