Flow sight glass type 551 with female thread G (BSP) or NPT

Short description - flow sight glass type 551 with female thread

The ACI flow sight glass with threaded connection Type 551 is a flow sight glass with cast body and female thread connection.

The sight glasses are made of the proven borosilicate industrial glass according to DIN 7080 and are highly resistant to most media. For alkaline media and high temperatures we recommend the installation of a mica screen to additionally protect the glass surface.

For sealing, we use proven flat gaskets from well-known manufacturers. In addition to aramid fibre material such as KlingerSIL C4400, the flow sight glass type 551 can also be equipped with graphite sealants such as Frenzelit novaphit MST. Soft sealing materials such as silicone, EPDM and NBR are also available from the factory.

Type 551 is available in many common steel and cast stainless steel alloys such as GG25 and 1.4408. Special materials such as Hastelloy can also be supplied on request.

Read on for more technical information and detailed product graphics.

Technical data


Body made of cast iron GG-25 (EN-JL 1040)

Body made of cast steel GS-C 25 (1.0619)

Body made of stainless steel 1.4408

Screws: 4.6/5.6 vz or A4-70

Gaskets: Graphite (or as desired)

NPT connection only possible for materials GS-C 25 or 1.4408.


Straight form

Nominal size:

1/4" - 2"

Nominal pressure:

PN 16 /25 / 40 depending on material and NPS


With borosilicate glass up to max. 280°C
With soda-lime glass up to max. 150°C

Optional versions are possible.

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Flow sight glass type 551