One of the primary uses of
bellows is to absorb dimensional changes due to thermal effects, which is very
useful when used high temperature flows such as steam. Bellows also serve to dampen vibration in the
system caused by rotating components, protect sensitive and brittle processing
equipment, and to absorb shock loadings.
Why is PTFE a popular choice for bellows?
PTFE (also known by its trade
name Teflon) is a popular choice for the bellows material. It is ideal for use in highly corrosive
environmentssuch as those involving strong oxidizing and reducing accents,
salts, high concentrations of acid, and chemically active organic
compounds. It has an extremely long flex
life (how many flexing cycles it can handle before it fails), and a very low
spring rate (amount of force needed to flex the bellows) – which means that it
can reliably handle the challenge of fluctuating and vibrational loadings.
What types of movements can bellows be used to absorb?
There are three types of
movement that bellow expansion joints can absorb: axial deflection, lateral deflection, and
angular deflection. Axial deflection
includes compression and extension affects along the longitudinal axis of the
bellows. Lateral deflection occurs when
the end joints of the bellows displace relative to each other. Also known as
parallel misalignment, this type of deflection can also be absorbed by a bellows
expansion joint. Angular deflection can be described as a rotational
displacement, or twisting displacement.
How does the number of convolutions affect bellow
performance?
Recall that a convolution is
the smallest flexible unit in a bellows.
The general heuristic for bellow convolutions is this: fewer convolutions will give you better
pressure and temperature ratings, BUT the amount of movement it can handle is
more limited than bellows with more convolutions. More convolutions, on the other hand, can
absorb more movement but at a cost in pressure/temperature ratings.
Are there other polymers used for bellows?
Yes, another polymer option
for bellows is UHMW PE, ultra-high molecular weight polyethylene. While not as chemically resistant as PTFE, it
currently has the highest impact strength of any polymer on the market
today. If the bellows are used in
connection with abrasive materials, UHMW PE would be a valid alternative to
PTFE because it has better abrasion resistance.
PFA, or Perfluoroalkoxy or
Teflon PFA, is similar to PTFE(teflon) in many ways and is someone chosen in place of
PTFE because it offers higher strength at extreme temperatures, even in the
presence of extremely aggressive chemicals.
TFM, or PTFE-TFM, is a
second-generation PTFE that has better fatigue properties than PTFE and offers
better stress recovery. It is well
adapted for situations that involve high temperatures and vacuum pressures.
Bellows Conclusion
Bellows serve a variety of
purposes – form absorbing displacement and shock to preventing sensitive
equipment of a brittle nature. They can
absorb axial, lateral, and angular displacements. The number of convolutions in a bellow is
related to both its strength and pressure rating as well as the maximum amount
of displacement it can absorb.
Finally, polymers such as PTFE(teflon),
UHMW PE, PFA, and TFM are popular choices for bellows materials, although PTFE
seems to remain the first choice for many engineers.Tags:teflon ptfe,teflon,bellows
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