ASME B16.5 Flange Guide: Dimensions and Ratings

Scope and Standard Coverage

ASME B16.5 defines dimensions, tolerances, and pressure class ratings for flanges and flanged fittings from NPS 1/2 through NPS 24. It covers the most common industrial flange types, facing styles, and bolting requirements used with valves, fittings, and pipe assemblies. For larger sizes, ASME B16.47 applies and should be referenced separately. B16.5 does not replace material specifications; it must be used together with ASTM grades and ASME B16.34 or project specifications for design pressure and temperature. For procurement, the standard matters because a small mismatch in bolt circle diameter, facing, or class can prevent assembly in the field. This guide highlights the key parameters buyers must confirm before issuing RFQs or approving substitutions.

Class Ratings at 100F for ASTM A105 (Group 1.1)

Pressure Class and Temperature Limits

Pressure class is not a fixed pressure; it is a pressure-temperature rating that changes with material group and operating temperature. As temperature rises, allowable pressure decreases, and different materials have different ratings for the same class. For example, carbon steel (ASTM A105) and stainless steel (A182 F316) share the same class numbers but have different pressure-temperature tables. Procurement should reference the correct material group table in B16.5 and verify the maximum operating temperature and design pressure. When projects specify a class, confirm whether any corrosion allowance or design factor has already been applied. This avoids underspecification in high temperature service and overbuying when moderate conditions are sufficient.

Facing Types and Gasket Compatibility

B16.5 allows several facing types, most commonly raised face (RF) and ring type joint (RTJ). RF is typical for general industrial service and uses soft or semi-metallic gaskets, while RTJ is used for higher pressure and temperature and requires matching ring grooves and RTJ gaskets. Flat face (FF) may be required for cast iron or low pressure systems where a full-face gasket is used. Facing choice affects bolting and gasket selection, and mixing faces can cause leakage or flange damage. Buyers should specify facing style, finish requirements, and gasket type together. For critical service, request flange drawings and confirm the facing dimensions match the mating equipment.

Material Grades and Documentation

Flange material must be specified separately from B16.5. Common forged grades include ASTM A105 for carbon steel, A350 LF2 for low temperature, and A182 F304 or F316 for stainless. Cast components often use ASTM A216 WCB or ASTM A351 CF8/CF8M. For sour service, NACE MR0175 compliance may be required. Procurement should request material test certificates, heat numbers, and impact test results if the project has low temperature requirements. Coatings, galvanizing, and corrosion allowances must also be clarified, especially for external corrosion control in marine or humid environments.

Procurement Checklist

A complete B16.5 flange specification includes NPS, pressure class, facing type, flange type (weld neck, slip-on, blind, or socket weld), material grade, and any supplementary testing. Confirm bolt material grade and size, gasket type, and whether coating or painting is required. Verify the standard edition used by the project and request dimensional drawings for approval when cross-standard equipment is involved. If flange is part of an assembly, confirm face-to-face and bolt circle alignment with the mating valve or equipment. A structured checklist reduces RFQ cycles and prevents costly field modifications during installation.