19-09 MECHANICAL SEAL FACE DAMAGED:
Corrosion damage. The elastomer swells, or the other seal parts become "sponge like" or pitted. We do not think of elastomers corroding we tend to call it chemical attack or chemical incompatibility, but regardless of the words, the affect is the same. Corrosion increases with temperature
- The product you are sealing is attacking one of the seal components.
- The attack is coming from the cleaner, or solvent used to flush the lines between batches or at the end of a "run".
- The attack is coming from lubricants put on the elastomers or seal faces. Petroleum grease on ethylene propylene (epr) O-rings will cause them to "swell up".
- Galvanic corrosion - happens with dissimilar materials located close to each other and connected by an electrolyte. As an example: stainless steel can attack the nickel binder in a tungsten carbide face.
- Oxidizers and halogens attack all forms of carbon including black O-rings.
Physical damage is the next cause of damage to a seal component:
- A discharge recirculation line can fracture a metal bellows and injure lapped seal faces, as well as interfere with the free movement of the seal.
- Back to back rotating seals experience damage at the inner seal face when pumping fluid contains abrasive solids
- Erosion from solids in the product you are pumping.
- Fatigue of the springs caused by severe misalignment.
- Fluid abrasion that can weaken materials and destroy critical tolerances.
- Fretting caused by the dynamic elastomer removing the passivated layer from the corrosion resistant shaft or sleeve.
- Problems at installation. These include mishandling, setting at the wrong compression, putting the wrong lubricant on the elastomer etc.
- Temperature extremes (both high and cryogenic) will destroy elastomers and some seal face materials.
- The elastomer can swell and breaks the face if it is mounted at the face inside diameter.
- The rotating seal hits something because of shaft deflection.
- Thermal expansion of the shaft/sleeve can break a stationary seal face or interfere with the free movement of a dynamic elastomer.
- Thermal shock of coated or plated seal face materials. Some ceramics also have this problem.
- Wear or rubbing of a flexible or moveable component.
Damage to individual seal components:
Springs/ metal bellows
- Compression set – austenitic materials
- Work harden with excessive flexing
Metal parts – Corrosion
- Chloride Stress corrosion. Three things necessary. The exact percentage of each is unknown
- 300 series. (The most popular)
- High temperature.
- Tensile stress.
- Forms Cr2O3 on metal surface. This is called pacification
- Corrosion slows down below 40°F (5°C) and above 285°F (140°C.
- Concentrated cell or crevice corrosion
- Salt water is low in Ph (8.0 to 9.0) and high in chlorides
- Erosion/ corrosion
- Doubles each 20°F rise in temperature
- At 1050°F (565°C) to 1600° F (870°C) Carbon in steel combines with the chrome to form chrome carbide.
- This is a normal welding temperature.
- How to fix the problem
- Anneal the piece after welding to remove stress.
- Use low carbon stainless steel.
- Add columbium to form columbium carbide. Metal is said to be stabilized.
- Aerobic, the kind that need oxygen.
- Anaerobic, the kind that do not need oxygen.
- Facultative, the type that goes both ways. They operate with or without oxygen
- Selective leaching
- Zinc embrittlement
Rotating part hitting something
- Protruding gasket,
- API bushing
- S/B support bushing
- Thermometer well
- Loose part-old spring/setscrew
- Rubbing part
- Stuffing box dry
CARBON/ GRAPHITE FACE
- Blistering, pitting
- Distortion (flatness)
- In backwards
- Phonograph finish
- Tapered stuffing box
- Corrosion— high pH
- Compression set
- Cutting on setscrew burr
- Ethylene Oxide blow-out
- Hardening - soak
- High Heat
- Ozone attack (buna)
- Using a metal pick for O-ring removal & damaging groove
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