SPECIAL SEAL DESIGNS SS001_5
If you prefer to solve your sealing problem by using a special seal rather than an environmental control the following thoughts might help in making your selection.
Seal design features that address the problems of extremes in hot and cold.
- Metal bellows seals. An excellent choice for cryogenic and high temperature, non-petroleum liquids. Petroleum products are the exception because they “coke” in the presence of high heat so cooling is necessary in petroleum applications. Heat treated Inconel 718 is the preferred bellows material
- Carbon pressed into metal holders to conduct heat away from the seal faces. Do not use “glued in carbon” versions.
- Low expansion metals such as Carpenter 42 and Invar 36 that will still retain the carbon or hard face in the holder even though the temperature changes greatly. Be aware that low expansion metals have poor chemical resistance so be careful in using them.
- Elastomers located some distance from the seal face to protect the elastomer (rubber part) from the additional heat generated at the seal faces
- Low friction face combinations. Carbon/ tungsten carbide or carbon/ silicon carbide are among the best. Some duplex material faces are showing good results in these applications. Carbon impregnated silicon carbide is an example of such a material that is finding wide use in hot water applications.
- Elastomers and plastics that have a wide range of operating temperature. Kalrez® and Chemraz are examples of these elastomer like materials.
- Stationary seal designs are subject to a differential temperature across the seal face and body if a recirculation line or flush is being used. This differential temperature can cause the face to go out of flat. You might be better off with a rotating design in this instance.
If you elect to solve only the sealing problem you must keep in mind that the extremes in heat and cold will also affect the bearing seals as well as the bearing oil. Unless you address these problems separately you will be better off controlling the temperature in the stuffing box area and solving most of the bearing area problems at the same time.
Seal design features that address the problem with slurries.
- Springs out of the fluid, the most common place to clog a seal.
- Vibration damping because the excessive wear of exposed components causes the rotating assembly to go out of balance.
- Be sure the dynamic elastomer moves to a clean surface as the seal carbon face wears.
- Take advantage of centrifugal force to clean the sliding seal components.
- Bake on non-stick coatings on the metal parts to prevent a build up of solids on the sliding components. These coatings are porous so do not use them for corrosion resistance.
- If possible, rotate the slurry in the stuffing box with the mechanical seal to reduce seal component wear.
Seal design features that address fugitive emission problems.
- Stationary dual seal designs that eliminate most of the seal movement caused by mis-alignment and operating off the pump’s best efficiency point (BEP).
- Dual hydrodynamic gas seals.
- Dual hydrostatic gas seals
Now we will take a look at some special seals that include:
- High speed applications. SS001
- High pressure sealing. SS02
- Sealing vacuum. SS003
- Sealing cryogenic fluids SS004.
- Sealing mixers and agitators with excessive shaft movement. SS005