SUBJECT: More about packing conversion.
Sales people constantly preach the virtues of converting that
nasty, greasy packing to a brand new, shiny, expensive mechanical
seal. Their presentation makes a lot of sense and besides that the
government is passing new legislation that is making conventional
packing more obsolete by the minute.
If you are about to make one of these conversion in your plant
there are some things that you should know to make the transition
easier. Packing conversion does have its down side. Let's look at a
few of the problems. We will then discuss some of the solutions, and
end this discussion with the advantages of converting to a better
mechanical seal .
First the problems with the
types of seal supplied by your pump company or the original equipment
- Unlike packing, mechanical seals are very sensitive to
- You have to measure carefully. If you use the improper face
load the seal will either overheat or lose its spring tension
before the carbon face is worn.
- The shaft or sleeve tolerance and finish are critical for
many seal designs. A typical shaft tolerance for a mechanical
seal should be +0.00 inches - 0.002 inches. A finish of at
least 32 rms. is required to prevent a sliding elastomer or
Teflon piece from hanging up.
- You cannot make the initial impeller setting for most ANSI
pumps without disturbing the seal setting.
- Many seal are damaged by poor packaging during shipment or
improper handling at the installation site. Seal faces are
lapped to a tolerance of less than one micron (0.000039
inches). There are not very many pieces of equipment in your
shop with that tight a tolerance.
- Without the radial support being supplied by the packing, the
shaft can deflect severely when the pump operates outside of its
very narrow operating window. This is a major problem at start up
when the pump is frequently running against a throttled discharge
- Packing is not sensitive to axial movement of the shaft. Seals
have a very small axial movement capability.
- Mechanical seals are very sensitive to pump/ driver
- Cavitation and other types of vibration are deadly for
mechanical seals. Although never desirable this type of shaft
deflection did not often cause catastrophic failure with packing
as it often does with mechanical seals.
- You could change packing without having to dismantle the pump.
Most seal installations require pump disassembly and all the
problems associated with it.
- Many pump and mixer applications alternate between a positive
stuffing box pressure and a vacuum (condensate pumps are typical).
OEM seals can be blown open during the vacuum cycle.
- Most mechanical seals require some sort of an environmental
control to function properly:
- Cooling to prevent a product from flashing or coking.
- Heating to prevent a liquid from solidifying or
- Flushing to wash away solid particles from the seal
- Quenching to prevent crystals and ice from forming outboard
of the seal interfering with its movement.
- Mechanical seals are very sensitive to fluctuating flushing
pressures. Lose the pressure and you often lose the seal. Needless
to say this is a very common failure.
- Seal failure is typically catastrophic and occurs at the worst
- Mechanical seals have to be centered in the stuffing box. You
never worried about that with packing. If you fail to center the
rotating seal's stationary face the rotating face can run off the
edge. With stationary seals the failure to center can cause a
wiping action across the seal faces.
- Impeller adjustment was easy with packing. With a
non-cartridge seal it is almost impossible. Remember that with
ANSI pumps the impeller has to be adjusted after the power end of
the pump has been installed into the wet end that was left hanging
on the piping. At this point the seal has already been set screwed
to the shaft or sleeve and impossible to move.
- Seal materials have to be selected for chemical compatibility
with the product you are sealing and any cleaners or solvents that
will be flushed through the lines. You need an in depth knowledge
of mechanical seals and chemicals to do this properly because of
the wide variety of materials currently being used by seal
manufacturers. This was seldom a problem with choosing packing
- Intermittent service pumps have more problems with mechanical
seals because the fluid can solidify, crystallize, become viscous,
etc. when the pump is not running.
- This change in the state of the product will almost
guarantee a failure the next time the pump is started. Packed
pumps had the same problem with the product changing state, but
the catastrophic failure at start up seldom happened. Standby
pumps experience the same difficulty.
- Spare parts are always a problem with mechanical seals. The
subject never came up with packing.
- The stuffing boxes of vertical pumps have to be vented after a
mechanical seal is installed. There is also a problem with venting
between dual seals when they have been installed in a vertical
application. Packed pump vent all the time.
- Horizontally split case pumps are a nightmare for mechanical
seals. You get massive misalignment between the rotating and
stationary seal faces because the top and bottom halves of the
casing are never lined up properly. This subject never came up
when packing was in the pump.
- Most original equipment seals will damage a shaft or sleeve so
you have the same problem with sleeve removal that you had with
packing. When the seal damages the shaft it is called "fretting",
but no matter what name you give it, it is still shaft damage
requiring a disassemble of the pump to replace either the shaft or
- Pumps of the same shaft size seldom take the same seal
Now that you know the bad news,
here is the good news. You can purchase mechanical seals that solve
many of the above problems.
- Hydraulically balanced seals do not generate a lot of heat so
they seldom require any cooling. A suction recirculation line is
almost always satisfactory.
- Cartridge seals solve most of the installation problems.
- The closer you move the seal to the bearing, the less affect
of vibration and shaft displacement. Newer cartridge designs are
located closer to the pump bearings.
- Stationary seals and self aligning designs solve most of the
problems you experience with horizontally split case pumps.
- There are plenty of non-fretting seal design on the market
today so shaft damage can be eliminated.
- Most modern cartridge seals have built in connections that
allow you to vent the stuffing box in vertical applications.
- Newer face materials and the latest elastomers are chemically
compatible with a wide cross section of chemicals and cleaners.
With few exceptions you should be able to put the same seal in
every pump of the same shaft size. The exceptions are non metallic
and exotic metal pumps that require a different seal
- Cartridge designs have solved the impeller adjustment and
centering problems that were common with the ANSI standard
- Split seal designs have solved the problem of having to
dismantle the pump to change a seal.
- Dual seals will solve the catastrophic seal failure
There is no
question that mechanical seals are better than packing if you know
how to deal with the problems mentioned above. The advantages of the
cartridge mounted balanced seal over conventional packings are
numerous, here are a few:
- Balanced seals consume one sixth the power of conventional
- Pollution and fugitive emission legislation prohibits the
leakage of even small amounts of many chemicals and packing does
- Most products are costly. Even a small steady leak represents
- Leakage always presents a safety and housekeeping
- The waste treatment of leakage can exceed the cost of the
leakage its self.
- Packing requires cooling that is often accomplished by the use
of a flush that will dilute your product. The cost of removing the
flush water at some later part in the process is another costly
- Packing leakage is a major cause of premature bearing failure.
Not only from the liquid leaking out of the pump getting into the
bearings, but because a water hose is often used to wash leakage
away from base plates, and that is the moisture that is our major
- Sleeve damage is costly not only because of the sleeve cost,
but also the costs involved in the sleeve removal. Seal removal
almost always means changing the bearings and wear rings also.
Also many parts get broken or lost when pumps are
- Balanced O-ring seals can seal either vacuum or pressure.
- Unlike packing, mechanical seals do not have to be adjusted to
compensate for wear.
- There are many seals available today that can be installed on
shafts that have been damaged by packing or fretted by other
mechanical seals. Many of these new designs install outside of the
stuffing box where the shaft is still in good shape.
- Split seal designs removed the last advantage of packing.
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