Subject: An overview of
metal bellows sealing.12-10
Metal bellows seals perform many functions. Unfortunately all the
functions are not available in the same seal.
- They can be used to eliminate elastomers (rubber like parts)
in the chemical process industry. Most chemicals and chemical
combinations can be sealed with either a good grade of Viton®
or ethylene propylene, but someone has to make the decision and
many responsible people are not capable, so mistakes are often
- Most solvents present a real problem for elastomer selection.
Expensive materials like Kalrez® and Chemraz are often the
only solution. You would be better off if you could eliminate
these special materials all together
- Metal bellows are also used to eliminate elastomers because of
temperature limits. All elastomers have both an upper and lower
temperature limit that prevents them from sealing many hot resins,
polymers and cryogenic applications. Hot oils are another high
temperature sealing problem, but their "coking" characteristics
dictates cooling of the stuffing box area.
The individual bellows convolutions can be formed in different
- Convoluted, stamped plates can be welded. This is the most
popular type used in mechanical seals. End fittings are welded to
the bellows to hold the seal faces, sleeve and gland
- The bellows can be formed by forcing a metal tube into a die.
Here you are limited to ductile material that have to be stretched
to conform to the die, leaving thin and thick cross sections.
"Crushed formed" techniques have helped, but they still lack the
reliability of welded bellows. You have seen many of these formed
bellows used in commercial expansion joints.
- The bellows configuration can be plated onto a wax mold that
can later be melted away to leave the bellows configuration. The
resultant thin bellows section eliminates this style for
mechanical seals, but they are frequently used in
In the following drawing we will learn the names of the individual
parts of a typical "nested convolution" bellows seal
Please take a look at the following diagram for some more bellows
- A convolution is two stamped plates welded together. You can
count the number of convolutions in the seal by counting the
spaces between the end fittings.
- The weld bead fusing the plates together is about 2.5 times
the thickness of an individual plate (0.004" or 0.10 mm).
- The span is the width of the plate. A 0.250 inch (6 mm) span
is the most popular but seldom the most sensible. Most bellows
seals come in this cross section because the tooling is readily
available. The wider the span the less convolutions you need to
get the desired spring rate for the proper face loading. If you
use too many convolutions you end up, with a "slinky toy".
- The pitch is the distance between the plates. You measure the
pitch from the center of a weld bead to the center of an adjacent
weld bead. 0.040 inches (0.10 mm) is typical in mechanical seal
The driving end of the bellows seal can be attached to the shaft
in several ways:
- It can be welded to a sleeve, and the sleeve clamped
and held to the shaft by the impeller. A stainless steel
gasket can prevent leakage between the sleeve and the
shaft. This method is also used to attach the bellows to
a stationary gland.
- Soft Aluminum gaskets have been tried in this
location, but they never worked out very well
- The end fitting can be sealed to the shaft with a
combination of set screws and a graphite wedge or V
- This is a popular attachment method in the chemical
industy. Sometimes a Teflon® wedge is substituted for
the graphite wedge
- The seal can be held and sealed to the shaft with
hydraulic force. As shown in the sketch, when you tighten
the cap screw the expanding fluid exerts a holding and
sealing force on the thin metal section touching the
- This method shows a lot of promise for elevated
temperature applications, but should not be used in
You have a choice of different
metals for the bellows plates:
- Hastelloy "C" is a good choice for most pumps because of its
chemical compatibility, but it may not be thick enough for a
Hastelloy "C" pump. Most bellows convolutions are only 0.004
inches (0.10 mm) thick and the definition of corrosion resistant
is that the material can corrode up to 0.002 inches (0.05 mm) per
- The 300 series of stainless steel should never be used because
of the probability of chloride stress corrosion problems.
- AM350 is a heat treatable form of stainless steel that has
been used successfully for many years in high temperature and
cryogenic seal applications. You need a heat treated material
because it has to retain its strength and spring rate at these
- Inconel 718 is a metal that has good corrosion resistant
properties in an annealed form and retains some of the corrosion
resistant properties after heat testament. It has become the
favorite of oil refinery people because of corrosion problems they
have experienced with AM350 after five or six years of
- Titanium, 17-4 PH and variety of other materials have been
used as bellows seals. In every case you are looking for high
strength and chemical resistance. A tough combination to put
There are several ways to retain the seal face in the bellows end
- Shrink fitting the carbon in a metal holder is not
usually a good idea. Both the holder and the face are out
of round to some degree. When the holder is expanded and
allowed to shrink around the seal face it will put uneven
stresses on the face outside diameter causing it to go
out of flat.
- If you install the carbon face this way you will have
to stress relieve the assembly to keep the seal face
flat. This can be done by taking the assembly through a
series of temperature transients or leaving the assembly
on the shelf for several months to relax and then relap
the seal face.
- A press fit makes sense with carbon because the
carbon will shear, to conform to the "out of roundness"
of the harder metal holder.
seals have been used successfully since the late 1950s, but they are
not trouble free. If they were, we would use them all the time. Here
are some of their limitations:
- Elastomer seals have a built in vibration damper. Metal
bellows seals lack this feature so a damper must be built in. The
most common method is to let the seal face holder come into
contact with the shaft when vibration starts. You can see this
feature in the first illustration
- "Slip stick" vibration is the most common. It occurs if the
product you are sealing is not a good lubricant (hot water as an
example). The resultant "slipping and sticking" between the lapped
faces causes the vibration.
- In the stationary version of the seal it is hard to get an
even cooling or heating of the bellows and seal faces unless you
have paid close attention to the location of the stuffing box
- In abrasive, slurry service the bellows plates may prove to be
too thin. Try to rotate the slurry with the bellows and you will
reduce the plate wear.
- Thicker plates are always desirable, but their higher spring
rate would cause the use of too many convolutions to get the
desirable spring load of about 30 psi on the seal faces. When the
carbon face is worn down there should still be a load of about 10
psi.on the faces to prevent vibration from causing them to
- Hard face retention in a holder is a persistent problem, and
there are times you really need two hard seal faces. Shrinking a
hard face in a metal holder has the same problems we discussed
about carbon a few paragraphs back.
- When bellows seals are used in temperature extremes they
should be provided with an API (American Petroleum Institute)
gland or back up seal.
- Since the face holder has a different expansion and shrink
rate than the seal face, high temperature applications require
that the face holder be manufactured from low expansion metals
such as Invar 36 or Carpenter 42 materials. These metals have poor
® Dupont Dow elastomer
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