- There are three important things to consider when selecting a mechanical seal design: the materials must be chemically compatible with the fluid, the seal faces must stay together to prevent leaks, and good seal life is defined as running until the carbon face wears away. - When selecting materials, it is important to choose ones that are compatible with both the pumped fluid and any cleaners or solvents used, and to select a material that does not exceed the temperature limits of either the seal components or the fluid. - Proper selection of mechanical seal design and materials is important to prevent chemical incompatibility, excessive temperatures, or solids from causing seal failure. The manufacturer should be consulted for specific recommendations.

1. 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
seals.
 The seal faces must stay together. If they open the seal will leak and allow
solids to penetrate between the faces where 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 twenty seven 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:
o 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.
o You must not exceed the temperature limit of the fluid you are 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 we had the seal failure
because we exceeded the temperature limit of petroleum products.
 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.

2.  Most Viton® compounds are attacked by water. Be sure to check if you have
the correct one. Remember that steam is another name for water and the steam
cleaning of lines is very common in the process industry. Caustic is another
common cleaner and caustic contains a high percentage of water also.
 Buna "N" (Nitrile) is an elastomer that 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 from 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 is 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 seal faces.
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. You will find a chart in the chart section of this web site
 Check to see if the plant has any experience with O-Rings, in this fluid, in
another seal application. O-Rings can also be found in filters, strainers, valves,
flanges, expansion joints etc..
 Test the O-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

3. 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 chemicals.
 If you are going to select 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 faces
also.
 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 as it can break
with as little as a 100 degree Fahrenheit (55 C) temperature difference. 99.5%
would be a much better choice.
 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 in 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 expansion problems.

4.  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.
Sealing Limits
 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 is not acceptable at ambient temperatures (it gets too hard).
Application
 A Balanced O-Ring seal will 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 :
o Products that crystallize (caustic or sugar solutions)
o Viscous products (asphalt or molasses)
o Products that solidify (polymers or chocolate)
o Products that vaporize (hot water or benzene)
o Film building liquids (hot petroleum or plating solutions)
o High temperature fluids (heat transfer oil or liquid sulfur)
o Dangerous products (fire hazard, explosive, radioactive, bacteria)
o Non lubricating liquids (solvents or hot water)
o Gases and dry running applications (hydrogen)
o Dry solids (cake mix or pharmaceuticals)
o Corrosive fluids (acids or strong bases)
o Cryogenics (liquid nitrogen)
o Slurries (river water, sewage, most raw products)
 In addition to these chemical categories there are other sealing problems that
include:

5. o High pressure
o Hard vacuum
o High speed
o 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 reliability.
 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.
 Do not use 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 bearings.
 The use of steam in a Quench gland is another solution, but not as good as the
jacketed stuffing box.
 It is all right to dead end fluid in a stuffing box if a jacketed stuffing box is
being used. 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 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 will penetrate the inner seal faces.
 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:
o 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.
o The elastomer (rubber part) has a temperature limit determined by the
compound used.

6. o The corrosion rate of all liquids increases with temperature.
o Thermal expansion can cause seal face loads to alter and seal face
flatness to change.
o Many products will change from a liquid to a solid or gas in the presence
of 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.
Technical
 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 severe problem.
 Carbon throat bushings should have a shaft clearance of 0.002 inches/inch
(0,002 mm/ millimeter) of shaft diameter. If they are to be used as a support
bearing you should cut the clearance down to 0.001 inches/ inch (0,001
mm/millimeter) of shaft diameter.
 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

7. 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 has 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 is 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 following:
o Use a stepped sleeve with rotating seals.
o Let the carbon slide in a case that is sealed to the shaft.
o Use a metal bellows. The balance is not perfect, but good enough.
o Use a stationary seal design, they require no stepped sleeves.
 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
choice.
 A convection tank can often be used between two balanced O-Ring seals. If
you use unbalanced seals the 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 is 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.

8.  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