Understanding ASME ANSI B16.5 Flange Types-A Quick Technical Guide

Selecting the right flange type is the first decision that determines whether a piping joint stays tight, safe and compliant. ASME/ANSI B16.5 organizes most industrial flanges by pressure class, facing type and end connection, and each design trades weld work, alignment effort and pressure capability in a different way. This guide compares the six common types, the specialized variants and the selection checks that matter before you specify the joint, the bolting and the flange gaskets that seal it.

How ASME B16.5 Groups Flanges

ASME B16.5 covers dimensions, tolerances, pressure-temperature ratings and marking for flanges and flanged fittings in piping. Almost every selection reduces to three parameters.

• Pressure class — 150#, 300#, 600#, 900#, 1500# and 2500#. Match the class to design pressure and design temperature, not to the pipe schedule alone.
• Facing type — raised face (RF) for general service, flat face (FF) for low-pressure or brittle mating parts, ring-type joint (RTJ) for high pressure and metal-to-metal sealing.
• Material grade — A105 carbon steel for standard duties, F304 and F316 austenitic stainless for corrosive media, checked against temperature and corrosion allowance.

Whatever the type, dimensions, tolerances and marking must follow ASME B16.5 so that parts from different suppliers remain interchangeable in the field.

Common Flange Types and Their Applications

The six types below cover most industrial piping duties. They differ mainly in how each one joins the line and how much load the joint can carry.

Practical limits are shown for reference only; final selection follows the ASME B16.5 pressure-temperature rating of the chosen class and material.

Bolt-hole pattern, outside diameter and thickness for each of these pipe flanges follow the ASME B16.5 tables, so any compliant gasket and bolting set fits without rework.

Specialized Types for Demanding Service

Beyond the six common designs, a smaller group of flanges solves specific layout, metering and vacuum problems.

• Long weld neck flange — extended hub for heavy-wall vessel nozzles, high thermal cycling and severe service.
• Orifice flange — built for flow metering with an orifice plate and pressure taps for instrumentation.
• Nipo flange — short welded branch for small connections and instrument take-offs.
• Reducer, expander and swivel flanges — change line size at the joint, or rotate for bolt-hole alignment during bolt-up.
• Vacuum flanges (KF/QF, ISO, CF) — for high-vacuum systems such as laboratories, semiconductor tools and space simulation.

Selection Checks Before You Specify

Most field problems start at specification time. Run these checks before the joint is defined.

• Match pressure class (150#, 300#, 600# and above) to the design pressure and design temperature of the line.
• Confirm facing compatibility (RF, FF or RTJ) with the mating flange and the gasket style.
• Verify material grade (A105, F304, F316) against the fluid, the temperature range and the corrosion allowance.
• Check dimensions, tolerances and marking against ASME B16.5; unmarked flanges mixed in one joint are a common leak source.
• Plan bolt-up: even cross-pattern torque, calibrated tools and a new gasket every time the joint is opened.

Frequently Asked Questions

What separates a weld neck flange from a slip-on flange?

A weld neck is butt-welded through a tapered hub that carries the load into the pipe wall, so it suits high pressure, high temperature and cyclic service. A slip-on slides over the pipe and is fillet welded inside and out; it costs less and aligns faster but is not recommended for severe cyclic loading.

When should a ring-type joint facing be specified?

RTJ seals with a solid metal ring gasket against metal, so it is the choice for high pressure and high temperature where a soft gasket would extrude. Check the groove for damage before fitting the ring.

Which flange types need no field welding?

Threaded flanges need no welding at all, and lap-joint flanges are only bolted around a stub end. Both help in hazardous zones or where hot work is restricted, although threaded joints are limited to about 1000 psi depending on material and size.

How is a blind flange used?

A blind flange closes a line for isolation, pressure testing or a future branch. Its rating matches the mating flange class, 150# through 2500#.

Why pair a lap-joint flange with a stub end?

The stub end carries the process fluid and can be a corrosion-resistant grade, while the backing flange is never welded and can stay carbon steel. The pair allows easy bolt-hole alignment and repeated dismantling for maintenance.