Blind Flange

ASME B16.5 Class 150 flange dimensions

Flange outside diameter, minimum flange thickness, bolt circle, bolting and raised face, in millimetres, for NPS 1/2 to 12 in. These dimensions are set by the size and the pressure class and are common to the flange types on this line; the bore, hub, socket or thread detail changes with the type and is confirmed on the drawing or the quotation. Thickness includes the 1.6 mm raised face.

Data to ASME B16.5; confirm against the project specification before fabrication. Sizes above 12 in and Class 300 to 2500 follow the same layout and are quoted against the specification. Send the size, class, facing and destination port and we will confirm availability and price within one business day.

What a Blind Flange Is

A blind flange is a solid disc with bolt holes and no bore at all. It is bolted against another flange to close an opening, so it contains pressure but does not connect to any pipe. Because it has no bore to machine and no hub, it is the simplest flange type – but it is also the heaviest per unit size, since the full disc has to carry the pressure that a pipe would otherwise contain. Blinds are supplied to ASME B16.5 from NPS 1/2 to 24 and to ASME B16.47 from NPS 26 to 60.

Where a Blind Flange Is Used

Closing a spare nozzle that may be used for a future tie-in, isolating a vessel, column or exchanger during a turnaround, forming the boundary of a hydrostatic or pneumatic test, closing vessel and tank manways and handholes, and blanking the end of a header or a line that is not yet in service. Because a blind is removable, it is the standard way to close an opening that may later have to be opened again.

How to Specify and What to Match

A blind flange must match the flange it is bolted to in four things: nominal size, pressure class, facing and bolt pattern. All four come from the same ASME table, so quoting the size and class of the mating flange is usually enough – but the series has to be stated above NPS 24, because ASME B16.47 Series A and Series B have different bolt circles. A blind flange can be supplied with a drilled and tapped hole for a vent, a drain or a test connection where the service calls for one.

Facing and Gasket for a Closed Joint

A blind flange carries the same facing options as any other flange, and the facing must match its partner: raised face to raised face with a sheet or spiral wound gasket, flat face to flat face, ring joint to ring joint with a metal ring gasket. The blind, the gasket and the bolting are three separate items in one joint, so when a blind is ordered to close an existing flange, the facing of that flange and the bolting size should be confirmed at the same time.

What the plate has to carry

A blind has no bore to machine and no hub, but it is the heaviest flange in the joint at the same size and class, because the full disc carries the pressure that the pipe would otherwise contain. Thickness follows the class rather than the pipe schedule, so a blind is ordered by size, class and facing — the wall thickness of the line does not change it. Four consequences are worth knowing before the order is placed.

• Bolting. A blind is thicker than a normal flange pair at the same class, so stud length differs. Where the studs and gaskets are part of the same package, the blind has to be on the list or the bolting comes out short.
• Gasket. The gasket is dimensioned by the facing and the class, and both sides of the joint take the same gasket. Ring joint gaskets are dimensioned separately from sheet and spiral wound types.
• Weight and handling. Large diameter blinds are lifted into place, and above NPS 24 the series governs the bolt circle, so Series A and Series B blinds are not interchangeable.
• Test use. A blind that closes a section for a test has to be checked against its rating at the test temperature, not against the line class alone, because the test is not always run at ambient.

Tapped connections on a blind

A blind can be supplied with a tapped hole where the line needs a vent, a drain, a gauge or a test connection. These are the usual options; each one has to be specified with its size, thread and position.

A tapped hole does not turn a blind into a rated branch connection. Where the connection has to be designed as a pressure retaining branch, it should be specified as a nozzle or a fitting, not as a hole in a blind.

Grades supplied and minimum properties

The grade fixes the minimum strength and the chemistry. The table below gives the requirements that separate the forged flange material grades ordered most often; the full composition and mechanical table for each grade is on its own page.

The complete chemistry and mechanical table for each grade, with the notes that qualify it, is on the individual grade page – for example ASTM A105 flange. The stainless grade has the higher tensile requirement but the lower minimum yield strength, so the two are not interchangeable on the strength table alone. The pressure class sets the flange dimensions, so the grade does not set the wall.

Source: specified minimum yield and tensile strength for each grade, per ASTM A105 / ASME SA105 and ASTM A182 / ASME SA182. The standard named against each grade is the governing document.

• A105
• A350 LF2
• F11
• F22
• F304
• F304L
• F316
• F316L
• F321
• F51

Download Blind Flange data sheet (PDF) One page: supply range, grades and key sizes, what to state on the enquiry, and the inspection documents that come with the order. The standard is the governing document.

What decides the price

There is no list price for flanges: the same drawing can be quoted at different numbers depending on seven things. Send them and the quotation comes back within one business day.

Reply within one business day. Send the seven items above with the quantity and the destination port. Send the specification or WhatsApp the specification.