SUBJECT : Environmental controls and
special seals 3-2
For any given seal application problem there are two generally
accepted solutions :
- Build a special seal that can compensate for the problem.
- Control the environment surrounding the seal to prevent the
problem from occurring in the first place. If you control the seal
environment you'll avoid the inventory and delivery problems
associated with special seals.
In the following paragraphs I'll be covering each of these
environmental controls in detail.
TEMPERATURE IN THE STUFFING BOX AREA.
- Flush the stuffing box (Port "C" in the illustrations at the
end of this paper) with a compatible, cool, clean liquid. Many
seal glands have this connection available in a more convenient
location than the lantern ring connection. Flush is a
misunderstood term. It describes six very different
- Discharge recirculation, where a line is connected from the
discharge side of the pump to the lantern ring connection in the
stuffing box (A), or an appropriate connection in the gland.
- Suction recirculation, The recirculation line is connected
from the bottom of the stuffing box to the suction side of the
- Jacketing fluid, The cooling or heating fluid flows through a
jacket that is surrounding the stuffing box (B).
- Barrier or buffer fluid, The fluid is circulated between two
seals either by convection, or by a separate circulation system
- Quench, The fluid is passed between the seal and a disaster
bushing that has been installed in the rear of the seal gland
- Flush, A liquid, from an outside source, is injected into the
stuffing box at one atmosphere above stuffing box pressure and
dilutes the product a small amount (C).
- Use two seals with a cool liquid circulating between them (E).
A two way balanced cartridge seal would be an excellent choice.
This arrangement provides cooling at the seal faces where it will
often do the most good.
- Use the jacketed stuffing box that came installed on the pump
(figure "B") or install one if it is missing. These jackets are
available as a replacement part for the back plate on most popular
pumps or as an after market bolt on accessory. To use the jacket
- Dead end the fluid you're trying to control. This means no
lines in or out of the stuffing box except those used to
circulate the jacketing fluid.
- Install a thermal bushing in the bottom of the stuffing
box. Carbon is a good choice because it is a poor conductor of
heat compared to the metal pump components. A typical clearance
over the shaft would be 0.002 inches per inch of shaft diameter
( 0,01 mm/mm of shaft diameter).
- Circulate the heating or cooling fluid, through the jacket,
to control the temperature. Six to eight gpm. (25 to 30 liters
/min.) is typical of the amount of cool water needed to cool
down heat transfer fluid to the point where it will stop
"coking" and Viton O-rings will be acceptable. If your water is
too hard you can substitute condensate or low pressure
- An A.P.I. Gland is available for most mechanical seals. The
gland has several features to provide various functions. It can be
- A quench connection (Q) to provide heating, cooling, or to
remove any vapors that might escape between the seal faces.
Steam can be injected to lower the seal temperature in the
event of a fire. In the event of a major seal failure, this
quench connection can be used, in conjunction with the gland
disaster bushing, to direct seal fluid leakage to point where
it can be collected.
- A flush connection (F) to provide clean fluid to the
stuffing box or it can be used to vent the stuffing box and
seal in a vertical pump application.
- A close fitting, non sparking, disaster bushing (DB) to
provide shaft support in the event of a bearing failure or to
protect personnel in the event of a massive seal failure.
- Heat tape or tracing lines can be installed around the
stuffing box to provide a small amount of temperature
- Install a cooler in the line between the pump discharge and
the stuffing box. Keep in mind that this system only works while
the pump is operating so it would be of no value if the problem
occurs during pump shut down
- Use only balanced seals in these applications to avoid the
heat problems associated with unbalanced seal designs. Elastomers
in the faces and two hard faces should also be avoided for the
PRESSURE IN THE STUFFING BOX AREA
- Increase stuffing box pressure by installing a recirculation
line from the pump discharge back to the stuffing box (figure "A")
with a close fitting bushing in the bottom of the stuffing box.
Try to avoid positioning the recirculation line so that it aimed
at the lapped seal faces or thin bellows seal plate
- Eliminate the pressure drop between seal faces by using two
seals with a higher pressure barrier fluid circulating between
them. This is very important in the sealing of chemicals such as
ethylene oxide that will penetrate into the elastomer, expand and
blow out the other side causing severe damage to the
- Flush the stuffing box with a high pressure liquid. This is
the best solution if the fluid contains solid particles that could
interfere with the seal movement.
- The only reason to lower stuffing box pressure is because your
seal does not have high pressure sealing capability. It's possible
to lower stuffing box pressure by the use of environmental
controls but a high pressure seal would be a much better choice.
In an emergency you could lower the pressure by one of the
following environmental controls:
- Equalize the pressure in the stuffing boxes of a double
ended pump, by connecting the stuffing boxes together. This is
a common application for a double ended centrifugal pump.
- It's possible to lower stuffing box pressure by installing
a close fitting bushing in the bottom of the stuffing box and
recirculate to the suction side of the pump. Be sure to "lock
in" the position of this bushing with either a snap ring or
some other retaining device to prevent it from moving towards
the seal. Be careful of using this control on a vertical
turbine pump because the high velocity liquid, recirculating to
the suction, can heat up the line to the point where it can
become "red hot".
- Lower the sealing pressure by utilizing an intermediate
fluid pressure between two tandem or "two way balanced
PROVIDING A LUBRICANT IF
THE SEALING PRODUCT IS A NON LUBRICANT ( Non lubricants have a film
thickness less than one micron)
- Use two seals with a higher pressure lubricant as the barrier
fluid. This is an excellent choice in most gas applications or
liquids that have little to no lubricating properties. This form
of lubrication will often solve the problems associated with seal
"slipstick" and some other types of vibration.
- Flush the stuffing box with a liquid lubricant.
- Cooling the product will sometimes turn a non lubricant such
as hot water into a lubricating liquid.
- For some vacuum applications it makes sense to install a
discharge recirculation line to destroy the vacuum in the stuffing
NOTE. If the impeller has been adjusted too close to the back
plate the "pump out vanes" can cause a vacuum to occur in the
stuffing box. This often happens if the impeller adjustment has been
made backwards (as is the case with Duriron pumps). The problem
exists with those open impeller designs that adjust towards the
volute (Goulds is an example).
THE AMOUNT OF LIQUID AGITATION IN THE STUFFING BOX.
This becomes very important if you have to seal a liquid that
increases its viscosity with agitation. We call these liquids
DILATANTS. Connect the bottom of the stuffing box to the suction side
of the pump to allow a single pass of the liquid through the stuffing
box. Make sure the connection is very close to the seal faces. You'll
be better off using the seal gland flush connection rather than the
stuffing box lantern ring connection.
Some liquids decrease their viscosity with agitation. We call
these liquids THIXOTROPHIC. In some instances the thinner liquid film
can cause more face wear and seal "slip stick". If this problem
exists, use one of the environmental controls mentioned above.
- Flush with a clean liquid. Check with your seal manufacture
for the minimum amount of flush that is needed. Balanced seal
designs with the springs located outside of the fluid and most
metal bellows designs require only one to two gallons (4 to 8
liters) per hour.
- Note: this is per hour not per minute. At shaft speeds
below 2900 rpm. filling the seal cavity with a compatible
grease is usually satisfactory.
- Increase the seal clearance in the stuffing box area.
Replacement back covers with extra large stuffing box designs are
available for most pumps. Bolt on, large diameter stuffing boxes
are also available in the after market. If your product is cool
you can probably run the fluid "dead ended" with no connections
coming into or out of the stuffing box.
- If you product contains sub-micron particles, as is the case
with Kaoline (china clay), you'll have to circulate a higher
pressure clean liquid between two seals to prevent solids
penetration between the faces. In some cases two hard faces also
helps. Almost any dual seal design is acceptable with the
exception of the "back to back" rotating design which is never
acceptable in any application.
- Recirculate to the suction side of the pump when possible.
This will circulate cleaner fluid from behind the impeller,
through the stuffing box, and then back to the suction side of the
pump. Original equipment manufacturers do just the opposite by
having the stuffing box fitting connected to the discharge side of
the pump. CAUTION! Do not connect to
the suction side of the pump if :
- The fluid is being pumped at or near its vapor point as
this could cause flashing in the stuffing box location.
- If the solids have a low specific gravity (they float on
the liquid) you may have to go to a clean liquid flush because
centrifugal force will work against you.
- Durco or Flowserve pumps use a stuffing box that is
pressurized close to suction pressure. so recirculaion cannot
- Single stage, double ended pumps position the stuffing
boxes at the pump suction. You will not be able to recirculae
the stuffing boxes to the suctionand get flow.
- Any time that you deal with a slurry application you're going
to have a couple of other problems as well ,so be prepared for
- Frequent impeller adjustment and excessive wear ring wear.
You'll need a cartridge seal or a sleeve mounted split seal to
compensate for the impeller adjustment. Cartridge seals can
generally be reused if the pump has been disassembled to
replace the wear rings.
- Vibration will increase as the impeller goes out of balance
due to abrasive wear. This can cause drive lug wear and carbon
face chipping. Vibration damping will become very important.
Seal designs that incorporate o-rings have a built in natural
vibration damper. Metal bellows seals require a separate
- Wear of the rotating components. This is especially true if
the seal rotates in the fluid. Better seals are designed to
cause rotation of the fluid in the seal chamber.
If you prefer to solve the application problem by using a special
seal. the following thoughts might help in deciding your
FEATURES THAT ADDRESS THE PROBLEMS OF EXTREMES IN HOT AND COLD.
- Metal bellows seals. An excellent choice for cryogenic and
high temperature, non petroleum liquids. Petroleum products "coke"
in the presence of high heat so cooling is necessary in these
- Carbon / metal composites to conduct heat away from the seal
faces. Do not use "glued in" versions.
- Elastomers located some distance from the seal face to protect
the elastomer (rubber part) from the additional heat generated at
the seal faces
- Low friction face combinations. Carbon / tungsten carbide or
Carbon/ silicone Carbide are among the best. Some duplex material
faces are showing good results in these applications. Carbon
impregnated silicone carbide is an example of such a
- Elastomers that have a wide range of operating temperature.
Kalrez® is a good example.
- Low expansion metals such as Carpenter 42 and Invar 36 that
will still retain the carbon or hard face in the holder even
though the temperature changes greatly. Be aware that low
expansion metals have poor chemical resistance so be careful in
- Stationary seal designs are subject to a differential
temperature across the seal face and body if a recirculation line
or flush is being used. This differential temperature can cause
the face to go out of flat. You will be better off with a rotating
design in this instance.
If you elect to solve only the sealing problem you must keep in
mind that the extremes in heat and cold will also affect the bearing
seals as well as the bearing oil. Unless you address these problems
separately you will be better off controlling the temperature in the
stuffing box area and solving most of the bearing area problems at
the same time.
FEATURES THAT ADDRESS THE PROBLEM WITH SLURRIES.
- Springs out of the fluid, the most common place to clog a
- Vibration damping because wear causes the rotating assembly to
go out of balance.
- Be sure the dynamic elastomer moves to a clean surface as the
seal carbon face wears.
- Use centrifugal force to clean the sliding seal components.
Rotating seals (the spring loaded face rotates) should be your
- Non stick coatings on the metal parts to prevent a build up of
solids on the sliding components. These coatings are porous so
don't use them for corrosion resistance.
- If possible, rotate the slurry to reduce seal component
®Dupont Dow elastomer
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