What Is a Blind Flange – Blind Flange Dimensions Chart in MM & Inch (Class 150 to 2500)

Whether you’re an engineer, technician, or someone involved in industrial operations, knowing what a blind flange is and how it works can make a significant difference in your projects. We’ll explore the definition, types, applications, and dimensions of blind flanges, ensuring you have all the necessary knowledge to select and use them effectively in your piping systems.

What Is a Blind Flange?

Blind flanges are crucial components in piping systems designed to block off sections of pipelines or vessel openings. They are essentially solid discs used to cover or close the end of a pipe, valve, fitting, or equipment nozzle. Blind flanges play a vital role in terminating or isolating piping systems, especially during non-destructive testing such as hydrostatic testing. They offer the flexibility to expand the piping system or install new equipment in the future. Proper selection and installation of blind flanges are essential to ensure the integrity and safety of the piping system, as they must withstand factors such as pressure, temperature, and pipe size.

Blind Flange vs Blank Flange, What Are the Differences

A blank flange is a flat, usually circular plate with no openings. It’s used to cover or close the end of a pipe, valve, or vessel. Blank flanges are commonly used in piping systems for maintenance, testing, or when a future connection might be required. They’re available in various materials and sizes to suit different applications. Below are some differences between blank flanges and blind flanges.

Types of Blind Flanges

Blind flanges come in many different designs, each has its own advantages or serves specific uses.
1. Spectacle Blind Flange
The spectacle blind flange, also known as a figure-8 flange or spectacle blind, is a specialized component used in piping systems to control and isolate fluid flow. It consists of two metal discs connected by a small section of steel, forming a figure-8 shape. One disc is solid, which blocks the flow when aligned with the pipeline, while the other disc has a hole that allows fluid to pass through. By rotating the spectacle blind flange, the solid disc can be aligned with the pipeline opening to block flow or positioned to allow flow. This design allows for easy switching between open and closed states without the need to remove bolts. This type is ideal for maintenance or shutdown purposes, enabling quick isolation of pipeline sections for inspection or repair.

2. Line Blind Flange
A line blind flange, also known as a paddle blind flange, features a paddle-like insert that can be inserted or removed without disassembling the entire piping system. Line blind flanges consist of two main components: a solid blind flange, which blocks the flow of fluids or gases, and a spacer that maintains alignment when the blind flange is removed. The solid blind flange is placed between two open flanges to stop the flow through a pipe, while the spacer allows for flow when the blind flange is removed. This design allows for frequent maintenance, cleaning, or inspection without the need for extensive disassembly. Line blind flanges are commonly used in applications where frequent access to the pipeline is required, such as in chemical plants or refineries. They are also used in compliance with standards like ASME B16.48, which covers pressure, temperature ratings, materials, dimensions, and testing for operational line blanks.

3. Raised Face Blind Flange
A raised-face blind flange (RF blind flange) is a specialized type of blind flange designed for high-pressure and high-temperature applications. It features a raised ring or face around the borehole, which enhances its sealing capability by concentrating pressure on a smaller gasket area when mated with a similarly raised face flange. This design allows for a stronger and more reliable seal, making it ideal for industries such as oil and gas, chemical processing, and power generation where robust sealing is crucial.

4. Flat Face Blind Flange
A flat-face blind flange is a type of flange with a completely flat surface, designed to create a seal when bolted to a mating flat-face flange. It lacks any raised or recessed features on the sealing surface, it is suitable for low-pressure applications where full contact with a gasket is needed. These flanges are commonly used in plumbing systems, water treatment facilities, and other industrial settings where simplicity and ease of installation are important. The straightforward installation process involves aligning the flange with the pipe’s bolt holes, placing a gasket between them, and tightening the bolts evenly to ensure a secure seal.

5. Ring Type Joint (RTJ) Blind Flange
A Ring-type joint blind flange features a precision-machined groove on its face that accommodates a metal ring gasket, typically made from materials like soft iron, stainless steel, titanium, or other alloys. When bolted to a mating flange, the metal ring gasket compresses into the groove, creating a reliable, leak-proof metal-to-metal seal that can withstand extreme conditions. RTJ Blind Flanges are particularly valuable in critical applications within industries like oil and gas, chemical processing, and power generation, where their ability to handle pressures up to 20,000 psi and temperatures exceeding 750°C makes them indispensable for maintaining system integrity and safety.

Blind Flange Dimensions & Size Chart in MM and Inches (ANSI/ASME B16.5)

Class 150 for a blind flange refers to its pressure rating classification according to the ASME B16.5 standard. This classification indicates that the flange is designed to withstand a specific pressure level, which is the lowest among the various classes (150, 300, 400, 600, 900, 1500, and 2500) defined by this standard.

On the drawing,

A = Overall Diameter

C = Flange Thickness

G = Face Diameter

I = Bolt Hole Diameter

J = Diameter of Circle of Holes

Blind Flange Dimensions Class 150

Blind Flange Dimensions Class 300

Blind Flange Dimensions Class 400

Blind Flange Dimensions Class 600

Blind Flange Dimensions Class 900

Blind Flange Dimensions Class 1500

Blind Flange Dimensions Class 2500