Flange Face Finish

The surface finish of a flange face directly controls gasket seating and seal integrity. Too smooth, and the gasket cannot grip the surface. Too rough, and leak paths form between the peaks. American Standard B16.5 specifies a standard serrated (concentric or spiral grooved) finish with a roughness average (Ra) of 3.2 to 6.3 micrometers for raised-face flanges.

Surface Roughness Terms

Term	Definition	Unit
Ra	Roughness Average—arithmetic mean deviation of the surface profile	micrometers (um) or microinches (uin)
AARH	Arithmetic Average Roughness Height—same as Ra (older term)	microinches
RMS	Root Mean Square roughness—approximately 1.11 x Ra	microinches
Rz	Average peak-to-valley height over sampling length	micrometers

Ra and AARH are effectively the same measurement. Ra is the ISO/metric term; AARH is the older American Standard/American term. The conversion: 1 micrometer = 39.37 microinches. So 3.2 um Ra = 125 AARH; 6.3 um Ra = 250 AARH.

American Standard B16.5 Requirements

Face Type	Finish Type	Ra Range	AARH Range
Raised Face (RF)	Serrated (concentric or spiral)	3.2-6.3 um	125-250 uin
Flat Face (FF)	Serrated or smooth	3.2-6.3 um	125-250 uin
RTJ groove	Smooth machine finish	0.8-1.6 um	32-63 uin
Tongue & groove	Smooth	0.8-3.2 um	32-125 uin

Pro Tip

The 3.2-6.3 um Ra range applies to standard spiral wound and compressed fiber gaskets. For PTFE-based gaskets, some manufacturers recommend a smoother finish (1.6-3.2 um) because PTFE does not conform to deep serrations as readily as graphite-filled gaskets.

Serration Types

Flange face serrations come in two patterns:

Type	Description	Use
Concentric serrated	Circular grooves centered on the flange bore	Preferred for hazardous and high-pressure service; no spiral leak path
Spiral serrated	Single continuous spiral groove (phonograph-record pattern)	Standard production method (easier to machine); creates a potential spiral leak path
Smooth (stock)	No deliberate serration	Used for RTJ grooves and gasket types requiring smooth contact

Concentric serrated finishes are specified in American Standard B16.5 for services where a spiral leak path is unacceptable (e.g., hydrogen, lethal fluids). Most standard production flanges have a spiral serrated finish unless concentric is specifically requested.

Common Mistake

Accepting flanges with damaged or corroded face finishes. Even light rust or a scratch across the serrations creates a leak path that the gasket may not seal. Always inspect flange faces before assembly and protect them with plastic face covers during storage and transport.

Practical Measurement

Surface roughness is measured with a profilometer (contact stylus or optical). On-site, visual and tactile comparison against reference standards (surface roughness comparators) is common. American Standard PCC-1 “Guidelines for Pressure Boundary Bolted Flange Joint Assembly” provides detailed guidance on flange face inspection before joint assembly.

Ra Value	Surface Appearance	Typical Application
0.8 um (32 uin)	Mirror-like, very smooth	RTJ grooves, metallic gaskets
3.2 um (125 uin)	Visible machining marks	Standard RF with spiral wound gaskets
6.3 um (250 uin)	Pronounced grooves	RF with soft-cut gaskets
>6.3 um (>250 uin)	Rough, out of spec	Reject—re-machine or replace

Read the full guide to flanges