Seal face lubrication
What is really happening between the mechanical seal faces? 9-7
The answer is, “lots of things”, and it’s hard to predict exactly what is going on in any particular application. As you can guess there are many theories about face lubrication, but few rules. Here are some of the more popular theories:
- Sometimes there is a film of lubricant between the seal faces. This is often refereed to as the “Pressure Wedge Theory”.
- Sometimes there is only vapor. This was determined in the nineteen sixties at The Battle Memorial Institute at Columbus, Ohio where they conducted a series of seal face lubrication tests for the aircraft industry. These tests introduced the “asperity theory” of face lubrication. This idea was later picked up by the British and introduced as “vapor phase sealing”.
- A lubricant is defined as any fluid that can maintain a film thickness of one micron or more at its operating temperature and load, so sometimes there is no lubricant between the lapped faces. They run dry. This happens with solvents, cryogenics, dry hot air and most dry gas applications.
- The three band theory is another popular theory. With this condition you can observe a band of lubricant at the face outside diameter, a band of vapor in the center of the seal faces, and a dry band at the inside diameter of the seal face.
Regardless of what is happening between the seal faces, the rules for operating mechanical seals successfully always remains the same. Keep the two lapped flat faces together and the seal will not leak.
Now, let’s define leakage:
- Any well designed mechanical seal can run with no visible leakage. If product is dripping from the seal something is wrong and the problem is always correctable.
- The ability to seal fugitive emissions is another story. Fugitive emission sealing requires the ability to seal as little as one hundred parts per million of product. You should be able to do this with any well designed stationary (the springs do not rotate) seal. The excessive movement involved in rotating designs (the springs rotate with the shaft) makes the sealing of fugitive emissions with a rotating seal almost impossible.
- It also means that you cannot seal fugitive emissions with cartridge mounted stationary seals either. When the cartridge sleeve is set screwed to the shaft it will cause the rotating face to become “not perpendicular” to the rotating shaft, and excessive seal movement will follow.
You will note that in a previous paragraph I said the seal faces had to be flat. The term flat is often confused with the term smooth. We want the seal faces flat, not smooth. Flatness can be measured with a straight edge, or by rubbing the part on a known flat that has been coated with machinist bluing or dye. Neither of these methods is satisfactory for checking the flatness of mechanical seal faces.
Flatness must be measured by an optical flat and a monochromatic light. This equipment generates helium light bands that measure distances of 0.0000116 inches (0,3 microns). If a seal face is lapped to less than three helium light bands (slightly less than one micron) and the faces kept in physical contact, the seal should be able to pass any fugitive emission test.
If you want to understand the relative size of one micron then realize that the smallest object that can be seen with the human eye is forty (40) microns. The average coffee filter mesh is some where between ten and fifteen microns, and we know that both water and coffee will pass through this small opening easily, but solids have a tough time getting through ten microns. This implies that the lapped seal faces make the best filter you own. In other words, dirt or solids will not penetrate between lapped seal faces unless they open for some reason. Testing has shown that visible seal leakage occurs at about five helium light bands or a space that measures slightly less than two microns.
Surface finish is a measure of smoothness or polish. In the United States we commonly measure surface finish with some sort of a comparative gauge that has been polished to various degrees of gloss. The readings are made using the units RMS (root mean square) which is a number representing the square root of the mean or middle distance between the peaks and the valleys in the surface of the seal face. The metric system commonly uses CLA or center line average which is the average distance between the peaks and valleys in the face. If a seal face is too smooth there is the possibility of galling or sticking of the seal faces.
People often ask if seal faces can run dry, and the answer is yes if you’re using a good grade of carbon graphite without any fillers or binders that would be injured by the additional heat caused by the friction between the faces. We all know that electric motors have run carbon brushes for years without lubrication.
Friction between the lapped seal faces will cause the softer graphite to coat or deposit on the hard face, leaving the harder carbon behind, automatically creating a non-smooth carbon surface. It is important to understand that this will only happen if moisture is present. This is the reason that children will lick the tip of a graphite pencil to make the writing darker. Dry gases, hot air, and cryogenic fluids do not have this moisture present, so a special carbon/ graphite must be used in these applications. This special grade has organics imbedded into the carbon/graphite mixture to release the softer graphite.
Experienced seal people know that excessive carbon wear is seldom a problem with mechanical seals. Most leaking seals have plenty of carbon/ graphite remaining when the seal begins to leak. The more common problem with running seal faces dry is that the friction heat will harden or destroy the elastomer or O-ring that is often located in, or close to the seal face.
Since there is a direct correlation between seal face flatness and seal face leakage it is important to know that here are a number of circumstances that can cause the seal faces to go “out of flat”. Here are some of them:
- Pressure distortion. Computer finite element analysis programs have helped a great deal with this problem. Pressure distortion is sometimes described as “hoop stress problems”.
- A differential temperature across the seal faces can cause face distortion. Excessive face load, faces isolated by a gasket or elastomer and the wrong percentage of hydraulic balance are some of the causes of this differential temperature.
- Uneven clamping of hard faces is another problem. Be sure to check for “equal and opposite clamping”. Different width gaskets on either side of the hard face is a condition that causes this problem.
- If the carbon is inserted into a metal holder it should be pressed in with an arbor press to allow the carbon to shear to the configuration of the metal part. Shrunk in carbons usually have flatness problems, especially if they have to operate over a range of temperature. Glued in carbon never did make any sense.
- Poor grades of carbon/graphite have too many voids or air pockets below the surface that will expand with heat, causing blistering and pits that will create face leakage. Stay with an unfilled carbon and, with the exception of hot oil applications, you should never have this face pitting problem. Hot oil is unique in that it creates coking solids at the seal interface that can pull out microscopic particles of carbon that can prevent the seal from sealing fugitive emissions. You are going to have to use two hard faces in hot oil applications if the seal must meet fugitive emission standards.
- Many hard faces are lapped only on one side. Some times the unlapped side is accidentally running against the lapped carbon face.
- Oxidizing agents and halogens attack all types of carbon/ graphite. Make sure you are not trying to seal one of these fluids with a carbon/ graphite face.
- Some products can cause seal faces to stick together and break the carbon at pump start up. If the product must be kept warm or cool to prevent solidifying, install the correct environmental control to provide the proper amount of heat and insure that it is functioning when the pump is stopped as well as when it is running.
- Poor lubricants can cause “slip stick” problems between the lapped faces that will chip and damage the carbon on the outside diameter.
- Some deionized water has been know to attack even good grades of carbon/graphite.
- Petroleum products can cause microscopic pits in the carbon/graphite. If your application is subject to fugitive emission standards you should specify two hard faces in this application. Tungsten carbide against silicone carbide would be a good selection.
- On February 18, 2018