AARH Flange Face Finish Explained

Quick Answer

AARH (Arithmetic Average Roughness Height) is the standard unit used to measure the surface roughness of flange faces. ASME B16.5 specifies a serrated finish of 125-250 AARH (3.2-6.3 micrometers Ra) for raised face and flat face flanges. This controlled roughness ensures proper gasket seating and leak-free seals.

AARH flange face finish is critical to the sealing performance of flanged joints. The surface roughness determines how effectively the gasket conforms to the flange face, creating a reliable seal under pressure. Too smooth, and the gasket cannot grip; too rough, and the gasket cannot fill the valleys.

What AARH Means

AARH measures the arithmetic average of all surface deviations from the mean line, expressed in microinches. It is functionally equivalent to Ra (Roughness Average) in metric units. The measurement is taken using a profilometer that traces across the serrated grooves on the flange face.

Parameter	Detail
Full name	Arithmetic Average Roughness Height
Unit	Microinches (or micrometers Ra)
Standard	ASME B16.5, MSS SP-6
Measurement tool	Surface profilometer
Typical range for flanges	125-250 AARH (3.2-6.3 Ra)
Groove type	Concentric serrated or phonographic spiral
Groove depth	1/64 in. (0.4 mm) per ASME B16.5

AARH Values for Different Flange Faces

The required AARH finish depends on the flange face type and the gasket to be used. ASME B16.5 and ASME B16.47 define the standard requirements:

Flange Face Type	AARH Range	Ra Equivalent	Gasket Compatibility
Raised face (RF)	125-250	3.2-6.3 um	Spiral wound, CNAF, graphite
Flat face (FF)	125-250	3.2-6.3 um	Full-face elastomeric, CNAF
RTJ groove	63 max	1.6 um max	Ring joint (oval/octagonal)
Tongue & groove	63 max	1.6 um max	Solid metal, spiral wound
Lens ring	32-63	0.8-1.6 um	Lens ring gaskets

Why Surface Finish Matters

A serrated concentric finish (also called a phonographic finish) creates tiny grooves that act as a labyrinth seal path. When the gasket is compressed between two flanges, the soft gasket material fills these grooves, creating a tortuous leak path that prevents fluid from escaping.

If the flange face is too smooth (below 125 AARH), the gasket has insufficient grip and may extrude under pressure. If the face is too rough (above 500 AARH), the gasket cannot fully seat into the deep valleys, leaving leak paths.

Common Mistake

Using a stock flange with a corroded or damaged face finish without re-machining. Even minor scratches across the serrations can create direct radial leak paths. Always inspect flange faces with a profilometer or surface comparator before gasket installation, especially for high-pressure rated flanges.

Measurement and Inspection

AARH is measured using a portable profilometer with a diamond stylus. The measurement is taken perpendicular to the serration grooves. At least four readings at 90-degree intervals around the flange face should be averaged for a representative result. Acceptance criteria follow ASME B16.5 or the project-specific piping class specification.

Read the full guide to flange face finishes