Seal Face Lubrication
The carbon face we see in mechanical seals is really a combination of carbon and graphite with the graphite being a good dry lubricant.
As the seal face wears, the graphite is deposited on the hard face (you can see the black ring) leaving the carbon behind. The function of the hard face is to give the graphite a place to deposit.
Testing has shown that when we seal a lubricating fluid the lubricant becomes trapped between the asperities (the peaks the graphite leaves when it deposits on the hard face) and in many cases becomes a vapor separating the two running surfaces. The fluid penetrates the lapped faces in two ways:
- Pressure forces the lubricant between the lapped faces
- Capillary action draws the vapor between the faces.
A lack of lubrication between the seal faces can cause a destructive form of vibration called slip-stick. Without proper lubrication the lapped seal faces try to stick together, but "slip" when the seal drive mechanism engages the drive lugs and inertia accelerates the faces off of these lugs. The faces then slow down as a result of the poor lubrication. This alternating "slipping" and "sticking" causes severe vibration with a resultant "chipping" at the outside diameter of the carbon face along with drive lug and slot wear.
The amount of wear experienced by the carbon, graphite mixture is affected by:
- The surface speed of the seal faces (a combination of shaft rpm and seal face diameter). PV numbers are not really valid because the carbon is sensitive to "P" but not to "V"
- The spring load on the seal faces and the area of the seal faces.
- The stuffing box pressure. Keep in mind that this number can vary during pump operation.
- The quality and grade of the carbon-graphite face.
- The surface finish and hardness of the hard face.
- The cleanliness of the sealing fluid.
- The accuracy of the initial installation dimension.
- The hydraulic balance designed into the face.
- The hardness of the carbon.
- The thickness of the lubricating film.
- The affect of centrifugal and hydrodynamic forces on the face loading.
- On February 17, 2018