FACE FLATNESS F003
There is often confusion between the terms “Seal face flatness” and “Seal face surface finish”. Seal face surface finish addresses the subject of roughness and is measured in terms of RMS (root mean square) or CLA (center line average). There are a couple of ways to make the measurement:
- We can do it manually by comparing our sample to metal standards that have been polished to different degrees of roughness.
- You can place the sample in piece of equipment that projects a blown up version of the sample on a screen and measures the roughness by a built in scale. This equipment is called a profilometer.
- You can use an instrument that drags a sensitive probe across the piece and measures finish in that manner.
Flatness is a different term that describes a level surface that has no elevations or depressions. We use terms like waviness, or concave and convex surfaces to describe the condition when we refer to mechanical seal faces. It is this flatness that is of the most concern to us because testing has shown that if the faces are separated by a space of about two microns or more, the seal faces will show visible leakage, and depending upon the separation, let solids penetrate that might score or in some way injure these lapped faces.
There are several ways you could measure flatness:
- You could place a straight edge on the surface and look for daylight between the straight edge and the sample. As you would guess this method is not accurate enough for our purposes.
- You could place “machinist’s bluing dye” on a known flat, rub the sample piece against it and look for transfer of the dye. Again this method would not be accurate enough for our purposes.
- You could read the flatness by using an optical flat and a monochromatic light source, and this is the method that is used by all of us in the sealing industry.
To understand this method of measurement you only have to know that it is a characteristic of light that when two lights of the same wave-length interfere with each other, the light disappears and the reflecting piece goes black. When you discuss visible light, color and wave length mean the same thing, so to make the measurement we use :
- A monochromatic or single wave-length light source (mono means one, and chromatic means color). Any color (wave-length) could be used, but most companies use a pink color that comes off a helium gas light source. This color has a wave-length of just about 0,6 microns (0.000023 inches).
- You will also need a precision ground and polished clear glass of optical quality (like the type you would find in a good pair of eye glasses or binoculars) that has been lapped flat on one at least one side.
The optical flat is placed on the piece to be measured. The monochromatic light is aimed at the piece and this light reflects off of the piece back through the optical flat causing interference light bands. If the distance between the optical flat and the piece we are measuring is one half the wave-length of helium, or an even multiple of the number, the band will show black. This is referred to as a helium light band and because it is one half the wave-length of helium it measures 0,3 microns or 0.0000116 inches.
To understand this measurement I might mention that the smallest object that can be seen with the human eye is forty (40) microns. Another way to understand this measurement is to know that the average coffee filter is in the range of ten to fifteen (10 to 15) microns. Sophisticated seal people know that this means that solids cannot penetrate between the seal faces unless they open.
We check the flatness of our seal face by comparing the pattern we see, to a chart that is supplied by the measuring equipment manufacturer. A copy of a typical flatness chart is shown in the charts and graphics section of this book.
There are some things that you should know about flatness readings:
- Hard seal faces should read less than three light bands for seal faces with a mean diameter up to four inches (100 millimeters). There should be no visible leakage. Leakage is always subject to definition, but three light bands of flatness will allow a mechanical seal to seal vacuum down to a measurement of one Torr (one millimeter of mercury).
- Carbon-graphite faces relax after lapping. Although lapped to less than one light band by the seal manufacturer, you will see readings as high as three light bands if you check the faces. These faces should return to flat once they are placed against a hard face that is flat.
- Most large seal manufacturers use finite element analysis techniques to design these faces and keep them flat over the operating range of the seal. Some repair and smaller seal facilities supply, replace or repair these faces with no provision for keeping them flat during temperature and pressure transients.
- Carbon/ graphite seal faces should not be relapped because the relapping procedure will drive the trapped solids further into these faces. It goes without saying that lapping powder or paste should not be used to lap carbon / graphite faces. They should be lapped dry on ceramic stones of varying grit or finish.
- Seals that are going to be used in cryogenic (cold) service should be lapped at the operating cryogenic temperature.
- Some seal companies use a concave taper to prevent the ingress of solids at start up. This is one of the reasons for the three light band allowable tolerance.
- Normal lapping produces a slight convex taper because the outer diameter of the seal face is larger than the inner diameter causing more wear as the piece rotates. Some seal companies use a convex lapping surface to compensate for this.
- If the seal faces stay flat within three helium light bands and the lapped seal faces stay in contact, a single stationary type mechanical seal can easily pass fugitive emission specifications of less than one hundred parts per million.
- Carbon faces that have been pressed into a metal holder have special flatness problems. The metal modulus of elasticity is almost ten times that of the carbon face, so the assembly must be stress relieved to keep the carbon flat. Stress relieving is done by taking the assembly through a series of temperature cycles or leaving the carbon holder assembly on the shelf for several months while the assembly relaxes,
- Carbon pressed into a metal holder is usually stays flatter than a design where the carbon is inserted into a holder that has been expanded with an induction coil.