Technical contribution: Flow sight glasses

In this technical contribution you get an overview of flow sight glasses, their typical designs and application possibilities.

Figure 1: Typical flow sight glasses. From left to right: Welded flanged body, pipe sight glass with impact shield, intermediate flanged pipe sight glass.

Figure 1: Typical flow sight glasses. From left to right: Welded flanged body, pipe sight glass with impact shield, intermediate flanged pipe sight glass.

Flow sight glasses are industrial sight glass devices that are equipped with common piping connections and can be installed in like any other industrial valve. Most types are available with flanges, welding ends or threaded connections.

The most basic type, as shown in Figure 1 on the left side (ACI Typ 530), has a straight main body and two opposite sight glass windows. Extrapolating from this basic type, three-way or four-way sight glass devices as well as sight glasses with multiple viewports are also available, some of which can be seen in Figure 2.

Figure 2: Additional variations of flow sight glasses

Figure 2: Additional variations of flow sight glasses

With an appropriate mechanical design and the right materials for body, glass, bolts and gaskets, this type of sight glass can be built for applications operating at 1000 °C or 1000 bar.

For improved observation of the process fluid, tubular sight glasses as shown in Figure 1 in the middle can be used.

This type of sight glass offers 360° view of the process fluid and is almost completely without wake spaces. These properties make this type of sight glass especially suited for applications requiring hygienic conditions at relatively low pressure (maximum 16 bar). The lack of wake space also reduces drag within the sight glass body. The relatively large open glass area frequently often necessitates the addition of an impact protection shell made of acrylic glass, as shown in Figure 1.

Due to manufacturing limitations the maximum allowable pressure for this type of tubular sight glass is significantly lower than for other flow sight glasses, especially in large nominal sizes, as shown in Figure 3.

For significantly higher temperatures and pressures, tubular sight glasses can also be manufactured in an intermediate flange design, which is available in quartz glass in addition to borosilicate glass. One such sight glass is shown in Figure 1 on the right, installed between welded neck flanges.

Figure 3: Maximum allowable operating pressure for ACI type 620 tubular sight glasses

Figure 3: Maximum allowable operating pressure for ACI type 620 tubular sight glasses