Is this a mechanical seal application?
You have three opportunities to do a mechanical seal
- You are purchasing a new piece of rotating equipment and you
would like it shipped with a mechanical seal that has a reasonable
chance of working.
- You are 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 are troubleshooting a premature mechanical seal
First we will look at the piece of equipment you will be sealing.
Sometimes mechanical seal life is directly related to the type of
hardware you are sealing:
- 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 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
- Some progressive cavity pumps are piped backwards causing a
high pressure in the stuffing box.
- Many PD pumps run at slower speeds making sealing easier.
- Submersible pumps sometimes lack the room for a good seal
installation. Often, dual seal applications can be converted to a
single seal, sealing the bearings in these applications.
- 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 also
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 will be sealing.
- Many times 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 a critical pump speed.
- Steam, water and gas turbines sometimes run at speeds that are
to high for some mechanical seals.
- Pulley driven equipment can have problems with shaft
Does the equipment you will 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 200° C)?
- 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 temperatures.
- An environmental control to prevent ice formation outboard
the mechanical seal that can interfere with the seal
- 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
- Harder vacuums can out-gas rubber parts casing them to
- Higher pressures can distort some seal faces and extrude some
Electric motor speeds or slower?
- High speeds require a lowering of the balance ratio, less
spring load at the seal faces, low friction faces, and a
stationary design to prevent the lapped faces from
Always consider 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 are troubleshooting a seal failure and think you might have
an application problem, consider the following:
- 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 or the seal installation as the problem, not the seal
- When did the failure occur? Did something change in the
operation? Rubber parts normally fail in five to ten days after
being exposed to the incompatible fluid.
- Has anything changed in the operation of this piece of
- Continuous service has been changed to intermittent service,
with frequent starts and stops.
- A different solvent or cleaner is being used to flush out the
- The temperature or concentration of the pumping fluid has
- Is the environmental control operating when the pump is shut
down or between batches?
- Does a control valve in the pump discharge sometimes cause the
pump to operate off its BEP, causing shaft deflection or
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