Choosing Sight Glass for Elevated Temperature Processing: Sight glasses are often used for equipment vessels used for processing of various chemicals, oils and gasses at elevated temperatures. Read Consulting Materials Failure Analysis and Process Engineering has been involved in evaluation of numerous sight glasses and their failures. As a result, it was determined that the one type of sight glass that appears to be the most reliable are round ones that use pipe threads to affect the sealing . An example is shown below (See Figure #1).
This sight glass has an external “nut” geometry that allows tightening along with the pipe thread for sealing at the opposite end. This is a glass to metal seal that relies on compression stresses on the glass to create sealing between the metal body (normally 1215 steel or stainless steel) and the glass window (normally soda lime or borosilicate glass). In the sealing process the glass and metal body are assembled and heated to temperatures in excess of 900°C. At these temperatures, the glass “melts” and flows to fill the opening in the metal body. Because the glass has a lower coefficient of thermal expansion, cooling creates a tight seal and puts the glass in high compressive stresses (See Figure #2). Because glass only fails in tension, these seals are virtually indestructible. In addition, no compliant material is involved in sealing. Finally, all the stresses are symmetrical, thus, there is no tendency for scaling or cracking.