SUBJECT: Classifying chemicals for mechanical sealing.

I. Fluids sensitive to small changes in temperature and or pressure. The temperature change can cause the fluid to:

• Vaporize
• Crystallize
• Become viscous
• Solidify
• Build a film on the seal sliding surfaces
• Increase the corrosion rate of the fluid. A 10°C. increase can double the corrosion rate of most fluids.

II. Fluids sensitive to agitation (non Newtonian fluids)

• Dilatants increase their viscosity with agitation
• Thixothrophic fluids decrease in viscosity with agitation.
• Plastic fluids release their viscosity suddenly. Catsup is an example.

III. Fluids that need two seals

• Dangerous fluids
• Pollutants
• Carcinogens
• Fluids that emit fugitive emissions
• Expensive fluids
• Any time a premature seal failure would cause an expensive down time.

IV. Slurries

• You must pump the fluid at a velocity of 4 to 7 ft/sec. (1,2 to 2,3 meters / sec. to prevent a bed of solids in the pipe.
• Slurries are defined as solids in liquid. The number and size of the solids is not important.
• Abrasion becomes a problem when you pump at a velocity greater than 10 Ft./sec. (3 Meters/ sec.). The abrasive fluids can:
  • Clog internal seal parts
  • Throw the impeller out of balance
  • Cause frequent open impeller adjustments
  • Can cause excessive wear of the metal bellows plates

V. Non Lubricating fluids (in order of difficulty to seal)

• Liquids
• Gases
• Dry solids

VI. Fluids that react together to form a solid. Often a mixer or blender application

• Polymers
• Epoxies

VII. Clean lubricating liquids

• Most finished products fall into this category

VIII. Extreme operating conditions. These are non-chemical, but present seal problems.

• The fluid is too hot for a seal component
  • The O-ring or any elastomer is the most sensitive to high temperature.
  • Some filled carbon have a low temperature limit.
• The fluid is a cryogenic. It is too cold for a seal component
  • The O-ring is sensitive to temperature changes
  • The carbon must be lapped at cryogenic temperature.
• The pressure in the stuffing box (not the discharge pressure) is too high. It is greater than 400 psi (28 bar)
  • The load on the seal faces can be too high causing excessive heat.
  • The elastomer (O-ring) can extrude and become damaged.
  • The lapped seal faces can deform and go out of flat.
  • The seal can push off the set screws.
• Hard vacuum. Less than 1 torr
  • The elastomer (O-ring) will shrink and leak.
• High shaft speed. Greater than 5000 fpm. (25 m/sec.)
• Excessive shaft axial & radial motion
  • Axial movement causes changes in the seal face loading. Radial movement can separate the lapped faces.
• Excessive vibration

IX. Not listed in the above categories.

• Polishing liquid
  • Antimony Peroxide
• Anaerobic fluids
  • Super glue

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