Stainless Steel Sight Glass | Flanged & Threaded

Stainless steel and PFA-lined flow sight glasses with flange and threaded connections – visual monitoring for industrial pipelines

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Frequently Asked Questions – Flow Sight Glasses

A flow sight glass – also called a sight glass fitting, sight flow indicator, or bull's eye – is a pipeline fitting that is installed directly in a pipeline to enable direct, visual monitoring of the medium flowing through it. Through a pressure- and temperature-resistant observation window made from toughened glass, the operator can determine at any time:

Whether a medium is flowing – flow present or absent (e.g. pump status, suction line leakage, flow failure)
What the medium looks like – colour, turbidity, bubble formation, foaming, contamination, or phase changes (liquid/gaseous) can all be directly observed
Whether the system is operating correctly – with no sensors, electronics, or external power – purely optical and maintenance-free

Sight glasses are used across a wide range of industries: drinking water treatment, food and pharmaceutical manufacturing, chemical processing, oil and gas installations, steam systems, HVAC, and general pipeline monitoring. FERGO offers sight glasses in stainless steel (1.4408 / V4A) and as PFA-lined versions for aggressive media, with flanged or threaded connections.

The choice of glass type is critical for chemical resistance, temperature capability, and safety. FERGO offers three glass options:

Borosilicate glass (DIN 7080) – The most chemically resistant of the three. Borosilicate glass withstands most acids (except hydrofluoric acid and hot phosphoric acid), alkalis, and organic solvents, while also offering excellent thermal stability (low coefficient of thermal expansion, good resistance to thermal shock). First choice for aggressive chemical media, process chemicals, and wherever the highest chemical resistance is required.
Soda-lime toughened glass (Kalknatron-Hartglas) – A thermally toughened soda-lime glass with good mechanical strength and pressure resistance. Slightly more economical than borosilicate, with lower chemical resistance to strongly alkaline media. Suitable for water, oil, neutral liquids, and general process media. Operating temperature up to 240 °C at 10 bar.
Pressed toughened glass (Presshartglas) – Press-moulded and subsequently toughened glass with high mechanical impact resistance. Particularly robust against mechanical loads. Used in flanged sight glasses for larger nominal sizes where the glass disc is exposed to larger pressure surfaces.

As a general rule: borosilicate for chemically aggressive media – soda-lime toughened for water and neutral media – pressed toughened for mechanically demanding flanged applications in larger nominal sizes.

Stainless steel 1.4408 (V4A / AISI 316) is an excellent material choice for the vast majority of industrial media – however, it has limitations with highly corrosive media:

Stainless steel sight glass (1.4408) – Suitable for water, steam, oil, neutral to moderately aggressive chemicals, food and pharmaceutical processes, and general industry. Chemically resistant to alkalis, many organic acids, and solvents. Not suitable for concentrated hydrochloric acid, hydrofluoric acid, highly oxidising acids (concentrated nitric acid), and chloride solutions at elevated temperatures (risk of crevice corrosion).
PFA-lined sight glass (GGG40.3+PFA) – The ductile iron body is fully lined internally with PFA (perfluoroalkoxy alkane) – a fluoropolymer with near-universal chemical resistance comparable to PTFE but with superior processing properties. Suitable for highly aggressive acids, alkalis, solvents, oxidising agents, and other chemicals where stainless steel would corrode. The ideal complement to PFA valves and fittings in fully chemically resistant pipeline systems.

If your medium is a highly aggressive chemical and you are unsure which version to choose, the FERGO team is happy to advise.

FERGO sight glasses are available in both connection types. The choice depends on the nominal size, pipeline system, and pressure requirements:

Threaded sight glass (socket sight glass) – Female thread connection to ISO 7-1 (EN 10226-1) for quick and easy installation in small nominal sizes (¼" to 2", corresponding to DN8–DN50). Ideal for building services, HVAC, laboratories, and wherever threaded pipelines are the standard. Minimal space requirements, simple installation without flanges or bolted connections.
Flanged sight glass – Flanged connection to DIN 2501 PN10/PN16 for medium to large nominal sizes (DN15 to DN250). A detachable, robust connection ideal for industrial pipelines, process plants, and wherever regular glass disc inspection or replacement needs to be straightforward. Face-to-face length to EN 558-1 Series 1 (F1) for direct compatibility with existing flanged systems.

As a general rule: female thread for small nominal sizes and simple installations – flanged for medium to large nominal sizes and industrial process plants.

The FERGO sight glass range comprises 29 models with the following specifications:

Nominal sizes – DN15 (½") to DN250 (10") for all common pipeline sizes, from domestic installations to large industrial process lines. Custom sizes available on request.
Connection pressure ratings – PN10 and PN16 for flanged connections; up to max. 16 bar operating pressure for threaded versions.
Operating temperature – Up to max. 240 °C at 10 bar for soda-lime toughened glass with PTFE gasket (stainless steel body). The exact pressure-temperature combination depends on the glass type selected.
Gasket material – Standard PTFE – near-universally chemically resistant, food-safe, and heat-resistant to over 200 °C.
Face-to-face length (flanged versions) – EN 558-1 Series 1 (F1) – standard face-to-face for maximum compatibility with existing flange systems.

Flow sight glasses are universal monitoring fittings used wherever the condition of a medium in a pipeline needs to be directly visible without sensors or electronics:

Water treatment and drinking water – Monitoring flow and water quality (turbidity, discolouration, bubbles) in filtration, softening, and disinfection systems.
Food and beverage industry – Hygiene-tested stainless steel versions for visual monitoring of liquids, oils, syrups, and process media. PTFE gaskets are food-safe.
Pharmaceutical industry – High-grade stainless steel (1.4408) sight glasses for cleanroom applications, bioreactor lines, and wherever hygienic visual monitoring is required.
Chemical process industry – PFA-lined sight glasses for aggressive acids, alkalis, and solvents; borosilicate glass versions for maximum chemical resistance.
Oil and gas installations – Monitoring oil, hydraulic, and fuel lines for flow, contamination, and phase changes.
Steam and HVAC systems – Monitoring steam, condensate, and heating media for phase state, flow, and fouling.
General industry – Wherever simple, maintenance-free visual inspection of media flow provides valuable process information without the cost or complexity of flow sensors.

FERGO uses exclusively stainless steel 1.4408 (also known as CF8M, V4A, or AISI 316) for its standard sight glasses – a high-alloy austenitic cast steel with the following key properties:

High corrosion resistance – The molybdenum content (2–3%) makes 1.4408 significantly more resistant to chlorides, acids, and alkalis than the more economical 1.4301 (V2A / AISI 304). Suitable for salt water, moderately aggressive chemicals, and all common process media.
Hygienic suitability – Stainless steel 1.4408 is approved for food and pharmaceutical contact, releases no ions into the medium, and is easy to clean (CIP/SIP compatible).
Mechanical robustness – Good resistance to pressure fluctuations, temperature differentials, and mechanical loads throughout the service life.
Wide temperature range – Usable from cryogenic temperatures up to 240 °C operating temperature (glass-dependent).
Attractive appearance and low maintenance – Stainless steel does not rust or tarnish and permanently retains its clean appearance – ideal also for visible installations in the food and pharmaceutical industry.

The standard FERGO sight glass range covers DN15 to DN250 with threaded or flanged connections, in stainless steel or PFA-lined versions – most models are available directly from stock for fast delivery. Beyond the standard range, individual requirements can be accommodated on request:

Custom sizes – Nominal sizes outside the standard range (below DN15 or above DN250), as well as non-standard face-to-face lengths, can be manufactured to order.
Special materials – Bodies in alternative materials (e.g. Hastelloy, titanium, duplex stainless steel) for extreme corrosion requirements are available on request.
Special glass types – Specific glass grades for extreme temperatures, elevated pressures, or particular chemical requirements are available on request.
Special connections – ASME/ANSI flanges, Tri-Clamp connections for hygienic applications, or other custom flanges to customer specification are available on request.

For all special requirements, please contact the FERGO team directly – our technical experts are happy to advise on the optimal sight glass selection for your application.

Stainless Steel Sight Glasses: Reliable Flow Monitoring for Pipelines

Materials and Glass Types for Maximum Safety

Sizes, Connections and Custom Solutions

Your Expert for Flow Sight Glasses

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Frequently Asked Questions – Flow Sight Glasses