PN3 Socket Head Bolt Flange Sight Glass

The Borlan Fluid PN3 socket head bolt flange sight glass is a compact circular observation window for tanks, vessels, pipelines and process equipment. Its low-profile cover uses recessed internal-hex fasteners, helping keep the top surface compact while securing the glass and sealing components. The 2025 Borlan catalog lists sizes from DN40 to DN300 in 304 or 316L stainless steel.

PN3 is the catalog pressure-class designation for this product family. Final suitability still depends on nominal size, glass, gasket, body material, operating temperature, vacuum, pressure cycling, installation interface and applicable project rules. Borlan Fluid reviews these conditions before confirming the quotation and production drawing.

What Is a PN3 Socket Head Bolt Flange Sight Glass?

This product is a bolted flange observation assembly. A transparent circular window is retained between machined stainless-steel sections and sealed for direct viewing into the equipment. The fasteners are socket head screws, sometimes described as internal-hex bolts. Compared with protruding external-hex nuts, the recessed fastening form gives the cover a compact, orderly profile.

The PN3 model belongs to Borlan Fluid’s conventional flange sight glass range. It is different from a fused glass-to-metal window and from a sanitary clamp sight glass. Buyers selecting among these constructions can compare them in our industrial sight glass types guide.

Key Product Features

• Compact socket head fasteners: recessed internal-hex screws secure the cover while limiting protrusions above the flange face.
• Direct visual access: the circular window supports routine checks of liquid level, flow, foam, color and process condition.
• DN40-DN300 catalog range: ten catalog size rows cover compact and larger observation points.
• 304 or 316L stainless steel: the catalog lists both grades; the order must state the required grade and wetted-part scope.
• Replaceable bolted construction: glass and sealing components can be accessed during planned maintenance after the equipment is safely isolated and depressurized.
• Project-specific review: the sight glass is checked against the medium, temperature, pressure, vacuum, cleaning method and installation drawing before production.
• Optional variants: wiper or other customized configurations may be reviewed separately and are not included in the standard product unless stated in the quotation.

Technical Specifications

Catalog Reference Dimensions

All dimensions are catalog references in millimeters. Confirm the latest approved drawing, glass thickness, gasket, fastener specification, weld or mounting interface and required clear opening before fabrication or installation.

How the Socket Head Bolt Design Helps

Socket head screws place the driving feature inside the fastener head. This supports a lower-profile top surface and gives technicians direct tool access during controlled assembly or maintenance. The design does not remove the need for correct bolt loading. Tightening must follow the approved sequence and torque or assembly procedure for the selected size, glass and gasket.

Uneven tightening can create local stress in the window or gasket. Fasteners should be tightened progressively in a cross pattern, with the mating surfaces clean, flat and aligned. Do not use the bolts to pull a distorted flange into position. After commissioning, inspect the assembly according to the equipment maintenance plan and replace damaged glass, seals or fasteners with approved components.

Material, Glass and Seal Selection

The catalog lists 304 and 316L stainless steel. The correct choice depends on the process medium, cleaning chemicals, chloride exposure, corrosion allowance and required material documents. State whether the material requirement applies to every metal component or only to wetted parts. If the project needs a material certificate or heat-number traceability, include that requirement in the inquiry.

Glass and gasket selection are equally important. Provide the minimum and maximum process temperature, operating and design pressure, vacuum, temperature cycling, cleaning procedure and chemical exposure. A gasket must be compatible with the medium and remain stable across the expected temperature range. The window must not be exposed to point loading, impact, severe misalignment or an uncontrolled thermal shock.

Typical Applications

• Mixing and batching tanks
• Reactors, receivers and process vessels
• Pipeline and equipment observation points
• Food, beverage and cosmetic processing equipment, subject to hygiene requirements
• Chemical and water-treatment equipment, subject to material compatibility
• General industrial systems requiring a compact circular viewing window

These examples describe common uses rather than guaranteed suitability. Hygienic service, corrosive media, vacuum, pressure cycling or rapid temperature change may require a different construction or additional documentation. Our sanitary versus industrial sight glass guide explains the extra selection checks for clean-process applications.

Installation and Maintenance Guidance

1. Verify the sight glass model, size, materials and approved drawing before installation.
2. Check that the mounting surface is flat, clean, aligned and free from weld distortion.
3. Inspect the glass and gasket for chips, cracks, contamination, hardening or deformation.
4. Assemble using the specified fasteners and tighten progressively in a cross pattern.
5. Prevent pipe strain, vessel distortion or nearby loads from acting on the observation window.
6. Pressure-test and inspect according to the approved equipment procedure.
7. Before maintenance, isolate, drain, cool and fully depressurize the equipment.

Inspection intervals should reflect the medium, temperature, pressure cycles, cleaning frequency and consequence of leakage. Replace any window that shows cracks, edge damage, deep scratches, clouding or other unacceptable deterioration. Do not reuse a gasket unless the approved maintenance procedure expressly permits it.

Why Work with Borlan Fluid?

Borlan Fluid manufactures conventional flange, union, pipeline and fused sight glass products as well as sight glass lights and customized vessel accessories. For a PN3 flange sight glass inquiry, our team can review the equipment interface, viewing requirement, material, glass, gasket, operating conditions and documentation before confirming the supply scope.

For product demonstrations and factory updates, visit the Borlan Fluid YouTube channel and LinkedIn page. Product-specific videos should be matched to the same size and configuration before they are used as technical evidence.

Frequently Asked Questions

What does PN3 mean on this product page?

PN3 is the Borlan catalog designation for this flange sight glass family. The allowable working conditions of a completed assembly must still be confirmed for the selected size, temperature, glass, gasket, material and installation.

Why is it called an internal-hex or socket head bolt sight glass?

The fastening screws have a recessed hexagonal drive. “Socket head bolt flange sight glass” is the clearer English product name; “internal hex flange sight glass” may also appear in buyer searches.

Which sizes are listed in the catalog?

The 2025 catalog lists DN40, DN50, DN65, DN80, DN100, DN125, DN150, DN200, DN250 and DN300.

Is the product available in 316L stainless steel?

Yes. The catalog lists 304 and 316L. Confirm the exact grade, wetted-part scope, surface requirement and material documentation in the order.

Can a wiper be added?

A wiper configuration may be reviewed separately for suitable sizes and applications. It is not part of the standard PN3 sight glass unless it is included in the quotation and approved drawing.

What information is needed for a quotation?

Send the DN size, equipment drawing, medium, normal and maximum pressure, vacuum, minimum and maximum temperature, material grade, glass and gasket requirements, cleaning method, viewing objective, quantity, destination country and required inspection documents.

Request a Quote

Send Borlan Fluid your DN size, equipment or mounting drawing, medium, pressure or vacuum, temperature range, material grade, glass and gasket requirements, cleaning method, quantity, destination country and documentation requirements. We will review the PN3 socket head bolt flange sight glass configuration and prepare a quotation based on the confirmed scope.