Stainless Steel Blind Flanges

A blind flange is the only flange that connects to nothing. There is no bore, no pipe, no weld, just a solid disc with a bolt pattern, and its job is to close an opening that may one day need to open again. That last part is what separates it from a welded cap, and it is usually why a blind is on the drawing rather than a cap. Solitaire Overseas supplies stainless steel blind flanges from India, Class 150 through 2500.

Part of our stainless steel flanges range.

Technical Data

Why a Blind Rather Than a Welded Cap? Specifications Grades

Why a Blind Rather Than a Welded Cap?

Both close off an opening. The difference is whether it ever reopens:

	Blind Flange	Welded Cap
Joint	Bolted to a mating flange	Welded to the pipe
Removal	Undo the bolts	Cut it off
Reopening	Routine	Hot work, and a new cap
Use when	The opening may be reopened, tested through, or extended later	The closure is permanent
Cost in place	Higher, needs a mating flange, gasket and bolting	Lower

The decision is about the future, not the present. A blind costs more to install because it needs a mating flange, gasket and bolt set. It earns that back the first time someone needs access without a hot work permit. If the drawing shows a blind where a cap would do, the designer is usually anticipating a tie-in, an inspection point or a test connection.

Specifications

Standard	Coverage
ASME B16.5	NPS 1/2 inch to 24 inch, Class 150 to 2500
ASME B16.47 Series A and B	NPS 26 inch to 60 inch. Series A and B are NOT interchangeable
ASTM A182	Forged stainless material specification
ASME B16.20 / B16.21	Gasket standards for RTJ and flat gaskets

Facing. Flat face, raised face and ring type joint, matching whatever the mating flange carries. A blind must match its partner in class, facing and drilling, so specify the mating flange when ordering if there is any doubt.

Grades

Grade	Typical Selection Reason
304 / 304L	General service. The volume grade
316 / 316L	Chloride exposure, marine atmosphere, chemical duty
321 / 347 and H variants	Elevated temperature, stabilized against sensitization
904L, duplex, super duplex	Aggressive chloride or high strength requirement

Applications

Closing a line end that may be extended later, on a header left for a future tie-in.
Pressure testing, blanking a spool or a system boundary so it can be hydrotested, then removed.
Isolation for maintenance, positive isolation of a vessel or line while work goes on downstream.
Vessel and equipment access, manway and inspection openings that are opened on a schedule.
Spare nozzles, closing an unused vessel or header connection while keeping it available.

BLIND OR SPECTACLE BLIND? Where a line needs to be switched between open and isolated repeatedly, a spectacle blind (a paddle and ring on one piece) is often the better fitting, because the changeover is a rotation rather than a removal and the isolation state is visible from a distance. If Solitaire supplies them, that is a separate page and should be cross-linked from here.

The One Dimensional Oddity

A blind flange is normally thicker than other flange types in the same size and class. The reason is straightforward once stated: every other flange transfers pressure load into the pipe it is welded to, while a blind has to carry the full pressure across an unsupported disc. The bending moment at the centre is carried by thickness alone, so the thickness has to rise, particularly in the larger sizes and higher classes where a blind can become a genuinely heavy item to handle.

Frequently Asked Questions

What is a blind flange used for?

Closing an opening that may need to be reopened. Common uses are terminating a line left for a future tie-in, blanking a system for pressure testing, positive isolation during maintenance, and closing spare nozzles on vessels and headers. It is the only flange type with no bore and no pipe connection.

What is the difference between a blind flange and a pipe cap?

How it is fixed. A blind flange bolts to a mating flange, so it comes off with a spanner. A welded cap is welded to the pipe and has to be cut off to remove. Use a blind where the closure may reopen and a cap where it is permanent. The blind costs more in place because it needs a mating flange, gasket and bolting, and earns that back the first time access is needed without hot work.

Why are blind flanges thicker than other flanges?

Because they carry the pressure load across an unsupported disc. Every other flange type transfers load into the pipe it is welded to, while a blind has to resist the bending moment at its centre by thickness alone. That is why blinds are heavier than other types in the same size and class, and why large bore high class blinds become a handling consideration.

Does a blind flange need to match the mating flange?

Yes, in class, facing and drilling. A raised face blind pairs with a raised face flange, an RTJ blind with an RTJ flange, and the bolt circle and hole count must match. If there is any doubt, specify the mating flange when ordering rather than the blind alone.