Seal application outline, chet sheet

SEAL APPLICATION OUTLINE 19-06 

SEAL MATERIALS

 UNFILLED CARBON/ GRAPHITE

  • The ideal soft face material
  • Do not use with:
    • Oxidizers or halogens
    • Color contamination problems
    • Hot oil – possible pitting
    • Deionized water. Carbon was often removed during the process

HARD FACES

  • Alpha sintered silicon carbide – no small cross section pieces
  • Reaction bonded silicon carbide – do not use in high pH fluids
  • 99.5 Ceramic – No hot oil or water
  • Nickel TC – Possible galvanic corrosion with stainless steel
  • Cobalt TC – Not as corrosion resistant as nickel tungsten carbide

FACE COMBINATIONS

  • Unfilled carbon/Alpha sintered – For most application
  • Unfilled carbon/99.5 – Avoid in hot oil or hot resin applications
  • Alpha sintered/ nickel TC – Good in hot oil, abrasive slurry
  • Alpha sintered/ reaction bonded – An alternative hard face

ELASTOMERS- Use the four step procedure to select the correct one for your application:

  • Look in an elastomer book. Watch out for flush fluids that are not compatible with the elastomer
    • Consider temperature limits of the elastomer
  • Look for elastomer experience in your plant for compounds and fluid mixtures that are not in the book.
  • Valves
  • Other seals
  • Gages
  • Covers
  • Flanges
  • Test for compatibility
    • Immerse in the fluid for 1 to 2 weeks
    • Look for change in weight, shape, appearance
  • Chemraz or Kalrez is a high priced alternative. Do not use Chemraz in high pH fluids
  • Miscellaneous elastomer problems:
    • Ethylene oxide blows out particles of the elastomer
    • Buna, shelf life (Subject to ozone attack)
    • EPR – not compatible with petroleum products.
    • Viton – possible compatibility with water problem

METAL COMPONENTS EXPOSED TO THE SEALING FLUID

  • Choose same metal as wet end of the pump, if not corroding.
  • Iron, steel, SS or bronze wetted parts, can probably use 316
  • Hastelloy “C” for seal springs and bellows to prevent chloride stress corrosion problems
  • Non metallic pump – use non metallic seal design

ENVIRONMENTAL CONTROLS

CONTROL TEMPERATURE IN THE SEAL AREA

  • Jacket on pump – circulating condensate or steam
  • Quench gland
    • Watch out for bearing contamination
  • Dual seals – avoid oil as a barrier or buffer fluid because of its low specific heat and conductivity
  • Place a pumping ring in stuffing box
  • Heat exchanger – only works while pump is running. Clogs easily
  • Flush with liquid at the correct temperature – 15 lbs (1 atmosphere). higher than stuffing box pressure
    • Water
    • Compatible fluid
    • One of the ingredients
    • Finished product
    • Additive
  • Heat tape around the stuffing box.
  • Build a dog house over the pump with a steam line inserted to heat the contents

CONTROL THE PRESSURE IN THE STUFFING BOX

  • Discharge recirculation to increase pressure. Use a restrictive bushing in bottom of the stuffing box.
  • Suction recirculation to lower pressure
  • Cross connect stuffing boxes in a double ended pump
  • Dual seals with a pressurized barrier fluid

REPLACE THE FLUID IN STUFFING BOX

  • Suction recirculation to remove solids.
  • Large stuffing box to centrifuge the fluid
    • No tapered designs
  • Pressurized barrier fluid
  • Flush with a compatible liquid
    • Water
    • Compatible fluid
    • One of the ingredients
    • Finished product
    • Additive that will be inserted down stream.
  • Grease for slow speed applications
  • Pump back seal

SEAL APPLICATIONS

 

FLUIDS SENSITIVE TO TEMPERATURE AND PRESSURE CHANGES’
They can:

  • Build a film on the seal sliding components and between the lapped faces
  • Change viscosity
  • Crystallize
  • Increase corrosion
  • Solidify
  • Vaporize

FLUIDS SENSITIVE TO AGGITATION

  • Dilatant – thickens
  • Newtonian – doesn’t change
  • Pseudo-plastic – change viscosity suddenly
  • Thixothrophic – thins out

SLURRIES
In slurry applications do any of the following:

  • Large bore stuffing box – not tapered
  • Suction recirculation
  • Flush with a compatible liquid using a restrictive bushing
  • Grease for slow applications
  • Pump back seal
  • What does not work:
    • Filter/strainer in the pump discharge recirculation line
    • Cyclone separator
    • Back to back dual seals with a clean liquid between them
    • Two hard faces
    • Mount outside to keep the springs out of the slurry

NON-LUBRICANTS

  • Non lubricating liquids (includes most solvents)
    • Use dual seals with a barrier fluid
  • Gases
    • Use dual seals with a barrier fluid
  • Dry solids
    • Use centrifugal force to throw the solids away from the sliding seal components
    • Blowing a gas in does not work. It channels through the solids

USE DUAL SEALS WITH:

Type of fluid:

  • Dangerous
  • Fire hazard
  • Toxic
  • Carcinogen
  • Explosive
  • Radiation
  • Pollutant
  • Must conform to the Clean Air Act
  • Fugitive emissions
  • Expensive products
  • Back up for predictive maintenance

FLUIDS THAT COMBINE TOGETHER TO FORM A SOLID

  • Usually mixed outside pump
  • Flush pump with tank cleaning liquid

CLEAN LUBRICATING LIQUIDS

  • No problem

EXTREME OPERATING CONDITIONS

TOO HOT FOR THE SEALING FLUID OR A SEAL COMPONENT

  • Metal bellows for non-coking fluids
  • Cool the product in the stuffing box
  • Problems when pump is not running:
    • Soak temperature
    • You must protect the bearings

CRYOGENIC FLUIDS

  • Metal bellows
  • Special carbon/ graphite containing an organic to release the graphite
  • Isolate moisture outboard, or it will freeze restricting movement of the seal

HIGH PRESSURE IN THE STUFFING BOX. >400psi or 27.5 bar

  • Seal face distortion
    • Increase mass
    • Use finite element analysis techniques in design
  • Face load
    • Hydraulic balance essential
  • Extrude dynamic elastomer
    • Higher durometer elastomer
    • Back up rings
  • Metal bellows
    • Thicker plates
    • Laminated plates

HARD VACUUM (less than 0.001 Torr)

  • Elastomer can “out gas” and leak
    • Non elastomer bellows
  • Face leakage
    • Prevent distortion with finite element analysis
    • Self-align cartridge seal design to prevent excessive axial movement
    • Stationary design non-cartridge seals
    • High density carbon/ graphite seal face

HIGH SPEED

  • Stationary seal design
  • Lower face load
  • Low friction faces
  • No elastomers in face
  • Move close to bearings

EXCESSIVE MOTION

  • Wide hard face
  • Increase internal clearances
  • Locate the seal close to bearings

EXCESSIVE VIBRATION

  • Sources of vibration:
    • Harmonic
    • Out of balance
    • Slipstick
  • Use vibration damping
    • O-ring is a good damper
    • Metal bellows designs need a damper device

Posted

  • On February 13, 2018