What is a blank flange?
In industrial piping systems, flanges are crucial connecting components, and a blind flange is a special type of flange. It looks like a regular flange, but it lacks a central opening and is primarily used to seal pipe or equipment connections. Simply put, the function of a blind flange is to “block” the pipe opening. It is widely used in industries such as petroleum, natural gas, chemical, power, water supply and drainage, and shipbuilding, and is an indispensable component of pressure piping systems.
Structural Characteristics of Blind Flanges
Blind flanges are similar in appearance to regular perforated flanges, but the biggest difference is:
| Item | Slip-On / Weld Neck Flange (Regular Flange) | Blind Flange |
| Center Hole | Yes | No |
| Function | Connects two pipe sections | Seals the end of a pipe |
| Fluid Passage | Allows fluid to pass through | Completely blocks flow |
The structural components include:
Flange body (solid)
Bolt holes (for fixing)
Sealing surface (for mating with gasket) Because there are no through holes, blind flanges bear end pressure when under pressure, therefore they are usually thicker and have higher strength requirements than ordinary flanges.
Main Functions of Blind Flanges
1. Sealing Pipe Ends
When a section of pipeline is not needed for further extension, a blind flange can be used to seal it. When expansion is needed later, simply remove the blind flange.
This method is more flexible than welding the pipe ends.
2. Equipment Maintenance Isolation
During equipment maintenance or repair, it is necessary to isolate a section of pipeline from the system to prevent media inflow. Blind flanges can:
✔ Block liquids or gases
✔ Improve maintenance safety
✔ Avoid dangers caused by misoperation
This is a standard safety measure in chemical plants and oil pipelines.
3. Pressure Testing Ends
After pipeline installation, water or air pressure tests are usually required. Blind flanges can be temporarily installed at the end of the pipeline to:
Seal the system
Withstand test pressure
Check for leaks
4. Temporary Sealing of Reserved Interfaces
In large-scale projects, many interfaces are reserved for future expansion. Blind flanges are typically used to seal these interfaces temporarily, and can be opened later if needed.
Common Types of Blind Flanges
Although blind flanges are inherently “closed,” they still come in different standards and sealing methods.
Classification by Pressure Rating
Class 150
Class 300
Class 600
Class 900
PN10 / PN16 / PN25, etc. (European Standard)
The higher the pressure, the thicker the blind flange usually is.
Classification by Standard
Common international standards include:
| Standard | Region | Characteristics |
| ASME B16.5 | United States / International | Inch-based system, commonly used for high-pressure applications |
| EN 1092-1 | Europe | Metric system, PN pressure rating series |
| JIS | Japan | JIS pressure classes such as 10K and 20K |
| GB/T | China | Chinese national standard system |
According to the sealing surface type:
The sealing surface of a blind flange determines its mating method with the gasket:
RF (Raised Face) – Most common
FF (Flat Face) – Flat face
RTJ (Ring Type Joint) – Ring type joint – High pressure and high temperature applications
Commonly Used Materials for Blind Flanges
Since blind flanges are often used in high-pressure or corrosive environments, the choice of materials is crucial.
| Material | Characteristics | Suitable Environment |
| Carbon Steel (A105) | High strength, low cost | General water, oil, and gas service |
| Stainless Steel 304/316 | Corrosion resistant | Chemical industry and seawater environments |
| Alloy Steel | Resistant to high temperature and high pressure | Power plant steam pipelines |
| Duplex Stainless Steel | Excellent resistance to chloride corrosion | Offshore platforms |
Why are blind flanges thicker than regular flanges?
This is a common question when people first see blind flanges.
Because regular flanges have a hole in the center, allowing fluid pressure to be transferred to downstream piping;
blind flanges, however, are “dead ends,” meaning all pressure is concentrated on the flange itself.
They act like a pressure-bearing “lid,”
therefore, they must be thicker to improve bending strength and safety.
Differences between Blind Flanges and Pipe Caps
Many people confuse blind flanges with pipe caps, but they have different uses:
| Item | Blind Flange | Pipe Cap |
| Installation Method | Bolted connection | Welded |
| Removability | ✅ Removable | ❌ Not easy to remove |
| Typical Application | Temporary or expandable systems | Permanent closure |
| Maintenance Convenience | High | Low |
Therefore, blind flanges are usually the preferred choice if the system can be modified in the future.
Precautions for installing blind flanges:
Select the correct pressure rating: It must not be lower than the pipeline design pressure.
Match suitable gaskets: such as graphite gaskets or spiral wound gaskets.
Tighten bolts symmetrically: to prevent uneven stress on the sealing surface.
Regularly check for corrosion and leaks.
A blind flange is a type of flange without holes, mainly used for sealing pipe ports, isolating equipment, pressure testing, and sealing pre-existing interfaces. It has advantages such as being removable, having high pressure resistance, and being easy to maintain, playing an important safety and functional role in industrial piping systems.
Simply remember: Ordinary flanges allow fluid to flow; blind flanges stop the fluid flow.
