Flow sight glass

DIN 11869:2016-08, "Flow Sight Glasses," is a key industrial standard published by the German Institute for Standardization in August 2016. This standard consolidates and replaces several previous related standards (DIN 3236-1:1994-10, DIN 3236-2:1996-02, DIN 3237-1:1994-10, DIN 3237-2:1994-10, and DIN 3237-3:1996-03), marking a significant advancement in the standardization of flow observation devices. This standard was jointly developed by the DIN Standardization Committee for Valves (NAA) and the Chemical Equipment Manufacturing Standardization Committee (FNCA). Although flow sight glasses are not considered valves according to the definition in DIN EN 736-1, their technical requirements can be applied in accordance with the requirements for metal valves in DIN EN 16668.

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Standard Scope and Core Parameters

This standard applies to circular flow sight glasses installed in piping systems, with connections including flanges, threads, welded sleeves, or butt welds. The standard covers pressure classes (PN and Class): PN 16, PN 25, PN 40, Class 150, and Class 300; and nominal diameters (DN) range from DN 15 to DN 100, for a total of nine sizes. This broad coverage ensures the standard's applicability in most industrial applications.

Pressure Rating	Maximum Allowable Pressure (bar)	Temperature Range (℃)	Applicable Medium
PN 16/Class 150	16	-20~120	Water, Steam, Oil
PN 25	25	-20~200	Chemical Medium, Gas
PN 40/Class 300	40	-20~400	High-temperature and high-pressure media

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Material Technical Requirements and Selection Guide

4.2.2 Material Specifications

The standard sets clear requirements for the materials of each component of the flow sight glass: All internal parts that come into contact with the media must be made of materials that are sufficiently corrosion-resistant to the media. Specific material options include:

Casing and mounting flange: According to DIN EN 16668, carbon steel, stainless steel, or special alloys are generally used. Material certification and corrosion performance data must be provided.

Sight glass: Three types of glass materials can be used:

• Borosilicate glass (designator X) in accordance with DIN 7080
• Soda-lime glass (designator Y) in accordance with DIN 8902
• Metal-sealed glass pane (designator Z) in accordance with DIN 7079-1

Bolts and nuts: According to DIN EN 16668, they must be compatible with the housing material to ensure sufficient preload under operating conditions.

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Sealing system design and structural requirements

4.2.1 Structural design

The standard specifies two main sealing system designs: the primary seal (in accordance with DIN 28120) and the secondary seal (in accordance with DIN 28121). While the primary sealing system achieves sealing through direct mechanical pressure, the secondary sealing system utilizes elastic sealing elements, such as the compensating ring and seal, shown in Figure 2, which form a durable elastic sealing system. In the secondary sealing design, a test hole (5mm diameter) must be provided in the fixed flange for monitoring the sealing status and performing pressure testing. All fixed flanges must be individually secured to the housing to ensure uniform sealing pressure and reliable performance.

Seal type	Standard basis	Features	Applicable scenarios
Main seal	DIN 28120	Mechanical pressure seal, simple structure	Normal temperature and pressure conditions
Auxiliary seal	DIN 28121	Elastic seal, strong compensation capability	Large temperature and pressure changes

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Pressure-temperature correspondence and restrictions

4.2.3 Pressure-temperature ratings

The standard has formulated detailed pressure-temperature correspondence according to different materials:

Steel housing: Must comply with DIN EN 12516-1 for the corresponding material group, the pressure-temperature rating is based on the material's high-temperature strength and low-temperature toughness.

Cast iron housing: Must comply with DIN EN 12516-4 requirements, but is not suitable for hydraulic or external impact conditions that may cause brittle fracture.

Low-temperature applications: When the intended minimum operating temperature is lower than the minimum value specified in DIN EN 12516-1, the maximum allowable pressure must not exceed the pressure corresponding to the lowest temperature in the corresponding table. When using a flow sight glass at temperatures below the corresponding table, the relevant requirements of DIN EN 16668 for pressure-containing parts must be met.

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Connection End Dimensions and Tolerance Requirements

4.2.4 Dimensions and Tolerances

The standard specifies the dimensions and tolerances for four connection methods:

Flange connection (designator FL): Must comply with the DIN EN 1092 series (PN designation) or DIN EN 1759 series (Class designation). The structural length (FTF) and its tolerances are selected in accordance with DIN EN 558.

Butt-weld connection (designator AE): Complies with DIN EN 12627, the groove form is in accordance with DIN EN ISO 9692-1, and the structural length (ETE) is selected in accordance with DIN EN 12982.

Welding sleeve connection (designator SM): Complies with DIN EN 12760. The structural length (ETE) is selected in accordance with the same standard.

Threaded connection end (identifier GE): The internal thread must comply with Rc and Rp types of ISO 7-1, G type of DIN EN ISO 228-1 or NPT type of ASME B1.20.1. The structural length (ETE) shall be selected in accordance with DIN EN 16722.

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Test Verification and Quality Assurance

Chapter 5 Test Methods

All flow sight glasses must be tested in accordance with DIN EN 16668, including:

Shell strength verification: Proof of the strength of pressure-containing parts shall be carried out in accordance with DIN EN 12516-1 (tabular method), DIN EN 12516-2 (calculation method) or DIN EN 12516-3 (test method).

Tightness test: The pressure shell must be tested for tightness in accordance with DIN EN 12266-1:2012-06 Appendix A.3 (P11) to ensure there is no leakage at the maximum allowable pressure.

Material verification: All materials must be accompanied by a certificate of conformity, and key components must undergo non-destructive testing.

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Manufacturer's Declaration and Labeling Requirements

Chapter 7 Labeling and Naming

The standard specifies a unified naming convention. For example, the full name of a flange-connected flow sight glass with a diameter of DN50, PN25, material GP240GH, PFA lining, and borosilicate glass (X) is: Durchflussschauglas DIN 11869—DN50—FL—PN25—GP240GH—PFA—X

Labeling Requirements: All flow sight glasses must be labeled in accordance with DIN EN 19, including key information such as the manufacturer's logo, material designation, and rated pressure value. Fixed flanges must be permanently marked with the manufacturer's logo, material name, and nominal pressure value.

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Implementation Recommendations and Application Guidelines

Purchasing Technical Specification Recommendations

According to the recommendations in Appendix A of the standard, the following technical information should be provided during procurement:

1. Standard Number DIN 11869
2. Housing Construction Type (Flange or Jacket Connection)
3. Nominal Diameter DN
4. Pressure Class (PN or Class)
5. Operating Temperature Range
6. Flow Rate or Volume Flow
7. Housing, Mounting Flange, and Cover Material
8. Sight Glass Material Type
9. Connection Type and Length Requirements
10. Auxiliary Connection Requirements
11. Transported Media Characteristics

Installation and Maintenance Recommendations

During installation, ensure that the sealing surfaces are clean and undamaged, and tighten the bolts evenly and diagonally to the specified torque. Regular inspections should focus on the glass surface condition, the integrity of the sealing system, and the legibility of the markings. When used in low-temperature environments, special attention should be paid to the risk of low-temperature brittleness of the material.

Technological Innovation Trends

With the development of materials science, new composite materials and coating technologies are expanding the application range of flow sight glasses. The integration of intelligent monitoring technologies, such as built-in sensors and remote monitoring systems, represents the future development direction. This standard provides a basic framework for these technological innovations while maintaining compatibility with existing industrial systems.