Mechanical seal faces are commonly lapped flat to within two to three helium light bands. The exact figure is set for each seal design and shown on its face drawing. The tolerance is that tight because the faces run on a fluid film around a micron thick, so a flatness error of similar size is enough to let fluid through.
How Flatness Tolerance Is Stated
Seal face flatness is stated in helium light bands, a count of the face’s peak-to-valley departure from a true plane. One helium light band is 0.29 µm, or 11.6 microinches. A sodium lamp’s band is within 0.4 % of a helium one, so a count taken under either light reads against the same tolerance.
The bands are read with an optical flat laid on the face under monochromatic light. The flatness reading is how far the bands curve across the face, counted in band widths; the shape of the band pattern shows whether that curve is concave or convex.
Flatness and surface finish (Ra) are separate requirements: a face can be smooth and still out of flat.
On a drawing or purchase order, give the maximum count in helium light bands and state that it is read on the free face, out of its holder.
Flatness Tolerance by Face Material
Industry guidance does not split the flatness figure by face material. The Fluid Sealing Association gives two to three helium light bands for seal faces in general, with no separate number for carbon, silicon carbide, tungsten carbide or ceramic.
When a Tighter Tolerance Is Needed
High pressure, vacuum, service under tight emission limits, and large seals all call for a flatter face than the usual figure. How much flatter is set by the seal maker for that design and written on the face drawing.
Which Standard Sets the Tolerance
The seal maker’s face drawing sets it. Whatever standard the seal is bought to, API 682 included, the band count to inspect against is the one on the drawing for that face. If the drawing you hold does not show it, ask the seal supplier for the figure.
Seals made in China may also cite two Chinese industry standards: JB/T 4127.1, the specification for mechanical seals, and JB/T 7369, the test method for checking seal face flatness.
Do Out-of-Flat Carbon Faces Fail the Tolerance
Measured free on the optical flat, a carbon face that reads over its figure fails, the same as any other face. One that reads out of flat only after it is pressed into a metal holder has not failed the face tolerance, because that tolerance applies to the face out of its holder, which is where it is normally checked.
Read a carbon face free before it goes into a metal holder and again after. If it passes free and fails assembled, check the holder’s own flatness and the O-ring seating before relapping the carbon.
