A blind flange is a solid disc with no center hole, used to seal the end of a pipe, valve, or pressure vessel. Sometimes called a closure plate flange, it bolts onto an open pipe end to block flow completely. Unlike a cap that gets welded on permanently, a blind flange can be unbolted and removed whenever you need to reopen the line for inspection, maintenance, or future expansion.
How a Blind Flange Works
Every other type of flange has a bore (a hole through the center) so fluid or gas can pass through. A blind flange has none. It’s a flat, round plate with bolt holes around its perimeter that lines up with the matching flange on the pipe end. A gasket sits between the two faces, and when you tighten the bolts, the gasket compresses to create a leak-proof seal.
This makes blind flanges useful in several situations. They permanently terminate a pipeline that doesn’t connect to anything else. They temporarily isolate a section of piping during maintenance or modification. And they serve as test boundaries during pressure testing, where they cap off a section so it can be pressurized with water or air without stressing valves or other sensitive components.
Because they’re bolted rather than welded, blind flanges are the go-to choice when you might need access to the pipe interior later. If a pipeline needs to be extended in the future, removing a blind flange is far simpler than cutting off a welded cap.
Face Types and Sealing
The face of a blind flange is the flat surface where the gasket sits, and it comes in three main configurations. The right choice depends on the pressure, temperature, and what the pipe carries.
- Raised Face (RF) is the most common in process plants. The gasket surface sits slightly higher than the bolting circle, which concentrates bolt pressure onto a smaller area and improves the seal. For pressure classes 150 and 300, the raised portion is about 1.6 mm (1/16 inch) tall. For higher classes (400 through 2500), it increases to about 6.4 mm (1/4 inch).
- Flat Face (FF) has the gasket surface flush with the bolt circle. These are typically paired with cast iron flanges or fittings. A flat face flange should never be bolted to a raised face flange, because the height difference can crack the thinner cast iron piece. If you need to connect the two, the raised face must be machined down and a full-face gasket used.
- Ring Type Joint (RTJ) has a precision-machined groove cut into the face that accepts a metal ring gasket. When the bolts are tightened, the soft metal ring deforms into the groove, creating a metal-to-metal seal. RTJ faces are used in high-pressure applications (Class 600 and above) and high-temperature services above 800°F (427°C).
Sizes and Pressure Ratings
Blind flanges are manufactured to cover a wide range of pipe sizes and pressure classes. The two main standards that govern their dimensions are ASME B16.5 for sizes 1/2 inch through 24 inches, and ASME B16.47 for large-diameter flanges from 26 inches up to 60 inches.
Within each size, flanges are rated by pressure class: 150, 300, 400, 600, 900, 1500, and 2500. A higher class means a thicker, heavier flange that can handle greater pressure. For example, a 12-inch Class 300 blind flange has an outer diameter of about 520 mm, weighs roughly 82.5 kg (182 lbs), and uses 16 bolts. A 24-inch version in the same class jumps to about 914 mm across and 350 kg (772 lbs) with 24 bolts. Large-diameter blind flanges covered by ASME B16.47 can be made from cast, forged, or plate materials and carry class ratings from 75 up to 900.
The thickness of a blind flange matters more than on other flange types because it has no pipe wall to help resist internal pressure. The entire force of the system pushes against that solid disc, so blind flanges are noticeably thicker than their bored counterparts at the same pressure rating.
Common Materials
Most blind flanges in oil, gas, and industrial piping are made from carbon steel or stainless steel. Carbon steel (typically forged to the ASTM A105 specification) handles the majority of general-purpose applications where corrosion isn’t a major concern. For pipelines carrying corrosive fluids, acids, or seawater, stainless steel grades like 304L and 316L are standard choices. Alloy steel flanges (ASTM A182) fill the gap for high-temperature or high-pressure services where carbon steel alone isn’t strong enough.
Tapped Blind Flanges
Some blind flanges come with one or two small threaded holes drilled and tapped into the face. These connections, usually 1/2 inch, allow you to attach a pressure gauge, a drain valve, or a vent line without removing the flange. Tapped blind flanges used for metered connections are typically supplied with a minimum 300 class rating to ensure there’s enough metal thickness around the tapped hole to maintain structural integrity.
Installation Basics
Proper installation of a blind flange follows the same bolt-up procedure as any flanged joint, but because the flange takes the full brunt of line pressure, getting it right is especially important.
Before assembly, both flange faces need to be inspected for scratches, corrosion pitting, dirt, and tool marks. RTJ grooves in particular must be clean and undamaged. The bolt holes should align freely without forcing. Any bolts or nuts with damaged threads should be replaced, not reused. Gaskets (especially soft material and spiral wound types) should always be new when reassembling a joint.
Bolts are tightened using a torque wrench in a criss-cross pattern, not sequentially around the circle. This distributes pressure evenly across the gasket and prevents the flange from cocking to one side. The process uses a minimum of three passes: the first pass typically goes to about 30% of the final torque value, with each subsequent pass increasing until the target is reached. Anti-seize lubricant on the bolt threads ensures accurate torque readings and makes future disassembly easier.
Flange faces should be checked for parallel alignment before any bolts are tightened. Even small misalignment can prevent the gasket from sealing properly, which under high pressure can lead to a dangerous leak.