Class 300 Weld Neck Flange Dimensions

Standard

ASME B16.5

Flange type

Weld Neck Flange

Pressure class

Class 300

Size range

NPS 1/2 – NPS 24

Raised face height

0.06 in (1.5 mm)

Rating at 100 °F (A105)

740 psig (51.0 bar)

Where a Class 300 weld neck flange is used

Class 300 is the usual step up when Class 150 runs out of pressure or temperature. It is common on hydrocarbon process lines, steam and hot oil, and it is often chosen over Class 150 for a hazardous fluid at modest pressure simply for the heavier joint.

At this class a weld neck is a choice rather than a necessity. It is picked over a slip-on where the line sees thermal or pressure cycling, where the fluid is hazardous, or where the butt weld has to be radiographed.

The number 300 is a designation and not a pressure. A Class 300 flange in A105 carbon steel is rated 740 psig (51.0 bar) at 100 °F, 635 psig at 400 °F, 570 psig at 600 °F and 410 psig at 800 °F. In Type 304 stainless, Group 2.1, it starts at 720 psig and still holds 405 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 300, the flange grows from 3.75 in (95.2 mm) across and 0.50 in (12.7 mm) thick at NPS 1/2 to 36.00 in (914.4 mm) across and 2.69 in (68.3 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 1/2 in, first needed at NPS 24. Against Class 150, an NPS 6 flange in Class 300 is 1.50 in larger in diameter and 1.5 times as thick.

Bore by pipe schedule

A weld neck flange is bored to the inside diameter of the pipe it is butt-welded to. Everything in the table above is the same for every Class 300 weld neck of a given size; the bore is not. It follows the schedule of the pipe, so one size and class covers several different part numbers, and a requisition that leaves the schedule off is incomplete. A bore that does not match leaves a step inside the joint, which disturbs the flow and concentrates stress at the root of the weld.

Bore is the pipe inside diameter, OD − 2t. A dash means B36.10M publishes no such schedule in that size. Each size links to its pipe page, which lists every schedule.

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