Chemical classification

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

Posted

  • On February 09, 2018