SUBJECT : A new way of classifying
chemicals to assure effective sealing 2-12
The most common question asked by seal salesmen is "what are you
sealing?" This is usually followed by asking about shaft size,
product, temperature, speed, stuffing box pressure and any other
operating conditions they can think of.
The problem with this simplistic approach is that you'd have to
have a very large data bank of information to reference a particular
problem so as to be able to make a sensible seal recommendation.
There's a much more logical approach to the problem that we'll be
discussing in the following paragraphs.
A sensible approach to the sealing of various chemicals, mixtures,
and compounds would be divided into three parts:
- You must know how to select mechanical seal components that
will not corrode, or be attacked in any way by the fluid you are
sealing, or any other chemicals that might come into contact with
the seal as a result of cleaning the system, flushing the stuffing
box, using barrier fluids between double seals, quenching behind
the seal etc..
- You must be knowledgable about the total range of operating
conditions of the equipment and then select seal designs that can
handle this range.
- You need a method of classifying chemicals that puts them into
neat, logical categories that can be handled by the use of a
special seal design and/or environmental controls. It's important
to note that the sealing environment will affect the sealing
fluid, often preventing the lapped sealing faces from staying in
In this paper we'll concentrate on the classification of chemicals
and leave the selection of seal materials, types of seals and use of
various environmental controls to other papers on this site
A fluid can be classified as either a liquid or a gas, and can be
divided into seven categorizes:
1. Fluids sensitive to changes in temperature and/or
2. Fluids that require two mechanical seals.
3. Non lubricating liquids, gases and solids.
4. Slurries, classified as solids in liquid . The solids may or
may not be abrasive.
5. Liquids sensitive to agitation.
6. Liquids that react with each other to form a solid.
7. Lubricating liquids.
We will be investigating each of these categories in detail and
learn how they affect the life of a mechanical seal. In other papers,
we will learn the detailed methods of sealing each of these
Fluids that are
sensitive to changes in temperature and/or pressure.
- Corrosive liquids - Most
corrosives will double their corrosion rate with a 18 degree
Fahrenheit (10 C.) rise in temperature. The temperature at the
seal face is always hotter than the temperature recorded in the
stuffing box ,or seal chamber. Keep in mind that any contact
between the rotating shaft and a stationary component will cause
high heat and will be detected as localized corrosion. Wear rings
and throttle bushings are subject to this rubbing. If the
equipment is provided with a cooling jacket. and it is not being
utilized, the air inside can act as an insulation increasing the
heat in the stuffing box that can migrate down the shaft and
affect the bearing temperature.
- Liquids that vaporize - Most any
liquid will vaporize if it becomes hot enough, or if the stuffing
box pressure gets too low. It 's the product with a low specific
gravity that give us the most trouble. If the product vaporizes
between the lapped seal faces, it'll separate the faces as the
gases expand. When hot water vaporizes it leaves behind any
chemicals that were dissolved in the water. Most of these
chemicals are left in a hard crystal form that will damage the
- Fluids such as benzene and others with a low specific
gravity, will freeze as they vaporize. If any oil or lubricant
was placed on the seal face it'll freeze and possibly damage
the lapped faces. Moisture on the outboard side of the seal
will also freeze and restrict movement of the sliding or
flexing seal components
- Liquids that solidify - Some
solidify with an increase in temperature, others with a decrease.
Solvents vaporize with lower pressure, leaving any solids behind.
Paint is a good example of a product where the solvent will
vaporize at or below atmospheric pressure. In most cases you can
reference a vapor pressure chart to learn when the solvent or
carrier will flash off in your application.
- Viscous Products - Their
viscosity usually decreases with an increase in temperature and
increases with a decrease in temperature. Oil is a good example of
this type of fluid. High viscosity can interfere with free seal
movement and cause seal face contact problems. Lowering the
viscosity can often increase the seal face wear as there is not
enough film thickness to keep the surfaces separated. You need a
film thickness of at least one micron to keep the lapped seal
- Film building liquids - Petroleum
products will form a varnish when first heated and then gradually
form a layer of coke as the temperature is elevated. These
transformations are not reversible and the resultant hard film
restricts sliding and/or flexing of the seal components. Hard
water is another example of a film building fluid.
- Hot water systems pick up
magnetite (Ferric Oxide) from the inside of the pipes. It is black
or reddish in color and will be attracted by a magnet. This
abrasive material will collect on the seal components and destroy
the dynamic O-Ring as well as restrict the movement of the seal
causing the lapped faces to open. Magnetite is a severe problem in
new , hot water systems. The problem will lessen as the system
ages and the protective film stabilizes.
- Liquids that crystallize - Sugar
and salt solutions are two examples of these fluids. If the
crystals form between the faces they can destroy the carbon. If
they form in the sliding or flexing components they will open the
seal faces as the shaft moves. Any leakage across the seal faces
will form a solids build up on the other side of the seal causing
interference as the seal tries to move when it compensates for
The names of these chemicals is not important. If you knew how
to seal any one of them you could seal all of them. It is just a
matter of fitting the particular chemical into the right
categories and learning how to seal that category. Common sense
would dictate that the product temperature and/or pressure must be
controlled in the seal area to prevent any of the above from
occurring. In most cases you should try to avoid the use of two
hard faces in these applications because of the additional heat
that will be generated between the faces. Needless to say, only
hydraulically balanced seals are acceptable in any temperature/
pressure sensitive fluid.
Liquids that require
two mechanical seals : These seals are installed with a circulating
barrier fluid that can be a "forced circulation", or in many cases a
convection system with a "pumping ring." The pressure of the barrier
or buffer fluid can be regulated to indicate a failure in either of
the mechanical seals allowing time for a pump shut down, isolation
and no subsequent loss in the pumping fluid.
- Costly products - Some times the
product costs so much you just cannot afford to have it leak.
There are plenty of charts to show how much leakage you get from
various sized drips or steady streams. The smallest steady stream
you can produce will be between twenty five and thirty U.S.
gallons per day (95 to 115 liters/day)
- Dangerous products - these fluids
are given a special category because even small amounts of leakage
aren't acceptable. The danger could fall into many categories:
radiation, toxic, fire, explosion, bacteria, etc.. The United
States' "Right to know law" is having a major affect on how
mechanical seals used in these type of products will be
- Pollutants - Usually there is a
"penalty" involved and the bad publicity does no one any good. In
this day and age a responsible company will not let pollutants
leak to the atmosphere or into the soil for any reason. Fugitive
emission legislation has increased the need for these type of
- Any time an unexpected seal failure
would be inconvenient - Down time can be a very costly time
in many plants. Two seals can prevent the unexpected seal failure
shut down. This is especially important with batch operations or
when there is no back up pump installed. On the atomic submarine
NAUTILUS the back up shaft seal allowed us to get to the surface
if a main shaft seal failed while we were submerged.
Sealing non lubricants.
- Dry solids - They can clog the
sliding components and provide no lubrication for seal faces. Once
the faces are open the solids penetrate between the faces and
usually destroy the lapped surfaces. Pharmaceuticals, freeze dried
coffee and cake mix are examples of this category. You can think
of many more.
- Non lubricating fluids such as
solvents and hot water. We experience more rapid face wear with
these types of fluids. In most cases their film thickness is less
than one micron and cannot support a load between two sliding
- Dry gases- unlike non lubricating
liquids they will not conduct heat very well and often are
dangerous at the same time. This is a common problem if you forget
to vent the stuffing box of a vertical pump. A top entering mixer
is another example of this type of application.
abrasive slurries. Clog the seal components and destroy faces like
the dry solids mentioned above.
- The list of these products is without end. A slurry is defined
as solids in liquid that cannot be dissolved by normal control of
the temperature or pressure. The number of solids, or their size
is not important. They'll collect on or in the sliding or flexing
components of the seal causing the faces to open and then
penetrate between the lapped faces, causing leakage and damage. In
some designs the springs or bellows (metallic or elastomer) will
experience severe wear in a short period of time. In these designs
it's important to rotate the fluid rather than have the bellows
component rotate within the abrasive slurry.
Liquids sensitive to
- Dilatants - Their viscosity
increases with agitation. This is how cream becomes butter. Some
clay slurries have the same problem. The resulting high
viscosities will restrict the free movement of the seal. When
dealing with dilatants it is important that you do not continually
rotate the fluid in the stuffing box area.
- Thixotropic fluids lower their
viscosity with agitation. They seldom present a problem for
mechanical seals except for an increase in seal face wear.
- Plastic fluids change their
viscosity suddenly. Catsup is a good example of this type of
- Newtonian fluids do not change
viscosity with agitation. They present no problem for mechanical
Liquids that combine
together to form a solid.
- Epoxy is a combination of a Resin
and a hardener.
- Styrofoam is formed by combining
several liquids together.
We seldom have problems with these liquids in pumps because the
blending takes place outside of the pump, but the problem sometimes
comes up in mixer applications. You'll note that I have not included
anaerobic fluids (they solidify in the absence of air) in any of the
categories (super glue is the product that first comes to mind).
- This is the ideal application for a mechanical seal. More
often than not we're sealing raw product that falls into one or
more of the above categories. Back in the days when we were using
packing in pumps we didn't pay too much attention to these
categories because we were either prepared to let the product leak
on the ground, or we would flush in clean liquid and concentrate
on sealing the clean flush instead.
Now that leakage is no longer tolerable and product dilution is no
longer desirable you must have knowledge of these categories to
approach the job of effective sealing. In most cases the fluid you're
sealing will fall into several of the above mentioned categories.
Using heat transfer oil as an example we
note that it falls into the following :
- Hot - Normally pumped at 600 -700
Fahrenheit ( 315 -370 C.) the fluid is too hot for available
- Film Building - The product
"cokes" at these temperatures.
- Dangerous - You don't need this
temperature oil leaking out. It's not only a fire hazard, but a
personnel hazard as well. Recent information indicates that some
of these oils are also carcinogenic.
- Costly - Leakage gets
- Slurry - Because of the coking,
solids are always present.
To successfully seal heat transfer oil you'd have to address all
of these problems at the same time. As is the case with all slurry
applications, you'd also have to recognize the problems with
vibration (impeller imbalance), thermal growth, and frequent impeller
In addition to handling
various chemicals we're often faced with extreme or severe operating
conditions. These conditions would include:
- Hot products - Defined as too hot
for one of the seal components, or hot enough to cause the fluid
to change form. Heat transfer oil is a good example of a fluid
that will "coke" at elevated temperature.
- Cryogenic fluids - They present a
problem for elastomers and some carbon faces. Liquid Nitrogen or
Oxygen would be an example.
- High Pressure - Defined as stuffing box, (not discharge)
pressure in excess of 400 psi. (28 bar). Pipe line and boiler
circulating pumps can have stuffing box pressures of this
- Hard Vacuum - Defined as 10-2
Torr or below. This number is well below most condenser or
evaporator applications, but does come up every once in a
- High Speed - Defined as the seal
faces moving greater than 5000 Feet Per Minute or 25 meters per
second. Most process pumps do not approach this speed. The
Sundstrand "Sundyne" pump is typical of a high speed
- Excessive motion - defined as
more than 0.005 inches (0,15 mm.) in a radial or axial direction.
Mixers, agitators and specialized equipment have shaft movements
up to 1/8 inch (3 mm). Long shaft vertical pumps and pumps
equipped with sleeve or babbitt bearings, are another application
for excessive motion.
- Excessive vibration -
Unfortunately there are no reliable numbers for the vibration
limits of mechanical seals. Most vibration studies have addressed
the bearings. It's important to consider that excessive vibration
- Open the lapped seal faces.
- Chip the outside diameter of the carbon face.
- Break the metal bellows used in some seal designs.
- Wear the driving mechanism used to transmit torque from the
set screws to the seal faces.
- Loosen drive screws.
- Shorten bearing life
- Most seal designs can damage (frett) expensive sleeves and
- Some, but not all designs have built in vibration dampers
to relieve some of these problems.
In other papers on this site you can learn how to seal each of
these fluid categories and learn how to protect the mechanical seal
against the affects of these extreme operating conditions.
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