IS THIS A SEAL APPLICATION?
You have three opportunities to do a seal application:
- You're purchasing a new piece of rotating equipment and you'd like it shipped with a mechanical seal that has a reasonable
chance of working.
- You're converting a piece of rotating equipment that has been
packed with conventional jam packing to a mechanical seal, because
you want to stop leakage.
- You're troubleshooting a premature mechanical seal
First we'll look at the piece of equipment you'll be sealing.
Sometimes mechanical seal life is directly related to your rotating equipment
- A single stage centrifugal pump, running at electric motor
speeds, is a good seal application candidate.
- A multistage centrifugal pump can work well with mechanical
seals if you cross connect the stuffing boxes to equalize the
stuffing box pressures and avoid high pressure sealing on one end
of the pump.
- Vertical pumps work well with mechanical seals because there
is usually plenty of room between the stuffing box and the bearing
housing. Be sure to vent the stuffing box back to the pump suction to
prevent trapping air at the seal faces
- Positive displacement pumps work well with mechanical seals,
but there is sometimes a space problem when you try to fit the
seal into the stuffing box; especially split seals that mount
outside the stuffing box.
- Most positive displacement pumps have the stuffing box
mounted on the low-pressure side of the pump, but there are
- Many PD pumps run at slower speeds, making sealing
- Some progressive cavity pumps are piped backwards causing a
high pressure in the stuffing box.
- Sleeved bearing equipment can present problems for mechanical
seals because of excessive shaft axial and radial movement. You
should try to use motion seal designs in these applications.
- Many boiler feed and deep well pumps fall into this
- Mixers and agitators have lots of shaft displacement problems
that can affect seal life. Again motion seals are a logical choice
in these applications.
- Many of these mixers and agitators run at slow shaft speeds
making sealing easier.
- Bottom entering shafts have real seal problems if there are
solids in the liquid.
- Paper mill refiners and similar pieces of equipment
experience excessive axial movement problems.
Constant running equipment is easier to seal than intermittent
- The sealing liquids tend to stay in a liquid form rather than
crystallize, solidify or change state when the equipment shuts
down and the fluid changes temperature.
- Breakaway torque is a common cause of premature seal failure,
causing the lapped seal faces to open.
Pumps located inside buildings are not subject to the changes in
weather that affect some liquids you'll be sealing.
- The stuffing box has to be kept warm during cold
weather to prevent the liquid from becoming viscous or
The driver you choose can affect the life of the mechanical
- Electric motors generally work well with mechanical seals
- Gasoline or diesel engines, along with variable speed electric
motors, sometimes run at or pass through a critical pump speed.
- Steam, water and gas turbines sometimes run at speeds that are
to high for rotating seal designs. You'll need the sationary springdesign for these applications
- Pulley driven equipment often experiences problems with shaft
Does the equipment you'll be sealing, fall into the following
categories? If so, the seal has a good chance of surviving with an
environmental control. If your application falls outside these
categories you might consider a special seal design:
- A temperature range of - 40 to 400°F (-40 to
- Cryogenic or cold temperature sealing requires:
- A special cabon/graphite seal face.
- Metal bellows designs to eliminate O-rings and similar
rubber elastomers that are sensitive to cold
- An environmental control to prevent ice formation
outboard the mechanical seal that can interfere with the
- High temperature sealing requires:
- The elimination of elastomers in some applications. Metal
bellows seals are often a good choice for polymers and
- Vibration damping.
- Cooling of petroleum and most oil products to prevent coke
- A method of retaining the carbon/graphite seal face to
prevent it from falling out of a metal holder.
- A pressure range of one Torr to 400 psi (one Torr to 7 bar) in
the stuffing box?
- Harder vacuums can out-gas rubber parts casing them to
- Higher pressures can distort some seal faces and extrude
- Electric motor speeds or slower?
- High speeds require a lowering of the seal hydaulic balance ratio, less
spring load at the seal faces, low friction faces, and a
stationary spring design to prevent the lapped faces from
- Be aware of cleaners, steam and solvents that might
circulate in the lines after, or between batches. Whatever seal
you choose must seal these fluids also.
If you're troubleshooting a seal failure consider the
- Is the same seal running successfully, in this same
application in another location in the plant? If it is, you should
look at the pump for the problem, not the seal.
- When did the failure occur? Did something change in the
operation? Rubber components often fail in five to ten days after
being exposed to an incompatible fluid (They swell up).
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