Weld-neck sight glass installed on pressure vessel
Weld-neck sight glass installed on pressure vessel

Weld-Neck Sight Glass — Pressure Vessel Viewport Lumiglas®

Lumiglas® weld-on high pressure sight glass with flange. Rated 10–64 bar, DN25–DN200. Borosilicate & safety glass. EN 10204-3.1 cert. DIN 28120 compatible. Request quote →

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Description


✓ EN 10204-3.1 Certified
✓ DIN 28120 / DIN 28121
✈ Fast Global Shipping
🏭 Factory Direct
🔥 100% Quality Inspected

The Lumiglas® Weld-Neck Sight Glass is a high-pressure weld-on viewing assembly designed for permanent installation on pressure vessels, reactors, autoclaves and storage tanks. Specifically, the base flange welds directly to the vessel wall, while the cover flange clamps the sight glass disc and gasket into a pressure-rated, leak-free assembly. As a result, this design eliminates the risk of flange face corrosion and bolt relaxation that can occur with loose flange connections over time.

Furthermore, Lumiglas® weld-neck sight glasses strictly follow DIN 28120 and DIN 28121 dimensional conventions, ensuring full interchangeability with existing installations worldwide. Because both standards share the same base flange geometry, our weld-neck assemblies serve as direct replacements for all DIN 28120 and DIN 28121 compliant installations. Therefore, you can upgrade without any vessel modification.

Additionally, they are available from DN40 to DN200, with borosilicate glass (DIN 7080), tempered soda-lime glass (DIN 8902), or specialty high-pressure glass on request. Moreover, all assemblies ship with EN 10204-3.1 material certificates as standard.

Key Technical Specifications

Parameter Value / Standard
Execution Standard DIN 28120 / DIN 28121 (weld-neck / weld-on flanged)
Nominal Sizes DN 40 / 50 / 80 / 100 / 125 / 150 / 200
Pressure Rating PN 10 / PN 25 standard — custom up to 70 MPa on request
Glass Material Borosilicate 3.3 (DIN 7080) / Tempered Soda-Lime (DIN 8902) / Specialty glass on request
Temperature Range Up to +280°C (borosilicate) — higher temperature glass available on request
Body / Flange Material Carbon Steel RSt37-2 / Stainless Steel 1.4571 / 1.4541 / 304 / 316
Gasket Options KLINGERsil C-4400 / PTFE / Silicone / Corrugated ring seal (high PN)
Visible Diameter (d1) 48 mm (DN40) to 150 mm (DN200) — see dimension table below
Quality Certificate EN 10204-3.1 Material Certificate (standard) / 3.2 available on request
Connection Type Weld-on base flange (pre-welded to vessel) + bolt-together cover flange assembly

Why Choose a Weld-Neck Sight Glass?

Indeed, the weld-neck design offers several structural and operational advantages over bolt-on or threaded sight glass alternatives — particularly in high-pressure and high-temperature services.

Advantage Details
✅ Permanent Pressure Integrity The welded base flange becomes part of the vessel wall — no risk of flange face leakage, bolt relaxation, or corrosion at the joint under thermal cycling
✅ DIN 28120 / 28121 Compliant Fully interchangeable with existing DIN 28120 and DIN 28121 installations — no vessel modification required
✅ Field Serviceable Glass disc and gasket are replaceable on-site without welding — simply unbolt the cover flange, swap the consumable components, and reseal
✅ Accessory Compatible Threaded ports on the flange accept explosion-proof sight glass luminaires, wipe systems, and protective covers from the Lumiglas® accessory range
✅ Wide Material Range Carbon steel for hydrocarbon service, or stainless steel 316 / 1.4571 for corrosive media, food, and pharmaceutical applications
✅ Custom Pressure Grades Standard PN10/25 from stock. Custom pressure-rated assemblies up to 70 MPa manufactured to order with full documentation

Standard Dimensions — Weld-Neck Sight Glass (DIN 28120)

Generally, dimensions follow DIN 28120 conventions. However, confirm exact values with the datasheet for pressure-rated assemblies.

Nominal Size (DN) Visible Diameter (d1) Glass Disc Diameter Pressure Rating (PN) Glass Standard
DN 40 48 mm ~80 mm PN 10 / 25 DIN 7080
DN 50 58 mm ~95 mm PN 10 / 25 DIN 7080
DN 80 82 mm ~125 mm PN 10 / 25 DIN 7080
DN 100 100 mm ~150 mm PN 10 / 25 DIN 7080
DN 125 120 mm ~175 mm PN 10 / 25 DIN 7080
DN 150 135 mm ~200 mm PN 10 / 25 DIN 7080
DN 200 150 mm ~265 mm PN 10 / 25 DIN 7080

* Custom sizes and higher pressure ratings up to 70 MPa available on request. Download the full datasheet for complete P-T curves and dimensional drawings.

🔗 Related Lumiglas® Vessel Sight Glass Products

Specifically, for the complete DIN 28120 flanged vessel sight glass assembly — including the bolted cover flange, glass disc, gasket set, and protective cover:

→ DIN 28120 Vessel Sight Glass — Complete Flanged Assembly (DN50–DN200) →

Alternatively, for DIN 28121 double-glass safety sight glass assemblies on high-hazard vessels:

→ DIN 28121 Pressure Vessel Sight Glass — Double Glass Safety Design →

Industrial Applications

Lumiglas® weld-neck sight glasses serve across all major process industries. Furthermore, because the base flange permanently becomes part of the vessel, operators can replace the glass and gasket during scheduled shutdowns. Consequently, this avoids any welding or vessel modification entirely.

Industry Typical Application
⚙️ Oil & Gas Separator vessels, surge tanks, knock-out drums — permanent weld-on installation for high-cycle pressure service
🔬 Chemical Processing Reactors, mixing vessels and agitated tanks — stainless steel flanges for corrosive service
💊 Pharmaceutical Bioreactors and sterile process vessels — EN 10204-3.1 certificates required, electropolished flanges available
🧪 Autoclave / Research High-pressure lab autoclaves and pilot reactors — custom assemblies up to 70 MPa available
🍺 Food & Beverage Fermentation tanks, dairy vessels — 316L stainless flanges with hygienic surface finish
🏭 Power & Energy Steam drums, deaerators, heat exchangers — aluminosilicate glass available for elevated temperature service

Frequently Asked Questions

What is a weld-neck sight glass?

Fundamentally, a weld-neck sight glass is a round viewport assembly where the base flange permanently welds to the vessel or tank wall. The cover flange then bolts over it, clamping the glass disc and gasket into a pressure-rated viewing window. Because the base flange becomes part of the vessel structure, this design delivers superior pressure integrity compared to threaded or loose-flange alternatives. Consequently, Lumiglas® weld-neck sight glasses follow DIN 28120 and DIN 28121 dimensional standards for full interchangeability.

What is the difference between DIN 28120 and DIN 28121?

Firstly, both standards define weld-on vessel sight glass assemblies. However, DIN 28121 specifies a double-glass safety design with two glass discs separated by a drainage space. Therefore, this makes DIN 28121 the preferred choice for hazardous media where a glass fracture would release toxic or flammable fluid. Conversely, for standard process vessels with non-hazardous media, DIN 28120 single-glass assemblies are generally sufficient. See our DIN 28120 vessel sight glass and DIN 28121 vessel sight glass pages for full specifications.

Which glass should I choose for high-temperature or high-pressure service?

Generally, borosilicate glass (DIN 7080) handles continuous service up to 280°C and suits most chemical and process applications. Specifically, for steam service or temperatures above 280°C, aluminosilicate glass (DIN 7081) is the correct choice. Moreover, for extreme high-pressure applications above PN 25, contact our technical team with your operating conditions — we will specify the correct glass thickness and gasket combination and provide the appropriate P-T derating curve.

Can I replace the glass without welding?

Yes — indeed, this is one of the key advantages of the weld-neck design. Since the base flange is already welded to the vessel, replacing the glass disc and gasket only requires unbolting the cover flange, swapping the consumable components, and retorquing to specification. Consequently, maintenance teams can complete glass replacement during a planned shutdown without any hot work permit or welding operations.

Do Lumiglas® weld-neck sight glasses come with material certificates?

Yes. Specifically, all Lumiglas® weld-neck sight glass assemblies ship with EN 10204-3.1 material certificates as standard, covering both the flange body and the glass disc. In addition, EN 10204-3.2 third-party witnessed inspection reports are available on request. Furthermore, further certifications including CE PED, ATEX, BV and Lloyd’s Register documentation can also be arranged for specific project requirements.

📖 Need help selecting the right vessel sight glass?
Read our complete Pressure Vessel Sight Glass Selection Guide — covering DIN 28120, DIN 28121, ASME and JIS standards, glass material selection, and pressure-temperature rating guidance.Alternatively, browse the complete DIN 28120 vessel sight glass range →

Ready to Specify Your Weld-Neck Sight Glass?

Send us your DN size, pressure class, process fluid and temperature.
Lumiglas® will confirm the correct glass and gasket specification within 24 hours.

Request Quote — Respond Within 24 Hours →

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