The American National Standards Institute (ANSI) code for flanges and flanged fittings (ANSI B16.5) requires that the flange face has a specific roughness to ensure that this surface be compatible with the gasket and provide a high quality seal. A serrated finish, either concentric or spiral, is required with 30 to 55 grooves per inch and a resultant roughness between 125 and 500 microinches.
This allows for various grades of surface finish to be made available by flange manufactures for the gasket contact surface of metal flanges. These grades are often referred to by name e.g. stock finish.
The exact definition of each grade may differ between manufacturers, but can be generalised as follows:
The most widely used of any flange surface finish, because practically, is suitable for all ordinary service conditions. Under compression, the soft face from a gasket will embed into this finish, which helps create a seal, and a high level of friction is generated between the mating surfaces. The finish for these flanges is generated by a 1.6 mm radius round-nosed tool at a feed rate of 0.8 mm per revolution up to 12 inch. For sizes 14 inch and larger, the finish is made with 3.2 mm round-nosed tool at a feed of 1.2 mm per revolution.
This finish shows no visually apparent tool markings. These finishes are typically utilized for gaskets with metal facings such as double jacketed, flat steel and corrugated metal. The smooth surfaces mate to create a seal and depend on the flatness of the opposing faces to effect a seal. This is typically achieved by having the gasket contact surface formed by a continuous (sometimes called phonographic) spiral groove generated by a 0.8 mm radius round-nosed tool at a feed rate of 0.3 mm per revolution with a depth of 0.05 mm. This will result in a roughness between Ra 3.2 and 6.3 micrometers (125 – 250 micro inch).
Hydrogen Service Finish
The finish for flanges in hydrogen service is very smooth, typically between Ra 2 and 3.2 micrometers (79 – 125 microinch).
Cold Water Finish
The flange face appears as mirror like. This flange finish is usually expected to be used with metal to metal contact, i.e. without a gasket. It is seldom used in the oil, petrochem and related industries.
Measuring Surface Roughness
Flange finish is generally measured by visual and tactile means. Comparing the feel of the machined face with that of a surface finish comparator gauge, occasionally referred to as a Rupert gauge, is considered adequate.
Arithmetic Average Roughness Height– Calculation of the Arithmetic Average Roughness Height involves measuring the distance of the peaks and valleys and performing an arithmetic average of the measurements.
Root Mean Square Average – Calculation of the Root Mean Square Average involves measuring the distance of peaks and valleys, adding the square of these measurements and calculating the square root of the total.The RMS value is approximately 11% higher than the AARH value.
What is AARH in flange finish?
AARH stands for Arithmetic Average Roughness Height.
It is used to measure the roughness (rather smoothness) of surfaces. 125 AARH means 125 micro inches will be the average height of the ups and downs of the surface.
Smooth finish flanges are more common for low pressure and/or large diameter pipelines and primarily intended for use with solid metal or spiral wound gaskets.
Smooth finishes are usually found on machinery or flanged joints other than pipe flanges. When working with a smooth finish, it is important to consider using a thinner gasket to lessen the effects of creep and cold flow. It should be noted, however, that both a thinner gasket and the smooth finish, in and of themselves, require a higher compressive force (i.e. bolt torque) to achieve the seal.