What Is a Nozzle Flange?

A nozzle flange is a flanged connection welded to a pressure vessel, heat exchanger, column, or tank nozzle. It provides the interface between the static equipment and the connected piping. The nozzle flange must match the pressure class, facing, and dimensional standard of the mating pipe flange to form a leak-tight bolted joint with the correct gasket and stud bolts.

Key Specifications

Property	Detail
Flange standard	ASME B16.5 (NPS 1/2-24), ASME B16.47 (NPS 26-60), EN 1092-1
Vessel standard	ASME Section VIII Div. 1/2, EN 13445
Common types	Weld neck (most common), long weld neck
Pressure class	Must match piping class specification
Materials	A105 (CS), A182 F304/F316 (SS), A182 F11/F22 (alloy)
Face types	RF, RTJ, FF per piping spec
Rating basis	ASME B16.5 P/T tables or vessel design pressure (whichever governs)

Nozzle Flange Rating Selection

The nozzle flange rating is determined by matching the vessel design conditions with the piping specification:

Design Parameter	Source
Design pressure	Vessel data sheet
Design temperature	Vessel data sheet
Flange class	ASME B16.5 P/T rating at design conditions
Piping spec	Pipe class assigned by the piping engineer
Corrosion allowance	Vessel design (may differ from piping)

Common Mistake

Assuming the nozzle flange class always matches the vessel design pressure. The flange must be rated for the higher of the vessel design pressure or the connected piping class. If the piping spec calls for Class 300 but the vessel design pressure only requires Class 150, the nozzle flange is still typically Class 300 to match the piping.

Nozzle Flange vs Standard Pipe Flange

Feature	Nozzle Flange	Standard Pipe Flange
Welded to	Vessel shell or nozzle neck	Pipe spool
Supplied by	Vessel manufacturer	Piping contractor or material supplier
Inspection	Per vessel code (ASME VIII)	Per piping code (ASME B31.3)
PWHT	With vessel (if required)	With pipe spool (if required)
Dimensions	Identical (same ASME B16.5 dimensions)	Identical
Marking	Vessel tag + flange markings	Flange markings only

Pro Tip

On P&ID reviews, always verify that every vessel nozzle flange rating matches the connecting piping class. Mismatches are among the most common engineering errors and are expensive to fix after fabrication. The piping material engineer and the vessel engineer must coordinate ratings during FEED.

Long Weld Neck Nozzles

For nozzles requiring extended reach through insulation or vessel skirts, long weld neck flanges are used. The extended hub acts as both the nozzle neck and the flanged connection, eliminating one weld joint between a separate nozzle pipe and a standard weld neck flange.

Read the full guide to flanges