Class 900 Blind Flange Dimensions

Standard

ASME B16.5

Flange type

Blind Flange

Pressure class

Class 900

Size range

NPS 1/2 – NPS 24

Raised face height

0.25 in (6.3 mm)

Rating at 100 °F (A105)

2220 psig (153.1 bar)

Where a Class 900 blind flange is used

Class 900 belongs to high-pressure process, injection and power piping. From this class upward the ring-type joint facing becomes a common alternative to the raised face.

A blind flange closes the end of a line or a nozzle. At this class the pressure load on the plate is large, and both the weight of the blind and the load on its bolts need to be allowed for in handling and in the joint design.

The number 900 is a designation and not a pressure. A Class 900 flange in A105 carbon steel is rated 2220 psig (153.1 bar) at 100 °F, 1900 psig at 400 °F, 1705 psig at 600 °F and 1235 psig at 800 °F. In Type 304 stainless, Group 2.1, it starts at 2160 psig and still holds 1215 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 19 sizes published in Class 900, the flange grows from 4.75 in (120.6 mm) across and 0.88 in (22.4 mm) thick at NPS 1/2 to 41.00 in (1041.4 mm) across and 5.50 in (139.7 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 and 20 from NPS 12, always in multiples of four so that the holes can straddle the centrelines. The largest bolt in the class is 2 1/2 in, first needed at NPS 24. Against Class 150, an NPS 6 flange in Class 900 is 4.00 in larger in diameter and 2.3 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 2 1/2 the Class 900 flange is dimensionally identical to the Class 1500 flange.

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