Weld-in circular sight glass fittings
Circular fittings welded into the vessel wall; the typical construction is DIN 28120.
A weld-in sight glass fitting is welded into the vessel wall with its body and carries the load through the glass element. It is historically the most common construction for reactors and jacketed vessels: minimal protruding parts, compact mounting, a direct view of the medium.
Responsibility for the compatibility of the assembly rests with the fitting manufacturer and the vessel designer: this category explains the construction class and its boundaries, but does not replace the calculation of a specific version.
What belongs to this category
- Circular weld-in fittings of the DIN 28120 type (force-fit: the load passes through the glass).
- Fittings for welding into vessel walls and covers under design pressure.
- Replacement discs and gaskets belonging to these fittings.
What does not belong here
- Fittings mounted on an existing flange (see the “Flange-mounted” category, DIN 28121).
- Oblong windows and non-pressure constructions — they have their own categories.
- In-line flow indicators and boiler level gauges.
How to choose a weld-in fitting
- Step 1
Record the design and operating parameters of the vessel: pressure, temperature range, vacuum and thermal cycles.
- Step 2
Determine DN and the desired viewable diameter; remember these are different quantities.
- Step 3
Check the medium composition and seal requirements (the medium-side and compensating gaskets may differ).
- Step 4
Ask the fitting manufacturer to confirm the assembly for your conditions and request the documents you need (for example EN 10204, if required).
Series and constructions of the section
The key series of the category is DIN 28120; the differences from flanged constructions are covered in the comparison 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