Емкостекло.Sight glasses for industrial vessels
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Sight glass fittings for an existing flange

Fittings whose load path bypasses the glass (force shunt), mounted on a prepared flange; the typical construction is DIN 28121.

Technical review: 2026-11-04

In flanged constructions the effort bypasses the glass disc: the body and cover form a load frame, and the glass works mainly as a seal. This lowers the demands on the glass and simplifies disc replacement without dismantling the fitting.

The typical version is DIN 28121. Compatibility with the flange already installed on the vessel is verified separately by dimensions, PN and sealing-surface condition — the flange PN class alone does not guarantee it.

What belongs to this category

  • Circular fittings of the DIN 28121 type with a force shunt.
  • Hygienic versions with flange mounting — when the documentation confirms them.

What does not belong here

  • Weld-in fittings DIN 28120 — a separate category.
  • Claims of automatic compatibility with any flange of the same PN.
  • Manholes and other access openings without a sight element.
Diagram of a flanged sight glass fitting: load frame of body and cover, glass, gasketsF
Conventional diagram of mounting on an existing flange, not to scale.

How to choose a flange-mounted fitting

  1. Step 1

    Measure the actual dimensions of the existing flange: DN, bolt circle diameter, number and diameter of holes, sealing-surface type.

  2. Step 2

    Compare the operating conditions with the assembly limits, not only with the flange PN value.

  3. Step 3

    Check the material and condition of the sealing surfaces; the medium-side and compensating gaskets may differ.

  4. Step 4

    Ask the manufacturer to confirm compatibility and, if required, provide quality documents.

Series and constructions of the section

The key series of the category is DIN 28121; the comparison with the weld-in DIN 28120 helps avoid substitution mistakes.

DIN 28121 — fittings with a load path bypassing the glassCircular fittings where the effort bypasses the glass plate (force shunt), often mounted on a prepared flange.DIN 28120 and DIN 28121: construction and connection differencesReferencePTFE, EPDM, FKM, graphite gaskets: what must be clarifiedReference

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Sources

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