Class 400 Blind Flange Dimensions

Standard

ASME B16.5

Flange type

Blind Flange

Pressure class

Class 400

Size range

NPS 1/2 – NPS 24

Raised face height

0.25 in (6.3 mm)

Rating at 100 °F (A105)

985 psig (67.9 bar)

Where a Class 400 blind flange is used

Class 400 is published but seldom specified. Most piping specifications step from Class 300 straight to Class 600, so Class 400 flanges are rarely held in stock and usually have to be made to order.

A blind flange closes the end of a line or a nozzle. It is loaded in bending across its full face, which is why at this class it is already among the heaviest flanges in a given size.

The number 400 is a designation and not a pressure. A Class 400 flange in A105 carbon steel is rated 985 psig (67.9 bar) at 100 °F, 845 psig at 400 °F, 755 psig at 600 °F and 550 psig at 800 °F. In Type 304 stainless, Group 2.1, it starts at 960 psig and still holds 540 psig at 800 °F, so the material and the metal temperature have to be fixed before the class can be.

Dimensions in every size

Dimensions are given in inches, with the millimetre equivalent beneath in grey.

Flange thickness excludes the raised face. Bolt diameter is nominal; stud bolts are normally ASTM A193 B7 with A194 2H nuts.

Across the 20 sizes published in Class 400, the flange grows from 3.75 in (95.2 mm) across and 0.56 in (14.2 mm) thick at NPS 1/2 to 36.00 in (914.4 mm) across and 3.00 in (76.2 mm) thick at NPS 24. The bolt count goes 4 from NPS 1/2, 8 from NPS 2, 12 from NPS 6, 16 from NPS 10, 20 from NPS 14 and 24 from NPS 18, always in multiples of four so that the holes can straddle the centrelines. The largest bolt in the class is 1 3/4 in, first needed at NPS 24. Against Class 150, an NPS 6 flange in Class 400 is 1.50 in larger in diameter and 1.7 times as thick.

A blind flange has no bore and no hub, so the flange outside diameter, thickness, bolt circle and bolting are the whole of its dimensional definition. Thickness shown is the minimum required by B16.5 — many mills supply heavier.

In NPS 1/2 through NPS 3 1/2 the Class 400 flange is dimensionally identical to the Class 600 flange; ASME B16.5 does not publish a separate Class 400 table for those sizes.

Weight and end load

With a blind flange the two figures most often wanted are not in B16.5 at all: what the flange weighs, for lifting and for support loads, and how much force the contained pressure puts on it. Both are calculated here from the dimensions above. The end load is what the bolts must resist in addition to the load needed to seat the gasket.

Weight is a solid-disc estimate from the outside diameter and the minimum thickness at 0.2836 lb/in³, and ignores the raised face, so it runs slightly light against a real forging. End thrust is the 100 °F rating for A105 acting on the raised face area; it is not a bolt-load calculation, which must also cover gasket seating.

Bolt holes, spacing and torque

Bolt holes in ASME B16.5 flanges are drilled 1/8 in larger than the nominal bolt and are set to straddle the vertical and horizontal centrelines, never to sit on them. Spacing is the distance between neighbouring holes measured round the bolt circle, which shows how much room there is for a wrench or a tensioner. The torque column is a starting figure for a lubricated stud, not a joint-specific value.

Torque is T = K·F·d with nut factor K = 0.16 and a bolt stress of 50,000 psi on the tensile stress area. The bolt torque guide gives the method and other nut factors.

Guides and comparisons for this page

Same flange, other classes

Same class, other flange types

Every class in one size