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This article summarizes the definition, dimensions, types, functions, weight calculation, and applications of blind flanges.
A blind flange is a solid disk used to seal the end of a piping system, valve, or pressure vessel opening, effectively functioning as a removable cap or closure device. The blind flange is manufactured with no opening, featuring a solid face that provides positive isolation of the pipeline.
It is typically machined with the same bolt pattern and raised face dimensions as standard flanges, allowing it to bolt directly to another flange to close off a line permanently or temporarily.
The blind flange's design includes sealing faces in various configurations, such as flat face, raised face, ring-type joint, and tongue-and-groove, to accommodate different gasket types and pressure requirements.
The ASME B16.5 standard governs the dimensions of blind flanges for pipe sizes ranging from 1/2 inch to 24 inches, with pressure classes from 150 to 2500.
Below is a comprehensive dimension table for ASME B16.5 blind flanges.
Class 150 flanges are suitable for general service applications with lower pressure requirements. The following table provides complete dimensions for blind flanges according to ASME B16.5.
Nominal Pipe Size (NPS) | Outside Diameter (in) | Flange Thickness (in) | Raised Face Diameter (in) | Number of Bolt Holes | Bolt Hole Diameter (in) | Bolt Circle Diameter (in) | Approx. Weight (lbs) |
1/2 | 3.5 | 0.44 | 1.38 | 4 | 0.62 | 2.38 | 2 |
3/4 | 3.88 | 0.5 | 1.69 | 4 | 0.62 | 2.75 | 2 |
1 | 4.25 | 0.56 | 2.0 | 4 | 0.62 | 3.12 | 2 |
1 1/4 | 4.62 | 0.62 | 2.5 | 4 | 0.62 | 3.5 | 3 |
1 1/2 | 5.0 | 0.69 | 2.88 | 4 | 0.62 | 3.88 | 3 |
2 | 6.0 | 0.75 | 3.62 | 4 | 0.75 | 4.75 | 4 |
2 1/2 | 7.0 | 0.88 | 4.12 | 4 | 0.75 | 5.5 | 7 |
3 | 7.5 | 0.94 | 5.0 | 4 | 0.75 | 6.0 | 9 |
3 1/2 | 8.5 | 0.94 | 5.5 | 8 | 0.75 | 7.0 | 13 |
4 | 9.0 | 0.94 | 6.19 | 8 | 0.75 | 7.5 | 17 |
5 | 10.0 | 0.94 | 7.31 | 8 | 0.88 | 8.5 | 20 |
6 | 11.0 | 1.0 | 8.5 | 8 | 0.88 | 9.5 | 27 |
8 | 13.5 | 1.12 | 10.62 | 8 | 0.88 | 11.75 | 47 |
10 | 16.0 | 1.19 | 12.75 | 12 | 1.0 | 14.25 | 67 |
12 | 19.0 | 1.25 | 15.0 | 12 | 1.0 | 17.0 | 123 |
14 | 21.0 | 1.38 | 16.25 | 12 | 1.12 | 18.75 | 139 |
16 | 23.5 | 1.44 | 18.5 | 16 | 1.12 | 21.25 | 187 |
18 | 25.0 | 1.56 | 21.0 | 16 | 1.25 | 22.75 | 217 |
20 | 27.5 | 1.69 | 23.0 | 20 | 1.25 | 25.0 | 283 |
24 | 32.0 | 1.88 | 27.25 | 20 | 1.38 | 29.5 | 415 |
For higher pressure applications, Class 300 flanges are required. They feature thicker walls and different bolt patterns compared to Class 150.
Nominal Pipe Size (NPS) | Outside Diameter (in) | Flange Thickness (in) | Raised Face Diameter (in) | Number of Bolt Holes | Bolt Hole Diameter (in) | Bolt Circle Diameter (in) | Approx. Weight (lbs) |
1/2 | 3.75 | 0.56 | 1.38 | 4 | 0.63 | 2.62 | 2 |
3/4 | 4.62 | 0.63 | 1.69 | 4 | 0.75 | 3.25 | 3 |
1 | 4.88 | 0.69 | 2.00 | 4 | 0.75 | 3.50 | 3 |
1 1/4 | 5.25 | 0.75 | 2.50 | 4 | 0.75 | 3.88 | 4 |
1 1/2 | 6.13 | 0.81 | 2.88 | 4 | 0.88 | 4.50 | 6 |
2 | 6.50 | 0.88 | 3.63 | 8 | 0.75 | 5.00 | 8 |
2 1/2 | 7.50 | 1.00 | 4.13 | 8 | 0.88 | 5.88 | 12 |
3 | 8.25 | 1.13 | 5.00 | 8 | 0.88 | 6.63 | 16 |
3 1/2 | 9.00 | 1.19 | 5.50 | 8 | 0.88 | 7.25 | 21 |
4 | 10.00 | 1.25 | 6.19 | 8 | 0.88 | 7.88 | 27 |
5 | 11.00 | 1.38 | 7.31 | 8 | 0.88 | 9.25 | 35 |
6 | 12.50 | 1.44 | 8.50 | 12 | 0.88 | 10.63 | 50 |
8 | 15.00 | 1.63 | 10.63 | 12 | 1.00 | 13.00 | 81 |
10 | 17.50 | 1.88 | 12.75 | 16 | 1.13 | 15.25 | 124 |
12 | 20.50 | 2.00 | 15.00 | 16 | 1.25 | 17.75 | 185 |
14 | 23.00 | 2.13 | 16.25 | 20 | 1.25 | 20.25 | 250 |
16 | 25.50 | 2.25 | 18.50 | 20 | 1.38 | 22.50 | 295 |
18 | 28.00 | 2.38 | 21.00 | 24 | 1.38 | 24.75 | 395 |
20 | 30.50 | 2.50 | 23.00 | 24 | 1.38 | 27.00 | 505 |
24 | 36.00 | 2.75 | 27.25 | 24 | 1.63 | 32.00 | 790 |
Higher pressure classes (400, 600, 900, 1500, 2500) have significantly larger dimensions and thicker walls.
Class | NPS | Outside Diameter (in) | Thickness (in) | Bolt Circle (in) | Number of Holes | Hole Diameter (in) |
400 | 4 | 10.0 | 1.19 | 7.88 | 8 | 0.88 |
6 | 12.5 | 1.44 | 10.63 | 12 | 0.88 | |
8 | 15.0 | 1.63 | 13.0 | 12 | 1.0 | |
12 | 20.5 | 2.0 | 17.75 | 16 | 1.25 | |
600 | 4 | 10.75 | 1.5 | 8.5 | 8 | 0.88 |
6 | 14.0 | 1.56 | 11.5 | 12 | 0.88 | |
8 | 16.5 | 1.88 | 13.75 | 12 | 1.0 | |
12 | 22.0 | 2.38 | 19.25 | 20 | 1.25 | |
900 | 4 | 12.0 | 1.75 | 9.5 | 8 | 1.0 |
6 | 15.5 | 2.19 | 12.5 | 12 | 1.12 | |
8 | 18.5 | 2.5 | 15.5 | 12 | 1.25 | |
12 | 24.0 | 3.13 | 21.0 | 20 | 1.38 | |
1500 | 4 | 12.25 | 2.5 | 9.5 | 8 | 1.12 |
6 | 15.5 | 3.25 | 12.5 | 12 | 1.38 | |
8 | 19.0 | 3.63 | 15.5 | 12 | 1.62 | |
12 | 28.0 | 4.63 | 23.0 | 16 | 2.12 | |
2500 | 4 | 14.0 | 3.63 | 10.75 | 8 | 1.38 |
6 | 19.0 | 5.0 | 15.0 | 8 | 1.88 | |
8 | 21.5 | 5.75 | 17.25 | 12 | 2.0 | |
12 | 30.0 | 7.5 | 25.0 | 12 | 2.62 |
Standard Raised Face Blind Flange: The most common type features a raised surface around the sealing area to concentrate gasket compression and ensure a tight seal.
The raised face (RF) blind flange has a circular raised portion machined on the sealing surface, typically 1/16 inch high for Class 150 and 300 and 1/4 inch high for higher classes. It is suitable for a wide range of pressures and temperatures and accommodates various gasket materials, including spiral wound, ring joint, and soft cut gaskets.
Flat Face Blind Flange: Designed with a completely flat sealing surface for use with flat-faced mating flanges and soft gaskets, primarily in low-pressure applications.
The flat face (FF) blind flange features no raised portion, providing a full, flat contact surface. The complete contact area distributes bolt load more evenly, reducing stress on fragile flange materials. These are commonly found in water treatment, low-pressure HVAC, and fire protection systems.
Ring-Type Joint Blind Flange: Engineered with grooves that accommodate metal ring gaskets for ultra-high-pressure and high-temperature services.
The RTJ blind flange contains a specially machined groove that accepts a metal ring gasket. When bolt tension is applied, the gasket compresses into the groove walls, creating a tight metal-to-metal seal capable of withstanding extreme pressures and temperatures.
These flanges are essential in oil and gas production, refinery operations, and high-pressure chemical processing where leak integrity is paramount and where temperature fluctuations could compromise non-metallic gaskets.
Figure-8 Blind Flange: A combination unit with a solid blind section and a spacer section connected by a thin web, allowing quick conversion between open and closed positions.
The Figure-8 Blind Flange, also called spectacle blind, consists of two metal disks connected by a section of metal resembling the number eight. One disk is solid (the blind), while the other has a bore matching the pipe diameter (the spacer).
By removing, rotating, and reinstalling the assembly, operators can quickly switch between closed and open positions without searching for separate components. This design is invaluable in applications requiring frequent line isolation, such as filter systems, parallel pump arrangements, and maintenance bypass configurations.
Butt Weld Blind Flange: It features a tapered hub designed for welding directly to a pipe.
Although less common than other types, the butt weld blind flange incorporates a long tapered hub that transitions the flange to pipe wall thickness gradually. It minimizes stress concentrations at the weld joint, making it ideal for cyclic loading conditions, severe service applications, and extreme temperature variations. The weld preparation on the hub end matches standard pipe bevels, allowing full penetration welding for maximum joint integrity.
The primary function of a blind flange is isolation. However, its utility extends far beyond simply "plugging a hole."
System Termination: It provides a safe, pressure-rated end to a piping system.
Maintenance Access: By unbolting a blind flange, technicians can gain access to the interior of the pipeline for cleaning, inspection, or repairs without needing to cut or weld.
Pressure Testing: During the construction of a new pipeline, blind flanges are used to seal sections so that hydrotesting can be performed to verify the integrity of welds and joints.
Future Expansion: Engineers often install a "tee" with a blind flange at the end. This allows for a future pipeline connection to be made simply by removing the blind and bolting on a new pipe run, minimizing downtime.
Class 150 Blind Flange Weight Table (Stainless Steel, ASTM A182)
Below is a comprehensive weight table for stainless steel blind flanges based on ASME B16.5 Class 150 dimensions, with weights calculated using a density of 0.285 lb/in³ (7,890 kg/m³).
Nominal Pipe Size (inches) | Outside Diameter (mm) | Thickness (mm) | Weight (kg) - Stainless Steel |
1/2 | 90 | 11.2 | 0.9 |
3/4 | 100 | 12.7 | 1.2 |
1 | 110 | 14.2 | 1.5 |
1¼ | 115 | 15.7 | 1.9 |
1½ | 125 | 17.5 | 2.3 |
2 | 150 | 19.1 | 3.2 |
2½ | 180 | 22.4 | 5.0 |
3 | 190 | 23.9 | 6.5 |
4 | 230 | 23.9 | 9.5 |
6 | 280 | 25.4 | 17.0 |
8 | 345 | 28.4 | 29.0 |
10 | 405 | 30.2 | 42.0 |
12 | 485 | 31.8 | 68.0 |
14 | 535 | 35.1 | 86.0 |
16 | 595 | 36.6 | 116.0 |
18 | 635 | 39.6 | 145.0 |
20 | 700 | 42.9 | 185.0 |
24 | 815 | 47.8 | 270.0 |
For blind flanges not listed in standard tables, weight can be calculated using geometric formulas. The fundamental principle is weight = volume × material density.
Simplified Formula for Blind Flanges:
For quick estimation, use this empirical formula widely accepted in the industry:
Weight (kg) = (Outside Diameter² × 6.17 × Thickness) / 1,000,000
Where dimensions are in millimeters and the result is in kilograms. This formula assumes a solid disk without bolt hole adjustments.
Precise Calculation Method:
For more accurate calculations, use the following approach:
Calculate the volume of the solid disk:
Volume = π × (OD/2)² × T
Subtract volume of bolt holes (if significant):
Bolt hole volume = Number of holes × π × (Bolt Hole Diameter/ 2)² × T
Net Volume = Solid disk volume - Bolt hole volume
Weight = Net Volume × Material Density
Material density values :
Carbon steel: 7.85 g/cm³ (0.284 lb/in³)
Stainless steel (304/316): 7.90-8.03 g/cm³ (0.285-0.290 lb/in³)
Alloy steel: 7.85 g/cm³ (0.284 lb/in³)
Example Calculation:
For a 24-inch Class 150 stainless steel blind flange:
OD = 815 mm, T = 47.8 mm
Solid disk volume = π × (815/2)² × 47.8 = π × 166,056 × 47.8 = 24,920,000 mm³
Bolt hole volume (20 holes × 35 mm diameter) = 20 × π × (35/2)² × 47.8 = 20 × 962 × 47.8 = 919,000 mm³
Net volume = 24,001,000 mm³ = 24,001 cm³
Weight = 24,001 × 7.90 g/cm³ = 189,608 g = 189.6 kg
This closely matches the table value of 189 kg for this size.
Blind flanges are ubiquitous in industries where fluid or gas transport is central to operations.
Oil and Gas Industry: Used to seal off high-pressure wellheads and manifold systems. In these environments, duplex stainless steel or Nickel Alloy 625 is often preferred for its resistance to sour gas and chloride stress corrosion cracking.
Chemical Processing: Blind flanges act as "manways" on chemical reactors and storage tanks, allowing for periodic internal cleaning and vessel entry.
Water Treatment: Used in large-scale desalination and filtration plants to manage the flow of seawater and treated water.
Power Generation: In steam lines and cooling systems, blind flanges provide the necessary isolation points for safety.
How to Calculate Blind Flange Thickness
Blind flange thickness is calculated using the formula.
t = d √(CP/SE + 1.9 W hg / SE d³), where d is the inside diameter or short span, C = 0.3 for bolted flat covers, P is design pressure, S is allowable stress, E is joint efficiency (typically 1), W is bolt load, and h_g is gasket moment arm.
Do Blind Flanges Require Hydrotesting?
Blind flanges manufactured to ASME B16.5 do not require hydrotesting as part of standard production, as the code exempts flanges but not flanged fittings. However, if used in assemblies or specified by clients, hydrotesting may be performed to verify seals under pressure.
How Do You Describe a Blind Flange in Spanish?
A blind flange is described in Spanish as "brida ciega," referring to a solid disk that seals pipe ends without a central bore. It bolts in place for isolation or termination, ensuring no flow. Common in technical contexts, it's "brida ciega elevada" for raised face types.
How Much Does a 12,300mm Blind Flange Weigh?
Assuming "12,300mm" refers to a 12-inch Class 300 blind flange, the weight is approximately 115-184 pounds (52-83 kg) for carbon steel, depending on exact specifications.
How to Measure a Blind Flange?
Measure the outside diameter from edge to edge, the bolt circle diameter from the center of one bolt hole to the opposite, the raised face diameter if applicable, and the thickness from face to back, and count the bolt holes while noting their diameter and spacing.
Use calipers for precision, ensuring even distribution for ASME B16.5 compliance.
How to Make a Blind Flange?
Blind flanges are made by forging or cutting from plate stock, then machining to specifications. Start with material selection, heat to forging temperature, shape into a disk, drill bolt holes, machine faces, and finish with threading if needed. Quality checks include dimensional verification and nondestructive testing for defects, ensuring adherence to ASME B16.5.
