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:
- Become viscous
- 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
- Fluids that emit fugitive emissions
- Expensive fluids
- Any time a premature seal failure would cause an expensive down time.
- 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)
- Dry solids
VI. Fluids that react together to form a solid. Often a mixer or blender application
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