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The apparent simplicity of sightglasses has lured many process operators into costly, sometimes dangerous equipment failures. The potential hazards of a sightglass failure in a high-pressure or high temperature system is obvious but even in more moderate conditions the loss or contamination of process fluids and the cost of unscheduled downtime can be a major problem.
In essence, a sightglass is simply a transparent section of a vessel wall, a window on the process in real time. Of course care must be taken in the selection of a sightglass; the specification should be developed by a qualified engineer familiar with both the process and with sightglass technology. But even after the proper sightglass has been installed and the process is up and running there are numerous opportunities for sightglass integrity to be compromised.
Four rules for routine maintenance:
1) Use only commercial glass cleaners to keep sightglasses clean.
2) Never use wire brushed metal scrapers or harsh abrasives.
3) Do not attempt to clean glass while equipment is in operation.
4) Once a conventional sightglass has been removed from its mounting, regardless of the reason for its removal, discard the glass and gaskets and replace with all new elements. (An exception: Metaglas® safety windows may be reinstalled, following instructions provided for this procedure.)
Four rules for routine inspection:
1) To examine for scratches, shine a very bright concentrated light source at an angle of about 45 degrees on the glass. Anything that glistens should be closely inspected.
2) Scratches that catch the fingernail and any star or crescent-shaped marks that glisten are cause for replacement.
3) A sightglass that appears cloudy or roughened after cleaning should be replaced.
4) Inspect the sightglass frame and/or flanges for corrosion buildup.
Six rules for reinstalling glass:
1) Never use glass that has been scratched, chipped or otherwise damaged.
2) The glass seating surface must be flat to within 0.0005-inch.
3) Flanges must be rigid.
4) Do not allow the glass to come into contact with the metal when assembling.
5) Gaskets must be new, clean and smooth.
6) Use gaskets of the same diameter and fit them concentrically.
Five rules for tightening bolts:
1) Always follow a regular tightening sequence to ensure even loading of the glass.
2) Allow a maximum difference of 1.5 foot-pounds between bolts during tightening.
3) Tighten only enough to produce a positive seal against the process pressure, always following manufacturer’s recommendations.
4) Bolts may need tightening after initial cycling of the vessel.
5) Never tighten bolts when the system is in operation or the sightglass is still hot.