30 years ago, in the ammonia plant where I worked, the sight glass of a high pressure (200 Kg/cm2) ammonia separator leaked during start up. Luckily we managed to shut the plant down safely with no injury to anyone. The root cause was the wrong torquing procedure used. A good article about sight glasses mentions the following:
"Proper design, installation and maintenance of sight glasses are the keys to their safe and effective use....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 magnitude
from one million to 1000psi. 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".
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