SUBJECT : A few rules of thumb for
mechanical seals 2-5
Before selecting your mechanical seal design there are three
things you want to remember:
- All of the seal materials must be chemically compatible with
any fluids that will be pumped through the system and that
includes solvents, cleaners or steam that might be introduced into
the system to flush or clean the lines. It also includes any
barrier fluids that are used to circulate between dual mechanical
- The seal faces must stay together. If they open, the seal will
leak and allow solids to penetrate between the faces. The solids
will eventually destroy the lapped surfaces.
- Good seal life is defined as running the mechanical seal until
the carbon face is worn away. Any other condition is called a seal
failure and is always correctable
The following is offered as a guide when dealing with mechanical
seals in general. If possible you should contact the manufacturer for
specific recommendations and limits. I have spent the past
thirty-five years lecturing about seals and pumps and during that
time have picked up a number of rules that are worth remembering.
Here are some of the most important:
Selecting materials - The elastomer ( the rubber part)
- There are two temperature limits for a mechanical seal:
- You must not exceed the temperature of the seal components.
As an example, Ethylene Propylene rubber cannot seal hot fluids
in excess of 300° degrees Fahrenheit ( 150° C)
without taking a compression set and eventually leaking.
- You must not exceed the temperature limit of the fluid
you're pumping. Many fluids will change from a liquid to a gas,
solid or crystal at elevated temperature. In almost every case
this will cause a seal failure. As an example: petroleum
lubricating oil cokes between 250 and 300 degrees Fahrenheit
(120° C. to 150° C.) and restricts the movement of
the seal components. A Viton® O-ring, in this application
would not have been subjected to its temperature limit, but you
could have the seal failure because of the coking problem
- Halogens will attack Teflon® coated elastomers . Halogens
are easily identified because they end in the letters " INE". The
list would include Bromine, Chlorine. Astatine, Fluorine, and
Iodine. These Halogens will penetrate the Teflon® coating and
attack the base rubber material causing it to swell and split the
Teflon sleeve or coating.
- Most Viton® compounds are attacked by water. Be sure to
check if you have the recommended grade. Remember that steam is
another name for water. The steam cleaning of lines is very common
in the process industry. Caustic is another common cleaner tht
contains a high percentage of water.
- Buna "N" (Nitrile) i has a short shelf life. This is the
elastomer that is most often used in Rubber Bellows Seals. The
problem is Ozone attack. Ozone is produced by the sparking of
electric motors, so it is a very common problem. A typical shelf
life for most Buna compounds would be one year.
- If a round O-Ring becomes square in operation (compression
set) it's almost always caused by excessive heat. Chemical attack
is usually recognized by a swollen and soft elastomer while high
heat will produce a shrunken, hard one.
- Chemical attack of the elastomer will usually cause a seal
failure within five to ten days. The swollen elastomer will "lock
up" the mechanical seal and in some instances, open the lapped
Determine the correct O-Ring by one of the following methods:
- Look up the chemical in published o-ring charts provided by
all reputable seal companies. Look under "O-ring
compatability" in the chart section of this web site
- Check to see if your plant has any experience with o-rings ,
in this fluid,. O-ring can also be found in filters, strainers,
valves, flanges, expansion joints etc..
- Test theo-ring by immersing it into the sealing fluid for one
week. If the o-ring changes weight, shape, or appearance, it is
not compatible with the fluid.
- Use a universal o-ring compound such as Green Tweed's Chemraz,
Dupont's Kalrez® or a similar product.
- When choosing an o-ring , or any other elastomer, be sure to
consider any cleaners or solvents that might be flushed through
the lines, or that could come into contact with the seal. The
elastomer must be compatible with these fluids also.
- Never use " glued together" elastomers in a split seal or any
"dynamic" application. A hard spot will be created that will
interfere with the movement of the dynamic elastomer.
Selecting Materials - The Faces.
- Carbon and most hard face materials have an expansion rate of
about one third that of stainless steel.
- Use two hard faces if the product has a tendency to solidify
between the seal faces. Never use plated or coated hard faces in
these applications. Hard faces are recommended if you find that it
is impossible to keep the seal faces together and solids are
present in the sealing liquid. Two hard faces are also recommended
in the sealing of hydrocarbons that have to pass a "fugitive
emissions" test. Coke particles forming between the faces will
pull pieces of carbon out of the carbon/graphite face presenting a
leak path for fugitive emissions.
- Although many carbon graphite compounds are available,
unfilled carbons are the best because they are corrosion resistant
to almost all chemicals except oxidizing agents and some de
ionized water applications. These oxidizing agents will combine
with the carbon to form Carbon Monoxide and Carbon Dioxide. The
most common oxidizers are oleum, sulfur trioxide, strong bleaches
and nitric Acid. You cannot use any form of carbon in these
applications. Keep in mind that black elastomers will also be
attacked by oxidizing agents because of their carbon content.
- Ceramic vs. ceramic is a good choice for oxidizing
- If you're going to use plated Tungsten Carbide as a face
material, use only the nickel base Tungsten Carbide. Cobalt base
is too hard and can crack with normal seal face differential
temperatures. Nickel base, because of its superior corrosion
resistance, is the preferred material for solid Tungsten Carbide
- Reaction bonded Silicone Carbide has excellent wear
characteristics, but contains up to 17% free silica which can be
attacked by many chemicals including caustic. Alpha sintered
Silicone Carbide is also available and is Silica free.
- 85% ceramic should never be recommended as a hard seal face
because it can break with as little as a 100 degree Fahrenheit (55
C) temperature difference. 99.5% would be a much better
- Plating or coating a seal face will not give it corrosion
resistance. Coatings are used for wear resistance and low
friction. To get corrosion resistance the outer coating must be at
least 1/8" (3 mm) thick. If the base material is not corrosion
resistant to the pumping fluid and any cleaners or solvents used
in the lines the corrosive will go through the coating and attack
the base, causing the plating to come off in sheets.
Selecting Materials - The Metal Parts.
- Be sure to use low expansion metal such as Carpenter 42 or
Invar 36 for your metal bellows seal face holder if the product
temperature can exceed 400° Fahrenheit (205°C). These
low expansion steels will prevent the carbon or hard seal faces
from leaking between the face and the metal holder. Needless to
say glue or epoxy is not a sensible solution to differential
- If your pump is manufactured from Iron, steel, stainless
steel, or bronze, you can probably use a seal manufactured from
316 stainless steel components. The springs or bellows, however,
must be manufactured from Hastelloy "C" to avoid problems with
Chloride Stress Corrosion.
- Use only stationary mechanical seals (the springs do not
rotate with the shaft) if the face surface speed exceeds 5000 feet
per minute ( 25 M/sec.), but never in a cartridge design unless
some method has been provided to insure that the cartridge sleeve
is square to the shaft.
- Use o-ring balanced seals in vacuum applications down to
10-2 inches, or one millimeter of mercury (1 Torr.).
The o-ring is the only elastomer that can seal both vacuum and
pressure. Split seals will work in these applications, but they
must be turned around for best operation.
- Any good quality, balanced, o-ring seal can seal stuffing box
pressures to 400 psi (28 bar) and temperatures to 400 degrees
Fahrenheit (205° C). There is a compound of Dupont's
Kalrez® that is satisfactory to 600 degrees Fahrenheit
(370° C), but it 's not acceptable at ambient temperatures
(it gets too hard).
- A Balanced o-ring seal should not vaporize the product at the
seal face if the stuffing box pressure is at least one atmosphere
above the products vapor point.
- The easiest product to seal is a cool, clean, lubricating
liquid. All problem chemicals can be placed into several
categories. If you know how to seal these categories you should
have no trouble making seals work in your applications :
- Products that crystallize (caustic or sugar solutions)
- Viscous products (asphalt or molasses)
- Products that solidify (polymers or chocolate)
- Products that vaporize (hot water or benzene)
- Film building liquids (hot petroleum or plating
- High temperature fluids (heat transfer oil or liquid
- Dangerous products (fire hazard, explosive, radioactive,
- Non lubricating liquids (solvents or hot water)
- Gases and dry running applications (hydrogen)
- Dry solids (cake mix or pharmaceuticals)
- Corrosive fluids (acids or strong bases)
- Cryogenics (liquid nitrogen)
- Slurries (river water, sewage, most raw products)
- In addition to these chemical categories there are other
sealing problems that include:
- High pressure
- Hard vacuum
- High speed
- Excessive motion
- Dual seals should be balanced in both directions to prevent
failure when barrier fluid pressure changes. The practice of using
"one direction" seal balance is commonly employed by most seal
companies and should be avoided for both safety and
- Use motion seals on mixers, agitators, sleeve bearing
equipment and any rotating device that has motion greater than
0.005" (0,15 mm.) in a radial or axial direction. Pump seals do
not work well in these applications because the hard faces are too
narrow and the internal seal clearances are too tight.
- Avoid using flushing fluid as a coolant in stationary
mechanical seals. The coolant will be directed to only one side of
the seal and since a stationary seal does not rotate the sliding
components, the differential temperature can cause the faces to go
out of flat. In the case of stationary bellows seals, it could
cause a bellows rupture.
- The best way to cool a seal is to use the jacketed stuffing
box that came as a part of the pump. This jacket will not only
cool down the seal area, but will provide the necessary cooling to
the shaft so that it will not transmit stuffing box heat back to
- The use of steam in a Quench gland is another solution, but
not as good as the jacketed stuffing box.
- It 's all right to dead end fluid in a stuffing box if a
jacketed stuffing box is being used, but do not attempt to
recirculate back to the suction side and cool the stuffing box at
the same time. When using a jacketed stuffing box it is best to
install a carbon bushing in the bottom to act as a thermal barrier
between the pumping fluid and the seal.
- Do not use rotating, "Back to Back" double seals in dirt or
slurry service. The solids will prevent the inner seal from moving
forward as the faces wear and if the barrier fluid pressure is
lost, solids can open and penetrate between the inner seal
- Be sure to vent vertical pumps back to the suction side of the
pump. Air trapped in the stuffing box can cause the seal faces to
run hot and in some instances destroy the elastomer.
- Cyclone type separators, or "in line filters" are not a good
method of cleaning up the fluid in the stuffing box.
- Heat affects a seal several ways:
- The faces can be attacked. Plated faces can have the hard
coating crack off and filled carbons can have the binder melted
out in high heat.
- The elastomer (rubber part) has a temperature limit
determined by the compound used.
- The corrosion rate of all liquids increases with
- Thermal expansion can cause seal face loads to alter and
seal face flatness to change.
- Many products will change from a liquid to a solid or gas
at high temperature. If this should occur between the seal
faces, they can be blown open.
- Do not be tempted to put the mechanical seal outside of the
stuffing box to keep the springs out of the fluid. As the face
wears, the seal must move into the slurry where it will eventually
"hang up" and leak. In these applications centrifugal force is
throwing solids into the lapped faces and if there is excessive
pressure in the system the seal faces will be blown open.
- When choosing the pressure range of a mechanical seal be sure
to consider the stuffing box pressure not the pump discharge
pressure. Very few seals will ever see discharge pressure.
- Seals lapped to less than three helium light bands (
0.000034") inches or 1,0 microns) should not show visible leakage.
Visible leakage occurs at about 5 light bands.
- A typical mechanical seal face load would be 30 psi. (0,2
N/mm2) when the carbon is new and 10 psi. (0,07
N/mm2) when the carbon is fully worn away. You must
never guess as to how much to compress a mechanical seal. Either
take the information from the seal print or calculate the correct
length from the above information.
- Both rotating and stationary metal bellows seals require
vibration damping. Elastomer seals do not experience this
vibration problem because the elastomer touching the shaft is a
natural vibration damper. Vibration can be either harmonic or
caused by poor lubricating fluids (slip stick)
- Use only non-fretting seal
designs. Shafts and sleeves cost too much to ignore this
- Carbon throat bushings should have a shaft clearance of 0.002
inches/inch (0,002 mm/ millimeter) of shaft diameter. If they are
going to be used as a support bearing you should cut the clearance
down to 0.001 inches/ inch (0,001 mm/millimeter) of shaft
- It is not necessary to lubricate seal faces at installation.
If the product you are sealing can vaporize between the faces and
cause freezing then you must remove any lubricant that might have
been placed there by the manufacturer.
- Balanced mechanical seals consume about one sixth the
horsepower of packing. Packing a pump would be like running your
automobile with the emergency brake engaged. The car would run,
but the fuel consumption would be high.
- Single spring seals are wound in either a right or left handed
direction. Check to see if your seal will a problem in keeping the
faces together because of the spring winding.
- Open impeller pumps require impeller adjustment. Use only
cartridge or split seals in these applications. Do not use seals
that locate against a shoulder or set screw to the shaft, as the
face load will change when the impeller is adjusted.
- Do not relap the carbon face unless it's an emergency. Seal
face opening is a common seal failure. When the faces open solid
particles imbed them selves into the carbon face and will be
driven in even further during the lapping process. If you must
relap in an emergency never use lapping powder, as the abrasive
particles will imbed into the soft carbon.
- You cannot balance an inside seal by removing material from
the carbon face. To get seal balance you must do one of the
- Use a stepped sleeve with rotating seals.
- Let the carbon slide in a holder that is sealed to the
- Use a metal bellows. The balance is not perfect, but good
- Use a stationary seal design, they require no stepped
- Seal face hardness is a confusing subject because of the
various measuring scales employed. The two most common are
Rockwell "C" and Brinnell. If you divide the Brinnell scale by ten
(10) it is almost equal to the Rockwell "C" scale.
- Avoid oil as a barrier or buffer fluid between two mechanical
seals. Most petroleum base and other oils have a low specific heat
(0.2 - 0.4) and combined with poor conductivity (0.5 of water)
makes them a poor choice compared to fresh water. If oil is
mandatory, a clean heat transfer oil would be your best
- A convection tank can often be used between two balanced
o-ring seals. If you use unbalanced seals the extra heat generated
by this type of seal is usually excessive for convection cooling.
Contact the seal manufacturer for his recommendations concerning
speed, diameter, face combination and pressure limits for
convection cooling. If convection is not satisfactory, a pumping
ring or forced lubrication is another option.
- If you decide to repair your mechanical seals in house, be
sure to purchase the parts from the original manufacturer. If you
decide to have them repaired send them back to the original
manufacturer. It 's important that the seal be rebuilt with the
original materials and it must meet the original tolerances. This
information is not available from the manufacturer because of
product liability problems. It's more difficult to troubleshoot a
repaired seal because old marks on the comonents can confuse the
- O-ring seal designs can tolerate three to four times the "run
out" capability of sliding or pusher seals incorporating wedges,
chevrons, U- cups etc..
- Oil on the seal faces can cause the faces to stick together
during long periods of non running. If you do not intend to run
the equipment soon remove any oil that might be on the seal faces
during the assembly procedure.
® DuPont Dow elastomer
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