Processing Magazine

Routine sight glass inspection is critical for operator safety

December 19, 2006

Does your company perform routine maintenance checks on your process observation equipment? Are your observation ports designed properly, providing a safe assembly where the glass is not under any stress? For many companies, routine maintenance is simply checking the glass for any scratches or corrosion. While this is important, the entire assembly should also be carefully evaluated. Many sight glass failures are caused because of unsafe assemblies.

Besides checking for scratches on glass, there are many other important points to examine on sightglass assemblies. Flanges used to mount a sight glass must be flat in order to provide support of the glass and thick enough to withstand the bolting loads without any bending. The bolts should always be tightened using a torque wrench. Using such a precision tool insures that the glass disc is under uniform compression, which means no stress upon the glass. Always remember to tighten bolts using the manufacturer’s recommended bolt tightening sequence.

Gaskets seats on the assembly should be flat, smooth, and free of gasket residue. An uneven gasket seat causes stress on the glass, which may lead to failure. The gasket seats are safest when they are counter-bored to a depth of 3/16” to accept the glass and gaskets. This reduces gasket blowouts and helps to center the glass. Your choice of gasket material is also important. The cushion gasket (outer side) should be a hard gasket and only 1/32” thick. The sealing gasket (process side) should be a material suitable to your process and 1/16” thick. Be sure all employees in contact with your sight glasses are properly educated on sight glass safety.

Whether the pressure in your application is extreme or just atmospheric, safety is always a concern. Bad manufacturing practices can cause a glass disc to break under no pressure at all.

Select a vendor that offers a variety of approvals, including TÜV, 3A, and even UL Listings. Some suppliers will test the assembly prior to shipping. For example, L.J. Star submits its visual flow indicators to a hydrotest at 1.5 times their design rating before they leave the factory.