The sealing of non lubricants 4-8
When we are discussing mechanical seals, a lubricant is defined as a fluid that has a film thickness of at least one micron (0.000039 inches) at its operating temperature and load. If the product we’re sealing is not a lubricant we are forced to use the self-lubricating characteristics of the carbon/ graphite mixture used in the manufacture of the seal face.
The key to this self lubrication is that carbon can form strong chemical bonds with gases such as water vapor. The adsorbed gas then weaken the interlacing bonding forces, which in turn reduce the rubbing friction. Many other types of vapors and gases can be readily adsorbed by carbon/ graphite and in some instances inorganic compounds can be added to the carbon/ graphite if adsorbable gases are not present, or in short supply. Graphitizing of the carbon (heating it to 5000 degrees Fahrenheit or 2750 degree Centigrade) is another approach to self lubrication.
In the seal business we are faced with the challenge of sealing three types of non lubricants. I will address the problems in order of their difficulty :
THE NON LUBRICATING LIQUID
Hot water and many solvents fit into this category. The lack of lubrication at the seal faces causes more rapid wear of the carbon face.
This carbon face 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 these 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.
A lack of lubrication between the seal faces can also cause a destructive form of vibration called “slipstick”. 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 out side 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.
There is little chance of excessive heat developing between the seal faces and in the stuffing box area because the generated heat can be carried away by the conductivity of the non lubricating liquid surrounding the seal.
All of the above means that the elastomer (o-ring) will probably not be affected by the extra heat generated between the seal faces, as a result of the poor or no lubricating properties of the fluid you are sealing.
THE NON LUBRICATING GAS
This application has all of the problems associated with the sealing of non-lubricating liquids, but now you have the additional problem of heat because gases are for the most part good insulators and do not let the heat generated between the faces dissipate to the surrounding product and metal stuffing box. Heat can affect a seal several ways:
- Filled carbon faces can be damaged depending on the filler or binder that was selected. There are special filled carbons manufactured if the gas can not be adsorbed into the carbon/ graphite releasing the graphite to provide dry lubrication.
- The elastomer (rubber part) is probably the most sensitive to an increase in heat. Its proximity to the seal faces is very important in dry running applications. Heat can cause an initial compression-set of the elastomer and eventual complete destruction. Each elastomer compound has a temperature limit as well as sensitivity to certain chemicals and compounds.
- Most fluids are affected by an increase in heat. They can: crystallize, solidify, lose their viscosity, vaporize, or build a film. In each of these cases, seal life will be affected.
- The corrosion rate of most corrosive fluids will double with an 18° Fahrenheit (10°C) increase in temperature.
- Seal flatness, face load, carbon squeeze, elastomer interference and many other tolerances can be affected by a change in stuffing box temperature.
SEALING A DRY SOLID
You now have all of the problems associated with the sealing of a gas with the additional problem of a bunch of solids thrown into the mix. This application is seldom associated with pumps but is commonly found in mixer applications. The application is very similar to sealing a slurry, so you should try to select those seal designs that have non-clogging features. These features would include:
- Springs out of the fluid.
- You might consider rotating the seal in the powder to take advantage of centrifugal force, to throw the solids away from the sliding components.
- The elastomer must move to a clean surface as the seal face wears.
- Select non-fretting designs. They are especially important in dry solids applications.
- Teflon coating of the rotating parts helps to prevent the solids from sticking to the moving components.
Mixers and aggitators designed with bottom entering stuffing boxes are especially sensitive to this problem. Try to locate the seal inside of the mixer and out of the narrow stuffing box or you will have trouble with the solids packing around the outside diameter of the mechanical seal. A clean flush with air or a suitable gas seldom works in this application because the air channels through the dry solids, or the vessel pressure will equalize with the incoming air pressure stopping the flow.
Most of these applications are slow speed (less than 500 rpm.) so a non-clogging type seal works well. A non metallic, outside seal can be used if you are prepared to clean it out with air or some other gas between batches.
A split seal with air introduced into the bottom of the gland is getting good results in many applications. If the seal does clog up, it’s easy to disassemble the seal for cleaning between batches.
In some applications it;s acceptable to use a compatible grease in the stuffing box to prevent the ingress of solids. A balanced o-ring type seal, running at lower motor speeds should not generate enough heat to affect the lubricating qualities of the grease.