DIN 28120 — weld-in circular sight glass fittings
Circular sight glass fittings for welding into the vessel wall; the load passes through the glass element.
What this is
The fitting is welded into the wall or cover of the apparatus and provides a direct view of the medium: jacketed reactors, process vessels, apparatus where foam or the colour of the medium must be observed.
Weld-in mounting gives a compact fitting without a protruding flange and works well with jackets and insulation; in return it requires qualified welding and planned glass maintenance.
Verified characteristics
| Parameter | Value | Source |
|---|---|---|
| Mounting | welded in | Values without a confirmed source are not published on this site. |
| Glass | borosilicate DIN 7080 | din-7080 |
| Working temperature of the glass element | ≤ 280 °C (glass element only) | din-7080 |
Construction and cutaway of the fitting
The typical composition of a weld-in fitting is shown in the conventional diagram; actual versions and dimensions are defined by the manufacturer documentation.
Base (body)
the welded part that carries the load from the vessel.
Medium-side seal
the medium-contact gasket; the material follows the medium and temperature.
Glass element
a circular disc, usually borosilicate per DIN 7080.
Compensating gasket
relieves the glass from tightening moments and thermal movements.
Cover (clamp ring)
transfers the tightening load to the glass and gasket pack.
Fasteners
studs or bolts and nuts with a regulated tightening torque.
Mounting and installation
The fitting is welded into a prepared opening in the wall; the geometry and welding sequence are defined by the manufacturer. Vessel heat treatment and welding distortions affect the seal geometry and are accounted for at mounting.
Fasteners are tightened with a regulated torque, usually in several passes; the torque and the sequence follow the documentation of the specific product.
What limits pressure and temperature
Permissible conditions are defined by the whole assembly: the glass, the gaskets, the body material, the mounting and the medium. A nominal PN class by itself is not a guarantee of working pressure at all temperatures and media.
A glass element per DIN 7080 (borosilicate glass) is described up to 280 °C within its applicable scope, but an assembly with specific gaskets can be limited stricter — 280 °C must not be copied onto every card.
Vacuum and pressure fluctuations require separate confirmation.
Thermal cycles and shocks limit the glass and the gaskets — clarify the modes.
Pulsating pressure requires manufacturer confirmation.
Glasses and seals
The main glass element is a circular thermally toughened borosilicate disc per DIN 7080. In applicable versions soda-lime glasses per DIN 8902 occur; glass-to-metal sealed elements (DIN 7079) belong to separate constructions and do not replace the standard disc.
Gaskets are divided into medium-contact and compensating; materials (PTFE, EPDM, FKM, graphite) follow the medium, temperature and washing modes — see the gaskets article.
How to choose DN
DN is the nominal size of the connection, not the view diameter: a DN 100 fitting has a noticeably smaller free window. Choose DN by the required visibility and the strength of the wall, and check the viewable diameter in the documentation of the specific version.
The view area can be estimated with the calculator from the free diameter; to compare two variants, enter both values explicitly.
Difference from DIN 28121
DIN 28121 carries the effort bypassing the glass plate (force shunt) and is often mounted on a prepared flange. It is not an interchangeable analogue of DIN 28120: replacement requires verification of the connection and the assembly.
Frequent questions about DIN 28120
Can a DIN 28120 fitting be mounted on an existing flange?
No: DIN 28120 is designed for weld-in mounting. For an existing flange, consider DIN 28121-type versions with a compatibility check.
What pressure can a DIN 7080 glass withstand?
The standard describes the glass element, not the assembly. The permissible pressure of the fitting is confirmed by the manufacturer for a specific version and conditions.
Why can the glass not be replaced with ordinary heat-resistant glass?
Thermal treatment, geometry and defect control are part of a pressure-bearing construction; arbitrary replacement is not allowed.
Related material
Sources
- DIN Media / AENORDIN 28120:2021-04open the sourceTechnical review: 2026-11-04
- ACIopen the sourceTechnical review: 2026-11-04
- Alfa Lavalopen the sourceTechnical review: 2026-11-04
- DIN MediaDIN 7080:2025-05open the sourceTechnical review: 2026-11-04