CHOOSING THE CORRECT MECHANICAL SEAL
The following discussion should help you select the correct
type of mechanical seal, install the proper environmental
controls, choose the right materials for your application, and
install the seal correctly. I have written this course so that you
can click on any of the blue underlined
words to learn more about that particular subject.
If your plant does not have a set of sealing
specifications, I recommend that you learn how to specify the
mechanical seal, and not take the seal recommended
by the pump or original equipment manufacturer. You know your product
better than anyone, and with a little training, plus a lot of common
sense, you will be the best qualified to select a proper sealing
Let me mention here at the beginning that mechanical seals fail
for only two reasons:
Please keep that information in mind as you go through this
We will begin with the type of seal you should be using. Seals
fall into many categories:
- Hydraulically balanced
seals are the only type to use because they generate less heat
and they can handle a wide range of pressure as well as a
reasonable vacuum. If the pump suction pressure (not discharge)
exceeds 400 psi (28 bar) you might have to go to a special version
of a balanced seal. Likewise if a vacuum on the suction side of
the pump measurers less than one "torr" you will have to go to a
seal with no elastomers because elastomers "out gas" and shrink as
the air is pulled out of the compound. This shrinking will cause
the elastomers to leak and the vacuum can be destroyed.
- The stationary version of the
seal is the best design because it will compensate for
misalignment between the pump and its driver, as well as the
stuffing box not being square or perpendicular to the rotating
shaft. This type of seal also has some real advantages if you are
converting a packed pump to a
mechanical seal or running at greater than electric motor
- Metal bellows seals should be selected if the product
temperature is in excess of 400° F (200°C) and no
cooling of the stuffing box is allowed. Metal bellows seals are
also a good choice for cryogenic (cold) service. Remember that
petroleum products have to be
cooled because they will "coke or carbonize", open the seal faces,
and clog up seal components. Metal bellows seals are also a
logical choice if no elastomers are allowed in the system.
- Dual seals are becoming very
important because of fugitive
emissions laws. They can also act as a spare seal if you
cannot afford an unexpected shut down. Dual seals should always be
designed with a "two way" hydraulic
- Non metallic seals should be selected if the pump wetted parts
are manufactured from non-metallic material such as carbon,
Teflon®, plastic, etc. These seals are normally mounted as
outside seals and have no metal parts exposed to the fluid you are
- Do not depend upon a flush into the stuffing box to protect
the seal materials against corrosion, and do not try to second
guess the application engineer that selected a non-metallic
pump for this application. Stick with a non-metallic seal
design in all non-metallic pumps unless you are a corrosion
expert and willing to select the correct metal for the
- Cartridge seals are necessary for ease of installation and to
make pump impeller adjustments without changing the seal face
loading, but some of them have
problems. Cartridge seals also allow you to compensate for
thermal growth of the shaft.
- Split seals can be used in
most applications today, especially the larger shaft sizes. In
most cases you can solve the
packing conversion or seal leakage problem without taking the
pump apart and getting involved in a lot of "down time". Split
seals are the only real alternative to cartridge seal designs.
Today they are available in both rotating and stationary
- Most products contain solids so the seal should have built
in slurry features. With the
exception of most "finished" product, accept the fact that you are
pumping a slurry.
- Be sure that the brand of seal you select will not frett
or damage the pump shaft. This will allow you to use solid
shafts instead of the inferior sleeved version with its high
After choosing the seal type we must select the seal materials.
Most of the time you will be choosing three different materials. If
you do not know which ones to select, click on the appropriate
underlined blue heading:
- The metal components
- You are concerned only about the parts that get wet in the
fluid, unless the pump is in a very corrosive atmosphere. The
wetted parts include the pump volute, impeller, shaft or
sleeve, mechanical seal, stuffing box etc.
- The different grades of carbon/graphite
- Carbon/graphite manufacturers supply a
carbon/graphite chart that will give you the details of any
particular grade they manufacture. With few exceptions an
unfilled carbon such as Pure grade 658 RC is always a safe
choice if any grade of carbon is acceptable in your
- The hard face
- There are many to choose from, but self sintered silicone
carbide will probably be your first choice. Reaction bonded
silicone carbide is also popular, but can be attacked by many
high pH fluids. The common cleaner "caustic" is a fluid that
will attack reaction bonded silicone carbide.
- The rubber parts (elastomers) that are exposed to the fluid.
- The dynamic O-ring (the one that flexes and rolls) is the
most critical. You can learn about elastomers and a four step
procedure to select the correct elastomer for your application
by clicking here. Part of the
selection process is to refer to a good
chart that is also available in this site.
- Most chemicals and chemical combinations can be sealed with
either a good grade of Viton® or a good grade of ethylene
propylene. Most solvents and a few other chemical combinations
require something a little better. The super
elastomers have just about replaced Teflon® in those
applications, but they cannot be used every where.
- Remember also that all elastomers have an upper and lower
temperature limit that must not be exceeded.
You must choose seal materials that are chemically compatible with
all the fluids that might be passing through the equipment, and that
includes any cleaners or solvents that might flush through the lines.
This is especially critical with the choice of elastomer (O-ring).
The metal parts and face combination are usually compatible with most
cleaners and solvents.
The next step is to place your chemical into the proper sealing
categories. Keep in mind that most chemicals fall into several
sealing categories at the same time. If you are not familiar with
these divisions you can click on the following for details :
Click on the following topics for more detailed information about
particular sealing problems.
After you have placed your fluid in the proper sealing categories,
selecting the correct environmental
control comes next. If the equipment is going to operate twenty
four hours a day and seven days a week, without shut down, you have a
simple sealing problem. But if you are on batch or intermittent
service, or if a standby pump is run occasionally then an
environmental control will become critical. You need to know about
the three most important environmental controls :
- Control the temperature in the stuffing
- You can raise the temperature, lower it or keep it within
certain limits that will be dictated by the seal design and
product characteristics. Temperature
control is the most important when the pump is shut down
and the pumping fluid either cools down or heats up because of
- Control the pressure in the stuffing box
- There are many occasions where you will want to control
stuffing box pressure to stop a fluid from vaporizing,
flashing or evaporating. We seldom have to lower stuffing box
pressure, but if it is too high for a conventional balanced
seal you will be better off going to a special high
- Cleaning up the product in the stuffing
- Clean products are easier to seal .
Flushing is only one of your options, there are more.
Any of these environmental controls will work better if you have
installed an oversized stuffing box
on your pump. The seal needs lots of radial room to allow centrifugal
force to throw solids away from the lapped seal faces and to lessen
the generation of heat in the stuffing box. Heat
is a major cause of problems with both centrifugal pumps and
mechanical seals. Any thing you do that will help to remove
heat from the stuffing box will add to the life of your seal and
Check the paper "A few rules of
thumb" for a summary of the above with a few additional
If you have done all of the above, all that remains is to
install the seal correctly, adjust
the pump impeller (if it is an open, or semi-open design) and hook up
the environmental control you chose to keep the seal functioning.
With a little luck you should now get good seal life, but unless you
are prepared to do a few pump
modifications your seals may last a very long time, but they will
never wear out, and last as long
as they could.
® Dupont Dow elastomer
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