DIN 28121 — fittings with a load path bypassing the glass
Circular fittings where the effort bypasses the glass plate (force shunt), often mounted on a prepared flange.
What this is
The construction forms a load frame of body and cover, so the glass is protected from tightening loads and thermal movements; the fitting is bolted to a prepared flange.
This is convenient for maintenance: the disc is replaced without dismantling the load frame, and hygienic versions are easier to wash.
Verified characteristics
| Parameter | Value | Source |
|---|---|---|
| Mounting | onto an existing flange | Values without a confirmed source are not published on this site. |
| Glass | borosilicate DIN 7080 | din-7080 |
| Load path | flange force shunt | din-28121 |
Construction and cutaway of the fitting
The cutaway is conventional: the version, dimensions and PN of a specific product are confirmed by the manufacturer.
Flange base
bolted to the mating flange of the vessel.
Load frame
body and cover carry the effort bypassing the glass.
Glass element
the disc works mainly as a seal.
Gaskets
medium-contact and compensating; materials follow the medium and temperature.
Cover
provides the initial compression of the pack and protects the glass.
Bolted fasteners
sized to the bolt circle of the mating flange.
Mounting on an existing flange
Before mounting, measure the actual dimensions of the mating flange: DN, bolt circle diameter, number and diameter of holes, sealing surface type. Matching PN does not guarantee compatibility — all dimensions are verified.
Tightening follows a cross pattern with a regulated torque; the sequence and the torque come from the manufacturer documentation.
Operating limits
The assembly works as a unit: frame stiffness, glass, gaskets and the condition of the sealing surfaces define the permissible conditions together.
Compatibility with the existing flange is confirmed by the manufacturer or the vessel designer.
Vacuum, thermal cycles and sterilization are separate verification items.
Do not mount a flanged fitting on a vessel connection with unverified geometry.
Glasses and gaskets
The typical glass element is a borosilicate disc per DIN 7080; the applicability of a specific glass material follows the fitting documentation. Gaskets follow the medium and washing modes (see the gaskets article).
How to choose DN
The DN of the fitting must match the DN of the mating flange; the viewable diameter is a separate characteristic. When selecting a new fitting, take into account both the free window and the connection dimensions.
Difference from DIN 28120
DIN 28120 carries the load through the glass and is welded in; DIN 28121 uses a force shunt and bolted mounting on a flange. Direct replacement of one type with the other without verifying the connection is not allowed.
Frequent questions about DIN 28121
Why is DIN 28121 more convenient for maintenance?
The load frame relieves the glass: the disc and gaskets are replaced without dismantling the fitting from the vessel.
Will a DIN 28121 fitting fit any PN16 flange?
No: DN, bolt circle diameter, holes and the sealing surface of the specific flange are verified.
Is special glass needed for a force shunt?
The glass material and type follow the fitting documentation; arbitrary replacement is not allowed.
Related material
Sources
- DIN MediaDIN 28121:2021-04open the sourceTechnical review: 2026-11-04
- ACIopen the sourceTechnical review: 2026-11-04
- ACIopen the sourceTechnical review: 2026-11-04
- DIN MediaDIN 7080:2025-05open the sourceTechnical review: 2026-11-04